Johan Andrén
parent
dcfa56e547
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9961495fad
20 changed files with 922 additions and 466 deletions
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@ -9,6 +9,12 @@ is sometimes necessary to define new transformation stages either because some f
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stock operations, or for performance reasons. In this part we show how to build custom processing stages and graph
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junctions of various kinds.
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.. note::
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A custom graph stage should not be the first tool you reach for, defining graphs using flows
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and the graph DSL is in general easier and does to a larger extent protect you from mistakes that
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might be easy to make with a custom :class:`GraphStage`
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.. _graphstage-java:
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Custom processing with GraphStage
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@ -79,7 +85,7 @@ in that state.
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.. image:: ../images/outport_transitions.png
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:align: center
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:align: center
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@ -115,7 +121,7 @@ in that state.
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.. image:: ../images/inport_transitions.png
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:align: center
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:align: center
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@ -125,10 +131,150 @@ Finally, there are two methods available for convenience to complete the stage a
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* ``failStage(exception)`` is equivalent to failing all output ports and cancelling all input ports.
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In some cases it is inconvenient and error prone to react on the regular state machine events with the
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signal based API described above. For those cases there is a API which allows for a more declarative sequencing
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of actions which will greatly simplify some use cases at the cost of some extra allocations. The difference
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between the two APIs could be described as that the first one is signal driven from the outside, while this API
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is more active and drives its surroundings.
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The operations of this part of the :class:``GraphStage`` API are:
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* ``emit(out, elem)`` and ``emitMultiple(out, Iterable(elem1, elem2))`` replaces the ``OutHandler`` with a handler that emits
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one or more elements when there is demand, and then reinstalls the current handlers
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* ``read(in)(andThen)`` and ``readN(in, n)(andThen)`` replaces the ``InHandler`` with a handler that reads one or
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more elements as they are pushed and allows the handler to react once the requested number of elements has been read.
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* ``abortEmitting()`` and ``abortReading()`` which will cancel an ongoing emit or read
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Note that since the above methods are implemented by temporarily replacing the handlers of the stage you should never
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call ``setHandler`` while they are running ``emit`` or ``read`` as that interferes with how they are implemented.
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The following methods are safe to call after invoking ``emit`` and ``read`` (and will lead to actually running the
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operation when those are done): ``complete(out)``, ``completeStage()``, ``emit``, ``emitMultiple``, ``abortEmitting()``
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and ``abortReading()``
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An example of how this API simplifies a stage can be found below in the second version of the :class:``Duplicator``.
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Custom linear processing stages using GraphStage
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------------------------------------------------
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Graph stages allows for custom linear processing stages through letting them
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have one input and one output and using :class:`FlowShape` as their shape.
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Such a stage can be illustrated as a box with two flows as it is
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seen in the illustration below. Demand flowing upstream leading to elements
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flowing downstream.
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.. image:: ../images/graph_stage_conceptual.png
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:align: center
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:width: 500
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To illustrate these concepts we create a small :class:`GraphStage` that implements the ``map`` transformation.
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.. image:: ../images/graph_stage_map.png
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:align: center
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:width: 300
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Map calls ``push(out)`` from the ``onPush()`` handler and it also calls ``pull()`` from the ``onPull`` handler resulting in the
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conceptual wiring above, and fully expressed in code below:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/GraphStageDocTest.java#one-to-one
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Map is a typical example of a one-to-one transformation of a stream where
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demand is passed along upstream elements passed on downstream.
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To demonstrate a many-to-one stage we will implement
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filter. The conceptual wiring of ``Filter`` looks like this:
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.. image:: ../images/graph_stage_filter.png
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:align: center
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:width: 300
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As we see above, if the given predicate matches the current element we are propagating it downwards, otherwise
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we return the “ball” to our upstream so that we get the new element. This is achieved by modifying the map
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example by adding a conditional in the ``onPush`` handler and decide between a ``pull(in)`` or ``push(out)`` call
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(and of course not having a mapping ``f`` function).
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/GraphStageDocTest.java#many-to-one
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To complete the picture we define a one-to-many transformation as the next step. We chose a straightforward example stage
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that emits every upstream element twice downstream. The conceptual wiring of this stage looks like this:
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.. image:: ../images/graph_stage_duplicate.png
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:align: center
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:width: 300
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This is a stage that has state: an option with the last element it has seen indicating if it
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has duplicated this last element already or not. We must also make sure to emit the extra element
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if the upstream completes.
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/GraphStageDocTest.java#one-to-many
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In this case a pull from downstream might be consumed by the stage itself rather
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than passed along upstream as the stage might contain an element it wants to
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push. Note that we also need to handle the case where the upstream closes while
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the stage still has elements it wants to push downstream. This is done by
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overriding `onUpstreamFinish` in the `AbstractInHandler` and provide custom logic
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that should happen when the upstream has been finished.
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This example can be simplified by replacing the usage of a mutable state with calls to
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``emitMultiple`` which will replace the handlers, emit each of multiple elements and then
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reinstate the original handlers:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/GraphStageDocTest.java#simpler-one-to-many
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Finally, to demonstrate all of the stages above, we put them together into a processing chain,
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which conceptually would correspond to the following structure:
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.. image:: ../images/graph_stage_chain.png
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:align: center
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:width: 700
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In code this is only a few lines, using the ``via`` use our custom stages in a stream:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/GraphStageDocTest.java#graph-stage-chain
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If we attempt to draw the sequence of events, it shows that there is one "event token"
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in circulation in a potential chain of stages, just like our conceptual "railroad tracks" representation predicts.
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.. image:: ../images/graph_stage_tracks_1.png
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:align: center
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:width: 700
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Completion
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----------
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**This section is a stub and will be extended in the next release**
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Completion handling usually (but not exclusively) comes into the picture when processing stages need to emit
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a few more elements after their upstream source has been completed. We have seen an example of this in our
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first :class:`Duplicator` implementation where the last element needs to be doubled even after the upstream neighbor
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stage has been completed. This can be done by overriding the ``onUpstreamFinish`` method in ``AbstractInHandler``.
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Stages by default automatically stop once all of their ports (input and output) have been closed externally or internally.
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It is possible to opt out from this behavior by overriding ``keepGoingAfterAllPortsClosed`` and returning true in
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@ -139,8 +285,6 @@ with care.
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Using timers
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------------
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**This section is a stub and will be extended in the next release**
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It is possible to use timers in :class:`GraphStages` by using :class:`TimerGraphStageLogic` as the base class for
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the returned logic. Timers can be scheduled by calling one of ``scheduleOnce(key,delay)``, ``schedulePeriodically(key,period)`` or
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``schedulePeriodicallyWithInitialDelay(key,delay,period)`` and passing an object as a key for that timer (can be any object, for example
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@ -151,11 +295,14 @@ fires. It is possible to cancel a timer using ``cancelTimer(key)`` and check the
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Timers can not be scheduled from the constructor of the logic, but it is possible to schedule them from the
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``preStart()`` lifecycle hook.
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In this sample the stage toggles between open and closed, where open means no elements are passed through. The
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stage starts out as closed but as soon as an element is pushed downstream the gate becomes open for a duration
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of time during which it will consume and drop upstream messages:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/GraphStageDocTest.java#timed
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Using asynchronous side-channels
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--------------------------------
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**This section is a stub and will be extended in the next release**
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In order to receive asynchronous events that are not arriving as stream elements (for example a completion of a future
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or a callback from a 3rd party API) one must acquire a :class:`AsyncCallback` by calling ``getAsyncCallback()`` from the
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stage logic. The method ``getAsyncCallback`` takes as a parameter a callback that will be called once the asynchronous
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@ -167,6 +314,13 @@ implementation.
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Sharing the AsyncCallback from the constructor risks race conditions, therefore it is recommended to use the
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``preStart()`` lifecycle hook instead.
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This example shows an asynchronous side channel graph stage that starts dropping elements
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when a future completes:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/GraphStageDocTest.java#async-side-channel
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Integration with actors
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-----------------------
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@ -188,8 +342,6 @@ or ``unwatch(ref)`` methods. The reference can be also watched by external actor
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Custom materialized values
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--------------------------
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**This section is a stub and will be extended in the next release**
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Custom stages can return materialized values instead of ``Unit`` by inheriting from :class:`GraphStageWithMaterializedValue`
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instead of the simpler :class:`GraphStage`. The difference is that in this case the method
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``createLogicAndMaterializedValue(inheritedAttributes)`` needs to be overridden, overridden, and in addition to the
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@ -200,6 +352,10 @@ stage logic the materialized value must be provided
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the thread that got hold of the materialized value. It is the responsibility of the programmer to add the
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necessary (non-blocking) synchronization and visibility guarantees to this shared object.
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In this sample the materialized value is a future containing the first element to go through the stream:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/GraphStageDocTest.java#materialized
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Using attributes to affect the behavior of a stage
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--------------------------------------------------
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@ -213,237 +369,52 @@ decision.
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See :ref:`composition-java` for an explanation on how attributes work.
