74 lines
2.6 KiB
ReStructuredText
74 lines
2.6 KiB
ReStructuredText
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.. _migration-streams-2.0-2.4-scala:
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##############################
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Migration Guide 2.0.x to 2.4.x
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##############################
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General notes
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=============
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akka.Done and akka.NotUsed replacing Unit and BoxedUnit
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-------------------------------------------------------
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To provide more clear signatures and have a unified API for both
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Java and Scala two new types have been introduced:
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``akka.NotUsed`` is meant to be used instead of ``Unit`` in Scala
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and ``BoxedUnit`` in Java to signify that the type parameter is required
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but not actually used. This is commonly the case with ``Source``, ``Flow`` and ``Sink``
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that do not materialize into any value.
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``akka.Done`` is added for the use case where it is boxed inside another object to signify
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completion but there is no actual value attached to the completion. It is used to replace
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occurrences of ``Future<BoxedUnit>`` with ``Future<Done>`` in Java and ``Future[Unit]`` with
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``Future[Done]`` in Scala.
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All previous usage of ``Unit`` and ``BoxedUnit`` for these two cases in the akka streams APIs
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has been updated.
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This means that Scala code like this::
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Source[Int, Unit] source = Source.from(1 to 5)
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Sink[Int, Future[Unit]] sink = Sink.ignore()
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needs to be changed into::
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Source[Int, NotUsed] source = Source.from(1 to 5)
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Sink[Int, Future[Done]] sink = Sink.ignore()
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These changes apply to all the places where streams are used, which means that signatures
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in