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Custom linear processing stages
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===============================
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Rate decoupled graph stages
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---------------------------
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To extend the available transformations on a :class:`Flow` or :class:`Source` one can use the ``transform()`` method
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which takes a factory function returning a :class:`Stage`. Stages come in different flavors swhich we will introduce in this
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page.
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Sometimes it is desirable to *decouple* the rate of the upstream and downstream of a stage, synchronizing only
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when needed.
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.. _stream-using-push-pull-stage-java:
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This is achieved in the model by representing a :class:`GraphStage` as a *boundary* between two regions where the
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demand sent upstream is decoupled from the demand that arrives from downstream. One immediate consequence of this
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difference is that an ``onPush`` call does not always lead to calling ``push`` and an ``onPull`` call does not always
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lead to calling ``pull``.
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Using PushPullStage
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-------------------
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The most elementary transformation stage is the :class:`PushPullStage` which can express a large class of algorithms
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working on streams. A :class:`PushPullStage` can be illustrated as a box with two "input" and two "output ports" as it is
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seen in the illustration below.
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.. image:: ../images/stage_conceptual.png
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:align: center
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:width: 600
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The "input ports" are implemented as event handlers ``onPush(elem,ctx)`` and ``onPull(ctx)`` while "output ports"
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correspond to methods on the :class:`Context` object that is handed as a parameter to the event handlers. By calling
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exactly one "output port" method we wire up these four ports in various ways which we demonstrate shortly.
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.. warning::
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There is one very important rule to remember when working with a ``Stage``. **Exactly one** method should be called
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on the **currently passed** :class:`Context` **exactly once** and as the **last statement of the handler** where the return type
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of the called method **matches the expected return type of the handler**. Any violation of this rule will
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almost certainly result in unspecified behavior (in other words, it will break in spectacular ways). Exceptions
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to this rule are the query methods ``isHolding()`` and ``isFinishing()``
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To illustrate these concepts we create a small :class:`PushPullStage` that implements the ``map`` transformation.
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.. image:: ../images/stage_map.png
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:align: center
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:width: 300
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Map calls ``ctx.push()`` from the ``onPush()`` handler and it also calls ``ctx.pull()`` form the ``onPull``
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handler resulting in the conceptual wiring above, and fully expressed in code below:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/FlowStagesDocTest.java#one-to-one
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Map is a typical example of a one-to-one transformation of a stream. To demonstrate a many-to-one stage we will implement
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filter. The conceptual wiring of ``Filter`` looks like this:
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.. image:: ../images/stage_filter.png
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:align: center
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:width: 300
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|
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As we see above, if the given predicate matches the current element we are propagating it downwards, otherwise
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we return the "ball" to our upstream so that we get the new element. This is achieved by modifying the map
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example by adding a conditional in the ``onPush`` handler and decide between a ``ctx.pull()`` or ``ctx.push()`` call
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(and of course not having a mapping ``f`` function).
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/FlowStagesDocTest.java#many-to-one
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To complete the picture we define a one-to-many transformation as the next step. We chose a straightforward example stage
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that emits every upstream element twice downstream. The conceptual wiring of this stage looks like this:
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.. image:: ../images/stage_doubler.png
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:align: center
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:width: 300
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This is a stage that has state: the last element it has seen, and a flag ``oneLeft`` that indicates if we
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have duplicated this last element already or not. Looking at the code below, the reader might notice that our ``onPull``
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method is more complex than it is demonstrated by the figure above. The reason for this is completion handling, which we
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will explain a little bit later. For now it is enough to look at the ``if(!ctx.isFinishing)`` block which
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corresponds to the logic we expect by looking at the conceptual picture.
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/FlowStagesDocTest.java#one-to-many
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Finally, to demonstrate all of the stages above, we put them together into a processing chain, which conceptually
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would correspond to the following structure:
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|
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.. image:: ../images/stage_chain.png
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:align: center
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:width: 650
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|
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|
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In code this is only a few lines, using the ``transform`` method to inject our custom processing into a stream:
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.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/FlowStagesDocTest.java#stage-chain
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|
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If we attempt to draw the sequence of events, it shows that there is one "event token"
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in circulation in a potential chain of stages, just like our conceptual "railroad tracks" representation predicts.
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|
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|
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.. image:: ../images/stage_msc_general.png
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:align: center
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|
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Completion handling
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^^^^^^^^^^^^^^^^^^^
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Completion handling usually (but not exclusively) comes into the picture when processing stages need to emit a few
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||||
more elements after their upstream source has been completed. We have seen an example of this in our ``Duplicator`` class
|
||||
where the last element needs to be doubled even after the upstream neighbor stage has been completed. Since the
|
||||
``onUpstreamFinish()`` handler expects a :class:`TerminationDirective` as the return type we are only allowed to call
|
||||
``ctx.finish()``, ``ctx.fail()`` or ``ctx.absorbTermination()``. Since the first two of these available methods will
|
||||
immediately terminate, our only option is ``absorbTermination()``. It is also clear from the return type of
|
||||
``onUpstreamFinish`` that we cannot call ``ctx.push()`` but we need to emit elements somehow! The trick is that after
|
||||
calling ``absorbTermination()`` the ``onPull()`` handler will be called eventually, and at the same time
|
||||
``ctx.isFinishing`` will return true, indicating that ``ctx.pull()`` cannot be called anymore. Now we are free to
|
||||
emit additional elementss and call ``ctx.finish()`` or ``ctx.pushAndFinish()`` eventually to finish processing.
|
||||
|
||||
The reason for this slightly complex termination sequence is that the underlying ``onComplete`` signal of
|
||||
Reactive Streams may arrive without any pending demand, i.e. without respecting backpressure. This means that
|
||||
our push/pull structure that was illustrated in the figure of our custom processing chain does not
|
||||
apply to termination. Our neat model that is analogous to a ball that bounces back-and-forth in a
|
||||
pipe (it bounces back on ``Filter``, ``Duplicator`` for example) cannot describe the termination signals. By calling
|
||||
``absorbTermination()`` the execution environment checks if the conceptual token was *above* the current stage at
|
||||
that time (which means that it will never come back, so the environment immediately calls ``onPull``) or it was
|
||||
*below* (which means that it will come back eventually, so the environment does not need to call anything yet).
|
||||
|
||||
The first of the two scenarios is when a termination signal arrives after a stage passed the event to its downstream. As
|
||||
we can see in the following diagram, there is no need to do anything by ``absorbTermination()`` since the black arrows
|
||||
representing the movement of the "event token" is uninterrupted.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_msc_absorb_1.png
|
||||
:align: center
|
||||
|
||||
|
|
||||
|
||||
In the second scenario the "event token" is somewhere upstream when the termination signal arrives. In this case
|
||||
``absorbTermination`` needs to ensure that a new "event token" is generated replacing the old one that is forever gone
|
||||
(since the upstream finished). This is done by calling the ``onPull()`` event handler of the stage.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_msc_absorb_2.png
|
||||
:align: center
|
||||
|
||||
|
|
||||
|
||||
Observe, that in both scenarios ``onPull()`` kicks off the continuation of the processing logic, the only difference is
|
||||
whether it is the downstream or the ``absorbTermination()`` call that calls the event handler.
|
||||
|
||||
.. warning::
|
||||
It is not allowed to call ``absorbTermination()`` from ``onDownstreamFinish()``. If the method is called anyway,
|
||||
it will be logged at ``ERROR`` level, but no further action will be taken as at that point there is no active
|
||||
downstream to propagate the error to. Cancellation in the upstream direction will continue undisturbed.
|
||||
|
||||
Using PushStage
|
||||
---------------
|
||||
|
||||
Many one-to-one and many-to-one transformations do not need to override the ``onPull()`` handler at all since all
|
||||
they do is just propagate the pull upwards. For such transformations it is better to extend PushStage directly. For
|
||||
example our ``Map`` and ``Filter`` would look like this:
|
||||
|
||||
.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/FlowStagesDocTest.java#pushstage
|
||||
|
||||
The reason to use ``PushStage`` is not just cosmetic: internal optimizations rely on the fact that the onPull method
|
||||
only calls ``ctx.pull()`` and allow the environment do process elements faster than without this knowledge. By
|
||||
extending ``PushStage`` the environment can be sure that ``onPull()`` was not overridden since it is ``final`` on
|
||||
``PushStage``.
|
||||
|
||||
Using DetachedStage
|
||||
-------------------
|
||||
|
||||
The model described in previous sections, while conceptually simple, cannot describe all desired stages. The main
|
||||
limitation is the "single-ball" (single "event token") model which prevents independent progress of an upstream and
|
||||
downstream of a stage. Sometimes it is desirable to *detach* the progress (and therefore, rate) of the upstream and
|
||||
downstream of a stage, synchronizing only when needed.
|
||||
|
||||
This is achieved in the model by representing a :class:`DetachedStage` as a *boundary* between two "single-ball" regions.
|
||||
One immediate consequence of this difference is that **it is not allowed to call** ``ctx.pull()`` **from** ``onPull()`` **and
|
||||
it is not allowed to call** ``ctx.push()`` **from** ``onPush()`` as such combinations would "steal" a token from one region
|
||||
(resulting in zero tokens left) and would inject an unexpected second token to the other region. This is enforced
|
||||
by the expected return types of these callback functions.
|
||||
|
||||
One of the important use-cases for :class:`DetachedStage` is to build buffer-like entities, that allow independent progress
|
||||
One of the important use-case for this is to build buffer-like entities, that allow independent progress
|
||||
of upstream and downstream stages when the buffer is not full or empty, and slowing down the appropriate side if the
|
||||
buffer becomes empty or full. The next diagram illustrates the event sequence for a buffer with capacity of two elements.
|
||||
buffer becomes empty or full.
|
||||
|
||||
The next diagram illustrates the event sequence for a buffer with capacity of two elements in a setting where
|
||||
the downstream demand is slow to start and the buffer will fill up with upstream elements before any demand
|
||||
is seen from downstream.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_msc_buffer.png
|
||||
:align: center
|
||||
.. image:: ../images/graph_stage_detached_tracks_1.png
|
||||
:align: center
|
||||
:width: 500
|
||||
|
||||
|
|
||||
|
||||
The very first difference we can notice is that our ``Buffer`` stage is automatically pulling its upstream on
|
||||
initialization. Remember that it is forbidden to call ``ctx.pull`` from ``onPull``, therefore it is the task of the
|
||||
framework to kick off the first "event token" in the upstream region, which will remain there until the upstream stages
|
||||
stop. The diagram distinguishes between the actions of the two regions by colors: *purple* arrows indicate the actions
|
||||
involving the upstream "event token", while *red* arrows show the downstream region actions. This demonstrates the clear
|
||||
separation of these regions, and the invariant that the number of tokens in the two regions are kept unchanged.
|
||||
Another scenario would be where the demand from downstream starts coming in before any element is pushed
|
||||
into the buffer stage.
|
||||
|
||||
For buffer it is necessary to detach the two regions, but it is also necessary to sometimes hold back the upstream
|
||||
or downstream. The new API calls that are available for :class:`DetachedStage` s are the various ``ctx.holdXXX()`` methods
|
||||
, ``ctx.pushAndPull()`` and variants, and ``ctx.isHoldingXXX()``.
|
||||
Calling ``ctx.holdXXX()`` from ``onPull()`` or ``onPush`` results in suspending the corresponding
|
||||
region from progress, and temporarily taking ownership of the "event token". This state can be queried by ``ctx.isHolding()``
|
||||
which will tell if the stage is currently holding a token or not. It is only allowed to suspend one of the regions, not
|
||||
both, since that would disable all possible future events, resulting in a dead-lock. Releasing the held token is only
|
||||
possible by calling ``ctx.pushAndPull()``. This is to ensure that both the held token is released, and the triggering region
|
||||
gets its token back (one inbound token + one held token = two released tokens).
|
||||
|
||||
The following code example demonstrates the buffer class corresponding to the message sequence chart we discussed.
|
||||
|
|
||||
|
||||
.. includecode:: ../../../akka-samples/akka-docs-java-lambda/src/test/java/docs/stream/FlowStagesDocTest.java#detached
|
||||
.. image:: ../images/graph_stage_detached_tracks_2.png
|
||||
:align: center
|
||||
:width: 500
|
||||
|
||||
.. warning::
|
||||
If ``absorbTermination()`` is called on a :class:`DetachedStage` while it holds downstream (``isHoldingDownstream``
|
||||
returns true) then ``onPull()`` will be called on the stage. This ensures that the stage does not end up in a
|
||||
deadlocked case. Since at the point when the termination is absorbed there will be no way to get any callbacks because
|
||||
the downstream is held, so the framework invokes onPull() to avoid this situation. This is similar to the termination
|
||||
logic already shown for :class:`PushPullStage`.
|
||||
|
|
||||
|
||||
|
||||
The first difference we can notice is that our ``Buffer`` stage is automatically pulling its upstream on
|
||||
initialization. The buffer has demand for up to two elements without any downstream demand.
|
||||
|
||||
The following code example demonstrates a buffer class corresponding to the message sequence chart above.
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#detached
|
||||
|
||||
Thread safety of custom processing stages
|
||||
=========================================
|
||||
|
|
@ -457,7 +428,7 @@ In essence, the above guarantees are similar to what :class:`Actor` s provide, i
|
|||
stage as state of an actor, and the callbacks as the ``receive`` block of the actor.
|
||||
|
||||
.. warning::
|
||||
It is **not safe** to access the state of any custom stage outside of the callbacks that it provides, just like it
|
||||
is unsafe to access the state of an actor from the outside. This means that Future callbacks should **not close over**
|
||||
internal state of custom stages because such access can be concurrent with the provided callbacks, leading to undefined
|
||||
behavior.
|
||||
It is **not safe** to access the state of any custom stage outside of the callbacks that it provides, just like it
|
||||
is unsafe to access the state of an actor from the outside. This means that Future callbacks should **not close over**
|
||||
internal state of custom stages because such access can be concurrent with the provided callbacks, leading to undefined
|
||||
behavior.
|
||||
|
|
|
|||
|
|
@ -3,15 +3,16 @@
|
|||
*/
|
||||
package docs.stream
|
||||
|
||||
import akka.stream.javadsl.Sink
|
||||
import akka.stream.scaladsl.Source
|
||||
import akka.stream.stage.{ OutHandler, GraphStage, GraphStageLogic }
|
||||
import akka.stream.scaladsl.{ Keep, Sink, Flow, Source }
|
||||
import akka.stream.stage._
|
||||
import akka.stream._
|
||||
|
||||
import akka.stream.testkit.AkkaSpec
|
||||
import akka.stream.testkit.{ TestPublisher, TestSubscriber, AkkaSpec }
|
||||
|
||||
import scala.concurrent.{ Await, Future }
|
||||
import scala.collection.mutable
|
||||
import scala.concurrent.{ Promise, Await, Future }
|
||||
import scala.concurrent.duration._
|
||||
import scala.collection.immutable.Iterable
|
||||
|
||||
class GraphStageDocSpec extends AkkaSpec {
|
||||
|
||||
|
|
@ -83,4 +84,425 @@ class GraphStageDocSpec extends AkkaSpec {
|
|||
Await.result(result2, 3.seconds) should ===(5050)
|
||||
}
|
||||
|
||||
//#one-to-one
|
||||
class Map[A, B](f: A => B) extends GraphStage[FlowShape[A, B]] {
|
||||
|
||||
val in = Inlet[A]("Map.in")
|
||||
val out = Outlet[B]("Map.out")
|
||||
|
||||
override val shape = FlowShape.of(in, out)
|
||||
|
||||
override def createLogic(attr: Attributes): GraphStageLogic =
|
||||
new GraphStageLogic(shape) {
|
||||
setHandler(in, new InHandler {
|
||||
override def onPush(): Unit = {
|
||||
push(out, f(grab(in)))
|
||||
}
|
||||
})
|
||||
setHandler(out, new OutHandler {
|
||||
override def onPull(): Unit = {
|
||||
pull(in)
|
||||
}
|
||||
})
|
||||
}
|
||||
}
|
||||
//#one-to-one
|
||||
|
||||
"Demonstrate a one to one element GraphStage" in {
|
||||
// tests:
|
||||
val stringLength = Flow.fromGraph(new Map[String, Int](_.length))
|
||||
|
||||
val result =
|
||||
Source(Vector("one", "two", "three"))
|
||||
.via(stringLength)
|
||||
.runFold(Seq.empty[Int])((elem, acc) => elem :+ acc)
|
||||
|
||||
Await.result(result, 3.seconds) should ===(Seq(3, 3, 5))
|
||||
}
|
||||
|
||||
//#many-to-one
|
||||
class Filter[A](p: A => Boolean) extends GraphStage[FlowShape[A, A]] {
|
||||
|
||||
val in = Inlet[A]("Filter.in")
|
||||
val out = Outlet[A]("Filter.out")
|
||||
|
||||
val shape = FlowShape.of(in, out)
|
||||
|
||||
override def createLogic(inheritedAttributes: Attributes): GraphStageLogic =
|
||||
new GraphStageLogic(shape) {
|
||||
setHandler(in, new InHandler {
|
||||
override def onPush(): Unit = {
|
||||
val elem = grab(in)
|
||||
if (p(elem)) push(out, elem)
|
||||
else pull(in)
|
||||
}
|
||||
})
|
||||
setHandler(out, new OutHandler {
|
||||
override def onPull(): Unit = {
|
||||
pull(in)
|
||||
}
|
||||
})
|
||||
}
|
||||
}
|
||||
//#many-to-one
|
||||
|
||||
"Demonstrate a many to one element GraphStage" in {
|
||||
|
||||
// tests:
|
||||
val evenFilter = Flow.fromGraph(new Filter[Int](_ % 2 == 0))
|
||||
|
||||
val result =
|
||||
Source(Vector(1, 2, 3, 4, 5, 6))
|
||||
.via(evenFilter)
|
||||
.runFold(Seq.empty[Int])((elem, acc) => elem :+ acc)
|
||||
|
||||
Await.result(result, 3.seconds) should ===(Seq(2, 4, 6))
|
||||
}
|
||||
|
||||
//#one-to-many
|
||||
class Duplicator[A] extends GraphStage[FlowShape[A, A]] {
|
||||
|
||||
val in = Inlet[A]("Duplicator.in")
|
||||
val out = Outlet[A]("Duplicator.out")
|
||||
|
||||
val shape = FlowShape.of(in, out)
|
||||
|
||||
override def createLogic(inheritedAttributes: Attributes): GraphStageLogic =
|
||||
new GraphStageLogic(shape) {
|
||||
// Again: note that all mutable state
|
||||
// MUST be inside the GraphStageLogic
|
||||
var lastElem: Option[A] = None
|
||||
|
||||
setHandler(in, new InHandler {
|
||||
override def onPush(): Unit = {
|
||||
val elem = grab(in)
|
||||
lastElem = Some(elem)
|
||||
push(out, elem)
|
||||
}
|
||||
|
||||
override def onUpstreamFinish(): Unit = {
|
||||
if (lastElem.isDefined) emit(out, lastElem.get)
|
||||
complete(out)
|
||||
}
|
||||
|
||||
})
|
||||
setHandler(out, new OutHandler {
|
||||
override def onPull(): Unit = {
|
||||
if (lastElem.isDefined) {
|
||||
push(out, lastElem.get)
|
||||
lastElem = None
|
||||
} else {
|
||||
pull(in)
|
||||
}
|
||||
}
|
||||
})
|
||||
}
|
||||
}
|
||||
//#one-to-many
|
||||
|
||||
"Demonstrate a one to many element GraphStage" in {
|
||||
// tests:
|
||||
val duplicator = Flow.fromGraph(new Duplicator[Int])
|
||||
|
||||
val result =
|
||||
Source(Vector(1, 2, 3))
|
||||
.via(duplicator)
|
||||
.runFold(Seq.empty[Int])((elem, acc) => elem :+ acc)
|
||||
|
||||
Await.result(result, 3.seconds) should ===(Seq(1, 1, 2, 2, 3, 3))
|
||||
}
|
||||
|
||||
"Demonstrate a simpler one to many stage" in {
|
||||
//#simpler-one-to-many
|
||||
class Duplicator[A] extends GraphStage[FlowShape[A, A]] {
|
||||
|
||||
val in = Inlet[A]("Duplicator.in")
|
||||
val out = Outlet[A]("Duplicator.out")
|
||||
|
||||
val shape = FlowShape.of(in, out)
|
||||
|
||||
override def createLogic(inheritedAttributes: Attributes): GraphStageLogic =
|
||||
new GraphStageLogic(shape) {
|
||||
|
||||
setHandler(in, new InHandler {
|
||||
override def onPush(): Unit = {
|
||||
val elem = grab(in)
|
||||
// this will temporarily suspend this handler until the two elems
|
||||
// are emitted and then reinstates it
|
||||
emitMultiple(out, Iterable(elem, elem))
|
||||
}
|
||||
})
|
||||
setHandler(out, new OutHandler {
|
||||
override def onPull(): Unit = {
|
||||
pull(in)
|
||||
}
|
||||
})
|
||||
}
|
||||
}
|
||||
//#simpler-one-to-many
|
||||
|
||||
// tests:
|
||||
val duplicator = Flow.fromGraph(new Duplicator[Int])
|
||||
|
||||
val result =
|
||||
Source(Vector(1, 2, 3))
|
||||
.via(duplicator)
|
||||
.runFold(Seq.empty[Int])((elem, acc) => elem :+ acc)
|
||||
|
||||
Await.result(result, 3.seconds) should ===(Seq(1, 1, 2, 2, 3, 3))
|
||||
|
||||
}
|
||||
|
||||
"Demonstrate chaining of graph stages" in {
|
||||
val sink = Sink.fold[List[Int], Int](List.empty[Int])((acc, n) => acc :+ n)
|
||||
|
||||
//#graph-stage-chain
|
||||
val resultFuture = Source(1 to 5)
|
||||
.via(new Filter(_ % 2 == 0))
|
||||
.via(new Duplicator())
|
||||
.via(new Map(_ / 2))
|
||||
.runWith(sink)
|
||||
|
||||
//#graph-stage-chain
|
||||
|
||||
Await.result(resultFuture, 3.seconds) should ===(List(1, 1, 2, 2))
|
||||
}
|
||||
|
||||
"Demonstrate an asynchronous side channel" in {
|
||||
import system.dispatcher
|
||||
//#async-side-channel
|
||||
// will close upstream when the future completes
|
||||
class KillSwitch[A](switch: Future[Unit]) extends GraphStage[FlowShape[A, A]] {
|
||||
|
||||
val in = Inlet[A]("KillSwitch.in")
|
||||
val out = Outlet[A]("KillSwitch.out")
|
||||
|
||||
val shape = FlowShape.of(in, out)
|
||||
|
||||
override def createLogic(inheritedAttributes: Attributes): GraphStageLogic =
|
||||
new GraphStageLogic(shape) {
|
||||
|
||||
override def preStart(): Unit = {
|
||||
val callback = getAsyncCallback[Unit] { (_) =>
|
||||
completeStage()
|
||||
}
|
||||
switch.foreach(callback.invoke)
|
||||
}
|
||||
|
||||
setHandler(in, new InHandler {
|
||||
override def onPush(): Unit = { push(out, grab(in)) }
|
||||
})
|
||||
setHandler(out, new OutHandler {
|
||||
override def onPull(): Unit = { pull(in) }
|
||||
})
|
||||
}
|
||||
}
|
||||
//#async-side-channel
|
||||
|
||||
// tests:
|
||||
val switch = Promise[Unit]()
|
||||
val duplicator = Flow.fromGraph(new KillSwitch[Int](switch.future))
|
||||
|
||||
// TODO this is probably racey, is there a way to make sure it happens after?
|
||||
val valueAfterKill = switch.future.flatMap(_ => Future(4))
|
||||
|
||||
val result =
|
||||
Source(Vector(1, 2, 3)).concat(Source.fromFuture(valueAfterKill))
|
||||
.via(duplicator)
|
||||
.runFold(Seq.empty[Int])((elem, acc) => elem :+ acc)
|
||||
|
||||
switch.success(Unit)
|
||||
|
||||
Await.result(result, 3.seconds) should ===(Seq(1, 2, 3))
|
||||
}
|
||||
|
||||
"Demonstrate a graph stage with a timer" in {
|
||||
|
||||
//#timed
|
||||
// each time an event is pushed through it will trigger a period of silence
|
||||
class TimedGate[A](silencePeriod: FiniteDuration) extends GraphStage[FlowShape[A, A]] {
|
||||
|
||||
val in = Inlet[A]("TimedGate.in")
|
||||
val out = Outlet[A]("TimedGate.out")
|
||||
|
||||
val shape = FlowShape.of(in, out)
|
||||
|
||||
override def createLogic(inheritedAttributes: Attributes): GraphStageLogic =
|
||||
new TimerGraphStageLogic(shape) {
|
||||
|
||||
var open = false
|
||||
|
||||
setHandler(in, new InHandler {
|
||||
override def onPush(): Unit = {
|
||||
val elem = grab(in)
|
||||
if (open) pull(in)
|
||||
else {
|
||||
push(out, elem)
|
||||
open = true
|
||||
scheduleOnce(None, silencePeriod)
|
||||
}
|
||||
}
|
||||
})
|
||||
setHandler(out, new OutHandler {
|
||||
override def onPull(): Unit = { pull(in) }
|
||||
})
|
||||
|
||||
override protected def onTimer(timerKey: Any): Unit = {
|
||||
open = false
|
||||
}
|
||||
}
|
||||
}
|
||||
//#timed
|
||||
|
||||
// tests:
|
||||
val result =
|
||||
Source(Vector(1, 2, 3))
|
||||
.via(new TimedGate[Int](2.second))
|
||||
.takeWithin(250.millis)
|
||||
.runFold(Seq.empty[Int])((elem, acc) => elem :+ acc)
|
||||
|
||||
Await.result(result, 3.seconds) should ===(Seq(1))
|
||||
}
|
||||
|
||||
"Demonstrate a custom materialized value" in {
|
||||
|
||||
//#materialized
|
||||
class FirstValue[A] extends GraphStageWithMaterializedValue[FlowShape[A, A], Future[A]] {
|
||||
|
||||
val in = Inlet[A]("FirstValue.in")
|
||||
val out = Outlet[A]("FirstValue.out")
|
||||
|
||||
val shape = FlowShape.of(in, out)
|
||||
|
||||
override def createLogicAndMaterializedValue(inheritedAttributes: Attributes): (GraphStageLogic, Future[A]) = {
|
||||
val promise = Promise[A]()
|
||||
val logic = new GraphStageLogic(shape) {
|
||||
|
||||
setHandler(in, new InHandler {
|
||||
override def onPush(): Unit = {
|
||||
val elem = grab(in)
|
||||
promise.success(elem)
|
||||
push(out, elem)
|
||||
|
||||
// replace handler with one just forwarding
|
||||
setHandler(in, new InHandler {
|
||||
override def onPush(): Unit = {
|
||||
push(out, grab(in))
|
||||
}
|
||||
})
|
||||
}
|
||||
})
|
||||
|
||||
setHandler(out, new OutHandler {
|
||||
override def onPull(): Unit = {
|
||||
pull(in)
|
||||
}
|
||||
})
|
||||
|
||||
}
|
||||
|
||||
(logic, promise.future)
|
||||
}
|
||||
}
|
||||
//#materialized
|
||||
|
||||
// tests:
|
||||
val flow = Source(Vector(1, 2, 3))
|
||||
.viaMat(new FirstValue)(Keep.right)
|
||||
.to(Sink.ignore)
|
||||
|
||||
val result: Future[Int] = flow.run()
|
||||
|
||||
Await.result(result, 3.seconds) should ===(1)
|
||||
|
||||
}
|
||||
|
||||
"Demonstrate a detached graph stage" in {
|
||||
|
||||
//#detached
|
||||
class TwoBuffer[A] extends GraphStage[FlowShape[A, A]] {
|
||||
|
||||
val in = Inlet[A]("TwoBuffer.in")
|
||||
val out = Outlet[A]("TwoBuffer.out")
|
||||
|
||||
val shape = FlowShape.of(in, out)
|
||||
|
||||
override def createLogic(inheritedAttributes: Attributes): GraphStageLogic =
|
||||
new GraphStageLogic(shape) {
|
||||
|
||||
val buffer = mutable.Queue[A]()
|
||||
def bufferFull = buffer.size == 2
|
||||
var downstreamWaiting = false
|
||||
|
||||
override def preStart(): Unit = {
|
||||
// a detached stage needs to start upstream demand
|
||||
// itself as it is not triggered by downstream demand
|
||||
pull(in)
|
||||
}
|
||||
|
||||
setHandler(in, new InHandler {
|
||||
override def onPush(): Unit = {
|
||||
val elem = grab(in)
|
||||
buffer.enqueue(elem)
|
||||
if (downstreamWaiting) {
|
||||
downstreamWaiting = false
|
||||
val bufferedElem = buffer.dequeue()
|
||||
push(out, bufferedElem)
|
||||
}
|
||||
if (!bufferFull) {
|
||||
pull(in)
|
||||
}
|
||||
}
|
||||
|
||||
override def onUpstreamFinish(): Unit = {
|
||||
if (buffer.nonEmpty) {
|
||||
// emit the rest if possible
|
||||
emitMultiple(out, buffer.toIterator)
|
||||
}
|
||||
completeStage()
|
||||
}
|
||||
})
|
||||
|
||||
setHandler(out, new OutHandler {
|
||||
override def onPull(): Unit = {
|
||||
if (buffer.isEmpty) {
|
||||
downstreamWaiting = true
|
||||
} else {
|
||||
val elem = buffer.dequeue
|
||||
push(out, elem)
|
||||
}
|
||||
if (!bufferFull && !hasBeenPulled(in)) {
|
||||
pull(in)
|
||||
}
|
||||
}
|
||||
})
|
||||
}
|
||||
|
||||
}
|
||||
//#detached
|
||||
|
||||
// tests:
|
||||
val result1 = Source(Vector(1, 2, 3))
|
||||
.via(new TwoBuffer)
|
||||
.runFold(Vector.empty[Int])((acc, n) => acc :+ n)
|
||||
|
||||
Await.result(result1, 3.seconds) should ===(Vector(1, 2, 3))
|
||||
|
||||
val subscriber = TestSubscriber.manualProbe[Int]()
|
||||
val publisher = TestPublisher.probe[Int]()
|
||||
val flow2 =
|
||||
Source.fromPublisher(publisher)
|
||||
.via(new TwoBuffer)
|
||||
.to(Sink.fromSubscriber(subscriber))
|
||||
|
||||
val result2 = flow2.run()
|
||||
|
||||
val sub = subscriber.expectSubscription()
|
||||
// this happens even though the subscriber has not signalled any demand
|
||||
publisher.sendNext(1)
|
||||
publisher.sendNext(2)
|
||||
|
||||
sub.cancel()
|
||||
}
|
||||
|
||||
}
|
||||
|
|
@ -9,6 +9,12 @@ is sometimes necessary to define new transformation stages either because some f
|
|||
stock operations, or for performance reasons. In this part we show how to build custom processing stages and graph
|
||||
junctions of various kinds.
|
||||
|
||||
.. note::
|
||||
A custom graph stage should not be the first tool you reach for, defining graphs using flows
|
||||
and the graph DSL is in general easier and does to a larger extent protect you from mistakes that
|
||||
might be easy to make with a custom :class:`GraphStage`
|
||||
|
||||
|
||||
.. _graphstage-scala:
|
||||
|
||||
Custom processing with GraphStage
|
||||
|
|
@ -129,10 +135,151 @@ Finally, there are two methods available for convenience to complete the stage a
|
|||
* ``failStage(exception)`` is equivalent to failing all output ports and cancelling all input ports.
|
||||
|
||||
|
||||
In some cases it is inconvenient and error prone to react on the regular state machine events with the
|
||||
signal based API described above. For those cases there is a API which allows for a more declarative sequencing
|
||||
of actions which will greatly simplify some use cases at the cost of some extra allocations. The difference
|
||||
between the two APIs could be described as that the first one is signal driven from the outside, while this API
|
||||
is more active and drives its surroundings.
|
||||
|
||||
The operations of this part of the :class:``GraphStage`` API are:
|
||||
|
||||
* ``emit(out, elem)`` and ``emitMultiple(out, Iterable(elem1, elem2))`` replaces the ``OutHandler`` with a handler that emits
|
||||
one or more elements when there is demand, and then reinstalls the current handlers
|
||||
* ``read(in)(andThen)`` and ``readN(in, n)(andThen)`` replaces the ``InHandler`` with a handler that reads one or
|
||||
more elements as they are pushed and allows the handler to react once the requested number of elements has been read.
|
||||
* ``abortEmitting()`` and ``abortReading()`` which will cancel an ongoing emit or read
|
||||
|
||||
Note that since the above methods are implemented by temporarily replacing the handlers of the stage you should never
|
||||
call ``setHandler`` while they are running ``emit`` or ``read`` as that interferes with how they are implemented.
|
||||
The following methods are safe to call after invoking ``emit`` and ``read`` (and will lead to actually running the
|
||||
operation when those are done): ``complete(out)``, ``completeStage()``, ``emit``, ``emitMultiple``, ``abortEmitting()``
|
||||
and ``abortReading()``
|
||||
|
||||
An example of how this API simplifies a stage can be found below in the second version of the :class:``Duplicator``.
|
||||
|
||||
|
||||
Custom linear processing stages using GraphStage
|
||||
------------------------------------------------
|
||||
|
||||
Graph stages allows for custom linear processing stages through letting them
|
||||
have one input and one output and using :class:`FlowShape` as their shape.
|
||||
|
||||
Such a stage can be illustrated as a box with two flows as it is
|
||||
seen in the illustration below. Demand flowing upstream leading to elements
|
||||
flowing downstream.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/graph_stage_conceptual.png
|
||||
:align: center
|
||||
:width: 500
|
||||
|
||||
|
|
||||
|
||||
|
||||
To illustrate these concepts we create a small :class:`GraphStage` that implements the ``map`` transformation.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/graph_stage_map.png
|
||||
:align: center
|
||||
:width: 300
|
||||
|
||||
|
|
||||
|
||||
Map calls ``push(out)`` from the ``onPush()`` handler and it also calls ``pull()`` from the ``onPull`` handler resulting in the
|
||||
conceptual wiring above, and fully expressed in code below:
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#one-to-one
|
||||
|
||||
Map is a typical example of a one-to-one transformation of a stream where
|
||||
demand is passed along upstream elements passed on downstream.
|
||||
|
||||
To demonstrate a many-to-one stage we will implement
|
||||
filter. The conceptual wiring of ``Filter`` looks like this:
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/graph_stage_filter.png
|
||||
:align: center
|
||||
:width: 300
|
||||
|
||||
|
|
||||
|
||||
|
||||
As we see above, if the given predicate matches the current element we are propagating it downwards, otherwise
|
||||
we return the “ball” to our upstream so that we get the new element. This is achieved by modifying the map
|
||||
example by adding a conditional in the ``onPush`` handler and decide between a ``pull(in)`` or ``push(out)`` call
|
||||
(and of course not having a mapping ``f`` function).
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#many-to-one
|
||||
|
||||
To complete the picture we define a one-to-many transformation as the next step. We chose a straightforward example stage
|
||||
that emits every upstream element twice downstream. The conceptual wiring of this stage looks like this:
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/graph_stage_duplicate.png
|
||||
:align: center
|
||||
:width: 300
|
||||
|
||||
|
|
||||
|
||||
This is a stage that has state: an option with the last element it has seen indicating if it
|
||||
has duplicated this last element already or not. We must also make sure to emit the extra element
|
||||
if the upstream completes.
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#one-to-many
|
||||
|
||||
In this case a pull from downstream might be consumed by the stage itself rather
|
||||
than passed along upstream as the stage might contain an element it wants to
|
||||
push. Note that we also need to handle the case where the upstream closes while
|
||||
the stage still has elements it wants to push downstream. This is done by
|
||||
overriding `onUpstreamFinish` in the `InHandler` and provide custom logic
|
||||
that should happen when the upstream has been finished.
|
||||
|
||||
This example can be simplified by replacing the usage of a mutable state with calls to
|
||||
``emitMultiple`` which will replace the handlers, emit each of multiple elements and then
|
||||
reinstate the original handlers:
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#simpler-one-to-many
|
||||
|
||||
|
||||
Finally, to demonstrate all of the stages above, we put them together into a processing chain,
|
||||
which conceptually would correspond to the following structure:
|
||||
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/graph_stage_chain.png
|
||||
:align: center
|
||||
:width: 700
|
||||
|
||||
|
|
||||
|
||||
In code this is only a few lines, using the ``via`` use our custom stages in a stream:
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#graph-stage-chain
|
||||
|
||||
If we attempt to draw the sequence of events, it shows that there is one "event token"
|
||||
in circulation in a potential chain of stages, just like our conceptual "railroad tracks" representation predicts.
|
||||
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/graph_stage_tracks_1.png
|
||||
:align: center
|
||||
:width: 700
|
||||
|
||||
|
|
||||
|
||||
Completion
|
||||
----------
|
||||
|
||||
**This section is a stub and will be extended in the next release**
|
||||
Completion handling usually (but not exclusively) comes into the picture when processing stages need to emit
|
||||
a few more elements after their upstream source has been completed. We have seen an example of this in our
|
||||
first :class:`Duplicator` implementation where the last element needs to be doubled even after the upstream neighbor
|
||||
stage has been completed. This can be done by overriding the ``onUpstreamFinish`` method in ``InHandler``.
|
||||
|
||||
Stages by default automatically stop once all of their ports (input and output) have been closed externally or internally.
|
||||
It is possible to opt out from this behavior by overriding ``keepGoingAfterAllPortsClosed`` and returning true in
|
||||
|
|
@ -140,11 +287,10 @@ the :class:`GraphStageLogic` implementation. In this case the stage **must** be
|
|||
or ``failStage(exception)``. This feature carries the risk of leaking streams and actors, therefore it should be used
|
||||
with care.
|
||||
|
||||
|
||||
Using timers
|
||||
------------
|
||||
|
||||
**This section is a stub and will be extended in the next release**
|
||||
|
||||
It is possible to use timers in :class:`GraphStages` by using :class:`TimerGraphStageLogic` as the base class for
|
||||
the returned logic. Timers can be scheduled by calling one of ``scheduleOnce(key,delay)``, ``schedulePeriodically(key,period)`` or
|
||||
``schedulePeriodicallyWithInitialDelay(key,delay,period)`` and passing an object as a key for that timer (can be any object, for example
|
||||
|
|
@ -155,11 +301,15 @@ fires. It is possible to cancel a timer using ``cancelTimer(key)`` and check the
|
|||
Timers can not be scheduled from the constructor of the logic, but it is possible to schedule them from the
|
||||
``preStart()`` lifecycle hook.
|
||||
|
||||
In this sample the stage toggles between open and closed, where open means no elements are passed through. The
|
||||
stage starts out as closed but as soon as an element is pushed downstream the gate becomes open for a duration
|
||||
of time during which it will consume and drop upstream messages:
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#timed
|
||||
|
||||
|
||||
Using asynchronous side-channels
|
||||
--------------------------------
|
||||
|
||||
**This section is a stub and will be extended in the next release**
|
||||
|
||||
In order to receive asynchronous events that are not arriving as stream elements (for example a completion of a future
|
||||
or a callback from a 3rd party API) one must acquire a :class:`AsyncCallback` by calling ``getAsyncCallback()`` from the
|
||||
stage logic. The method ``getAsyncCallback`` takes as a parameter a callback that will be called once the asynchronous
|
||||
|
|
@ -171,6 +321,12 @@ implementation.
|
|||
Sharing the AsyncCallback from the constructor risks race conditions, therefore it is recommended to use the
|
||||
``preStart()`` lifecycle hook instead.
|
||||
|
||||
|
||||
This example shows an asynchronous side channel graph stage that starts dropping elements
|
||||
when a future completes:
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#async-side-channel
|
||||
|
||||
Integration with actors
|
||||
-----------------------
|
||||
|
||||
|
|
@ -189,11 +345,10 @@ or ``unwatch(ref)`` methods. The reference can be also watched by external actor
|
|||
- they cannot be accessed from the constructor of the :class:`GraphStageLogic`, but they can be accessed from the
|
||||
``preStart()`` method.
|
||||
|
||||
|
||||
Custom materialized values
|
||||
--------------------------
|
||||
|
||||
**This section is a stub and will be extended in the next release**
|
||||
|
||||
Custom stages can return materialized values instead of ``Unit`` by inheriting from :class:`GraphStageWithMaterializedValue`
|
||||
instead of the simpler :class:`GraphStage`. The difference is that in this case the method
|
||||
``createLogicAndMaterializedValue(inheritedAttributes)`` needs to be overridden, overridden, and in addition to the
|
||||
|
|
@ -204,6 +359,11 @@ stage logic the materialized value must be provided
|
|||
the thread that got hold of the materialized value. It is the responsibility of the programmer to add the
|
||||
necessary (non-blocking) synchronization and visibility guarantees to this shared object.
|
||||
|
||||
In this sample the materialized value is a future containing the first element to go through the stream:
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#materialized
|
||||
|
||||
|
||||
Using attributes to affect the behavior of a stage
|
||||
--------------------------------------------------
|
||||
|
||||
|
|
@ -217,239 +377,54 @@ decision.
|
|||
See :ref:`composition-scala` for an explanation on how attributes work.
|
||||
|
||||
|
||||
Custom linear processing stages with PushPullStage
|
||||
==================================================
|
||||
|
||||
To extend the available transformations on a :class:`Flow` or :class:`Source` one can use the ``transform()`` method
|
||||
which takes a factory function returning a :class:`Stage`. Stages come in different flavors swhich we will introduce in this
|
||||
page.
|
||||
|
||||
.. _stream-using-push-pull-stage-scala:
|
||||
|
||||
Using PushPullStage
|
||||
-------------------
|
||||
|
||||
The most elementary transformation stage is the :class:`PushPullStage` which can express a large class of algorithms
|
||||
working on streams. A :class:`PushPullStage` can be illustrated as a box with two "input" and two "output ports" as it is
|
||||
seen in the illustration below.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_conceptual.png
|
||||
:align: center
|
||||
:width: 600
|
||||
|
||||
|
|
||||
|
||||
The "input ports" are implemented as event handlers ``onPush(elem,ctx)`` and ``onPull(ctx)`` while "output ports"
|
||||
correspond to methods on the :class:`Context` object that is handed as a parameter to the event handlers. By calling
|
||||
exactly one "output port" method we wire up these four ports in various ways which we demonstrate shortly.
|
||||
|
||||
.. warning::
|
||||
There is one very important rule to remember when working with a ``Stage``. **Exactly one** method should be called
|
||||
on the **currently passed** :class:`Context` **exactly once** and as the **last statement of the handler** where the return type
|
||||
of the called method **matches the expected return type of the handler**. Any violation of this rule will
|
||||
almost certainly result in unspecified behavior (in other words, it will break in spectacular ways). Exceptions
|
||||
to this rule are the query methods ``isHolding()`` and ``isFinishing()``
|
||||
|
||||
To illustrate these concepts we create a small :class:`PushPullStage` that implements the ``map`` transformation.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_map.png
|
||||
:align: center
|
||||
:width: 300
|
||||
|
||||
|
|
||||
|
||||
Map calls ``ctx.push()`` from the ``onPush()`` handler and it also calls ``ctx.pull()`` form the ``onPull``
|
||||
handler resulting in the conceptual wiring above, and fully expressed in code below:
|
||||
|
||||
.. includecode:: code/docs/stream/FlowStagesSpec.scala#one-to-one
|
||||
|
||||
Map is a typical example of a one-to-one transformation of a stream. To demonstrate a many-to-one stage we will implement
|
||||
filter. The conceptual wiring of ``Filter`` looks like this:
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_filter.png
|
||||
:align: center
|
||||
:width: 300
|
||||
|
||||
|
|
||||
|
||||
As we see above, if the given predicate matches the current element we are propagating it downwards, otherwise
|
||||
we return the "ball" to our upstream so that we get the new element. This is achieved by modifying the map
|
||||
example by adding a conditional in the ``onPush`` handler and decide between a ``ctx.pull()`` or ``ctx.push()`` call
|
||||
(and of course not having a mapping ``f`` function).
|
||||
|
||||
.. includecode:: code/docs/stream/FlowStagesSpec.scala#many-to-one
|
||||
|
||||
To complete the picture we define a one-to-many transformation as the next step. We chose a straightforward example stage
|
||||
that emits every upstream element twice downstream. The conceptual wiring of this stage looks like this:
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_doubler.png
|
||||
:align: center
|
||||
:width: 300
|
||||
|
||||
|
|
||||
|
||||
This is a stage that has state: the last element it has seen, and a flag ``oneLeft`` that indicates if we
|
||||
have duplicated this last element already or not. Looking at the code below, the reader might notice that our ``onPull``
|
||||
method is more complex than it is demonstrated by the figure above. The reason for this is completion handling, which we
|
||||
will explain a little bit later. For now it is enough to look at the ``if(!ctx.isFinishing)`` block which
|
||||
corresponds to the logic we expect by looking at the conceptual picture.
|
||||
|
||||
.. includecode:: code/docs/stream/FlowStagesSpec.scala#one-to-many
|
||||
|
||||
Finally, to demonstrate all of the stages above, we put them together into a processing chain, which conceptually
|
||||
would correspond to the following structure:
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_chain.png
|
||||
:align: center
|
||||
:width: 650
|
||||
|
||||
|
|
||||
|
||||
In code this is only a few lines, using the ``transform`` method to inject our custom processing into a stream:
|
||||
|
||||
.. includecode:: code/docs/stream/FlowStagesSpec.scala#stage-chain
|
||||
|
||||
If we attempt to draw the sequence of events, it shows that there is one "event token"
|
||||
in circulation in a potential chain of stages, just like our conceptual "railroad tracks" representation predicts.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_msc_general.png
|
||||
:align: center
|
||||
|
||||
|
|
||||
|
||||
|
||||
Completion handling
|
||||
^^^^^^^^^^^^^^^^^^^
|
||||
Rate decoupled graph stages
|
||||
---------------------------
|
||||
|
||||
Completion handling usually (but not exclusively) comes into the picture when processing stages need to emit a few
|
||||
more elements after their upstream source has been completed. We have seen an example of this in our ``Duplicator`` class
|
||||
where the last element needs to be doubled even after the upstream neighbor stage has been completed. Since the
|
||||
``onUpstreamFinish()`` handler expects a :class:`TerminationDirective` as the return type we are only allowed to call
|
||||
``ctx.finish()``, ``ctx.fail()`` or ``ctx.absorbTermination()``. Since the first two of these available methods will
|
||||
immediately terminate, our only option is ``absorbTermination()``. It is also clear from the return type of
|
||||
``onUpstreamFinish`` that we cannot call ``ctx.push()`` but we need to emit elements somehow! The trick is that after
|
||||
calling ``absorbTermination()`` the ``onPull()`` handler will be called eventually, and at the same time
|
||||
``ctx.isFinishing`` will return true, indicating that ``ctx.pull()`` cannot be called anymore. Now we are free to
|
||||
emit additional elementss and call ``ctx.finish()`` or ``ctx.pushAndFinish()`` eventually to finish processing.
|
||||
Sometimes it is desirable to *decouple* the rate of the upstream and downstream of a stage, synchronizing only
|
||||
when needed.
|
||||
|
||||
The reason for this slightly complex termination sequence is that the underlying ``onComplete`` signal of
|
||||
Reactive Streams may arrive without any pending demand, i.e. without respecting backpressure. This means that
|
||||
our push/pull structure that was illustrated in the figure of our custom processing chain does not
|
||||
apply to termination. Our neat model that is analogous to a ball that bounces back-and-forth in a
|
||||
pipe (it bounces back on ``Filter``, ``Duplicator`` for example) cannot describe the termination signals. By calling
|
||||
``absorbTermination()`` the execution environment checks if the conceptual token was *above* the current stage at
|
||||
that time (which means that it will never come back, so the environment immediately calls ``onPull``) or it was
|
||||
*below* (which means that it will come back eventually, so the environment does not need to call anything yet).
|
||||
This is achieved in the model by representing a :class:`GraphStage` as a *boundary* between two regions where the
|
||||
demand sent upstream is decoupled from the demand that arrives from downstream. One immediate consequence of this
|
||||
difference is that an ``onPush`` call does not always lead to calling ``push`` and an ``onPull`` call does not always
|
||||
lead to calling ``pull``.
|
||||
|
||||
The first of the two scenarios is when a termination signal arrives after a stage passed the event to its downstream. As
|
||||
we can see in the following diagram, there is no need to do anything by ``absorbTermination()`` since the black arrows
|
||||
representing the movement of the "event token" is uninterrupted.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_msc_absorb_1.png
|
||||
:align: center
|
||||
|
||||
|
|
||||
|
||||
In the second scenario the "event token" is somewhere upstream when the termination signal arrives. In this case
|
||||
``absorbTermination`` needs to ensure that a new "event token" is generated replacing the old one that is forever gone
|
||||
(since the upstream finished). This is done by calling the ``onPull()`` event handler of the stage.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_msc_absorb_2.png
|
||||
:align: center
|
||||
|
||||
|
|
||||
|
||||
Observe, that in both scenarios ``onPull()`` kicks off the continuation of the processing logic, the only difference is
|
||||
whether it is the downstream or the ``absorbTermination()`` call that calls the event handler.
|
||||
|
||||
.. warning::
|
||||
It is not allowed to call ``absorbTermination()`` from ``onDownstreamFinish()``. If the method is called anyway,
|
||||
it will be logged at ``ERROR`` level, but no further action will be taken as at that point there is no active
|
||||
downstream to propagate the error to. Cancellation in the upstream direction will continue undisturbed.
|
||||
|
||||
Using PushStage
|
||||
---------------
|
||||
|
||||
Many one-to-one and many-to-one transformations do not need to override the ``onPull()`` handler at all since all
|
||||
they do is just propagate the pull upwards. For such transformations it is better to extend PushStage directly. For
|
||||
example our ``Map`` and ``Filter`` would look like this:
|
||||
|
||||
.. includecode:: code/docs/stream/FlowStagesSpec.scala#pushstage
|
||||
|
||||
The reason to use ``PushStage`` is not just cosmetic: internal optimizations rely on the fact that the onPull method
|
||||
only calls ``ctx.pull()`` and allow the environment do process elements faster than without this knowledge. By
|
||||
extending ``PushStage`` the environment can be sure that ``onPull()`` was not overridden since it is ``final`` on
|
||||
``PushStage``.
|
||||
|
||||
|
||||
Using DetachedStage
|
||||
-------------------
|
||||
|
||||
The model described in previous sections, while conceptually simple, cannot describe all desired stages. The main
|
||||
limitation is the "single-ball" (single "event token") model which prevents independent progress of an upstream and
|
||||
downstream of a stage. Sometimes it is desirable to *detach* the progress (and therefore, rate) of the upstream and
|
||||
downstream of a stage, synchronizing only when needed.
|
||||
|
||||
This is achieved in the model by representing a :class:`DetachedStage` as a *boundary* between two "single-ball" regions.
|
||||
One immediate consequence of this difference is that **it is not allowed to call** ``ctx.pull()`` **from** ``onPull()`` **and
|
||||
it is not allowed to call** ``ctx.push()`` **from** ``onPush()`` as such combinations would "steal" a token from one region
|
||||
(resulting in zero tokens left) and would inject an unexpected second token to the other region. This is enforced
|
||||
by the expected return types of these callback functions.
|
||||
|
||||
One of the important use-cases for :class:`DetachedStage` is to build buffer-like entities, that allow independent progress
|
||||
One of the important use-case for this is to build buffer-like entities, that allow independent progress
|
||||
of upstream and downstream stages when the buffer is not full or empty, and slowing down the appropriate side if the
|
||||
buffer becomes empty or full. The next diagram illustrates the event sequence for a buffer with capacity of two elements.
|
||||
buffer becomes empty or full.
|
||||
|
||||
The next diagram illustrates the event sequence for a buffer with capacity of two elements in a setting where
|
||||
the downstream demand is slow to start and the buffer will fill up with upstream elements before any demand
|
||||
is seen from downstream.
|
||||
|
||||
|
|
||||
|
||||
.. image:: ../images/stage_msc_buffer.png
|
||||
:align: center
|
||||
.. image:: ../images/graph_stage_detached_tracks_1.png
|
||||
:align: center
|
||||
:width: 500
|
||||
|
||||
|
|
||||
|
||||
The very first difference we can notice is that our ``Buffer`` stage is automatically pulling its upstream on
|
||||
initialization. Remember that it is forbidden to call ``ctx.pull`` from ``onPull``, therefore it is the task of the
|
||||
framework to kick off the first "event token" in the upstream region, which will remain there until the upstream stages
|
||||
stop. The diagram distinguishes between the actions of the two regions by colors: *purple* arrows indicate the actions
|
||||
involving the upstream "event token", while *red* arrows show the downstream region actions. This demonstrates the clear
|
||||
separation of these regions, and the invariant that the number of tokens in the two regions are kept unchanged.
|
||||
Another scenario would be where the demand from downstream starts coming in before any element is pushed
|
||||
into the buffer stage.
|
||||
|
||||
For buffer it is necessary to detach the two regions, but it is also necessary to sometimes hold back the upstream
|
||||
or downstream. The new API calls that are available for :class:`DetachedStage` s are the various ``ctx.holdXXX()`` methods
|
||||
, ``ctx.pushAndPull()`` and variants, and ``ctx.isHoldingXXX()``.
|
||||
Calling ``ctx.holdXXX()`` from ``onPull()`` or ``onPush`` results in suspending the corresponding
|
||||
region from progress, and temporarily taking ownership of the "event token". This state can be queried by ``ctx.isHolding()``
|
||||
which will tell if the stage is currently holding a token or not. It is only allowed to suspend one of the regions, not
|
||||
both, since that would disable all possible future events, resulting in a dead-lock. Releasing the held token is only
|
||||
possible by calling ``ctx.pushAndPull()``. This is to ensure that both the held token is released, and the triggering region
|
||||
gets its token back (one inbound token + one held token = two released tokens).
|
||||
|
||||
The following code example demonstrates the buffer class corresponding to the message sequence chart we discussed.
|
||||
|
|
||||
|
||||
.. includecode:: code/docs/stream/FlowStagesSpec.scala#detached
|
||||
.. image:: ../images/graph_stage_detached_tracks_2.png
|
||||
:align: center
|
||||
:width: 500
|
||||
|
||||
.. warning::
|
||||
If ``absorbTermination()`` is called on a :class:`DetachedStage` while it holds downstream (``isHoldingDownstream``
|
||||
returns true) then ``onPull()`` will be called on the stage. This ensures that the stage does not end up in a
|
||||
deadlocked case. Since at the point when the termination is absorbed there will be no way to get any callbacks because
|
||||
the downstream is held, so the framework invokes onPull() to avoid this situation. This is similar to the termination
|
||||
logic already shown for :class:`PushPullStage`.
|
||||
|
|
||||
|
||||
|
||||
The first difference we can notice is that our ``Buffer`` stage is automatically pulling its upstream on
|
||||
initialization. The buffer has demand for up to two elements without any downstream demand.
|
||||
|
||||
The following code example demonstrates a buffer class corresponding to the message sequence chart above.
|
||||
|
||||
.. includecode:: code/docs/stream/GraphStageDocSpec.scala#detached
|
||||
|
||||
|
||||
Thread safety of custom processing stages
|
||||
|
|
|
|||
|
|
@ -3,16 +3,15 @@
|
|||
*/
|
||||
package akka.stream.stage
|
||||
|
||||
import java.util
|
||||
import java.util.concurrent.atomic.{ AtomicReferenceFieldUpdater, AtomicReference }
|
||||
import java.util.concurrent.atomic.{ AtomicReference }
|
||||
|
||||
import akka.actor._
|
||||
import akka.dispatch.sysmsg.{ DeathWatchNotification, SystemMessage, Unwatch, Watch }
|
||||
import akka.event.LoggingAdapter
|
||||
import akka.japi.function.{ Effect, Procedure }
|
||||
import akka.stream._
|
||||
import akka.stream.impl.StreamLayout.Module
|
||||
import akka.stream.impl.fusing.GraphInterpreter.GraphAssembly
|
||||
import akka.stream.impl.fusing.{ GraphInterpreter, GraphModule, GraphStageModule }
|
||||
import akka.stream.impl.fusing.{ GraphInterpreter, GraphStageModule }
|
||||
import akka.stream.impl.{ ReactiveStreamsCompliance, SeqActorName }
|
||||
|
||||
import scala.annotation.tailrec
|
||||
|
|
@ -612,6 +611,17 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
|
|||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* Java API: Read a number of elements from the given inlet and continue with the given function,
|
||||
* suspending execution if necessary. This action replaces the [[InHandler]]
|
||||
* for the given inlet if suspension is needed and reinstalls the current
|
||||
* handler upon receiving the last `onPush()` signal (before invoking the `andThen` function).
|
||||
*/
|
||||
final protected def readN[T](in: Inlet[T], n: Int, andThen: Procedure[java.util.List[T]], onClose: Procedure[java.util.List[T]]): Unit = {
|
||||
import collection.JavaConverters._
|
||||
readN(in, n)(seq ⇒ andThen(seq.asJava), seq ⇒ onClose(seq.asJava))
|
||||
}
|
||||
|
||||
/**
|
||||
* Read an element from the given inlet and continue with the given function,
|
||||
* suspending execution if necessary. This action replaces the [[InHandler]]
|
||||
|
|
@ -631,6 +641,16 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
|
|||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* Java API: Read an element from the given inlet and continue with the given function,
|
||||
* suspending execution if necessary. This action replaces the [[InHandler]]
|
||||
* for the given inlet if suspension is needed and reinstalls the current
|
||||
* handler upon receiving the `onPush()` signal (before invoking the `andThen` function).
|
||||
*/
|
||||
final protected def read[T](in: Inlet[T], andThen: Procedure[T], onClose: Effect): Unit = {
|
||||
read(in)(andThen.apply, onClose.apply)
|
||||
}
|
||||
|
||||
/**
|
||||
* Abort outstanding (suspended) reading for the given inlet, if there is any.
|
||||
* This will reinstall the replaced handler that was in effect before the `read`
|
||||
|
|
@ -690,6 +710,32 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
|
|||
*/
|
||||
final protected def emitMultiple[T](out: Outlet[T], elems: immutable.Iterable[T]): Unit = emitMultiple(out, elems, DoNothing)
|
||||
|
||||
/**
|
||||
* Java API
|
||||
*
|
||||
* Emit a sequence of elements through the given outlet, suspending execution if necessary.
|
||||
* This action replaces the [[AbstractOutHandler]] for the given outlet if suspension
|
||||
* is needed and reinstalls the current handler upon receiving an `onPull()`
|
||||
* signal.
|
||||
*/
|
||||
final protected def emitMultiple[T](out: Outlet[T], elems: java.util.Iterator[T]): Unit = {
|
||||
import collection.JavaConverters._
|
||||
emitMultiple(out, elems.asScala, DoNothing)
|
||||
}
|
||||
|
||||
/**
|
||||
* Java API
|
||||
*
|
||||
* Emit a sequence of elements through the given outlet, suspending execution if necessary.
|
||||
* This action replaces the [[AbstractOutHandler]] for the given outlet if suspension
|
||||
* is needed and reinstalls the current handler upon receiving an `onPull()`
|
||||
* signal.
|
||||
*/
|
||||
final protected def emitMultiple[T](out: Outlet[T], elems: java.util.Iterator[T], andThen: Effect): Unit = {
|
||||
import collection.JavaConverters._
|
||||
emitMultiple(out, elems.asScala, andThen.apply _)
|
||||
}
|
||||
|
||||
/**
|
||||
* Emit a sequence of elements through the given outlet and continue with the given thunk
|
||||
* afterwards, suspending execution if necessary.
|
||||
|
|
@ -740,6 +786,10 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
|
|||
*/
|
||||
final protected def emit[T](out: Outlet[T], elem: T): Unit = emit(out, elem, DoNothing)
|
||||
|
||||
final protected def emit[T](out: Outlet[T], elem: T, andThen: Effect): Unit = {
|
||||
emit(out, elem, andThen.apply _)
|
||||
}
|
||||
|
||||
/**
|
||||
* Abort outstanding (suspended) emissions for the given outlet, if there are any.
|
||||
* This will reinstall the replaced handler that was in effect before the `emit`
|
||||
|
|
@ -875,6 +925,18 @@ abstract class GraphStageLogic private[stream] (val inCount: Int, val outCount:
|
|||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* Java API: Obtain a callback object that can be used asynchronously to re-enter the
|
||||
* current [[GraphStage]] with an asynchronous notification. The [[invoke()]] method of the returned
|
||||
* [[AsyncCallback]] is safe to be called from other threads and it will in the background thread-safely
|
||||
* delegate to the passed callback function. I.e. [[invoke()]] will be called by the external world and
|
||||
* the passed handler will be invoked eventually in a thread-safe way by the execution environment.
|
||||
*
|
||||
* This object can be cached and reused within the same [[GraphStageLogic]].
|
||||
*/
|
||||
final protected def createAsyncCallback[T](handler: Procedure[T]): AsyncCallback[T] =
|
||||
getAsyncCallback(handler.apply)
|
||||
|
||||
private var _stageActorRef: StageActorRef = _
|
||||
final def stageActorRef: ActorRef = _stageActorRef match {
|
||||
case null ⇒ throw StageActorRefNotInitializedException()
|
||||
|
|
@ -982,6 +1044,7 @@ abstract class TimerGraphStageLogic(_shape: Shape) extends GraphStageLogic(_shap
|
|||
|
||||
/**
|
||||
* Will be called when the scheduled timer is triggered.
|
||||
*
|
||||
* @param timerKey key of the scheduled timer
|
||||
*/
|
||||
protected def onTimer(timerKey: Any): Unit = ()
|
||||
|
|
@ -1029,6 +1092,7 @@ abstract class TimerGraphStageLogic(_shape: Shape) extends GraphStageLogic(_shap
|
|||
|
||||
/**
|
||||
* Cancel timer, ensuring that the [[#onTimer]] is not subsequently called.
|
||||
*
|
||||
* @param timerKey key of the timer to cancel
|
||||
*/
|
||||
final protected def cancelTimer(timerKey: Any): Unit =
|
||||
|
|
|
|||
Loading…
Add table
Add a link
Reference in a new issue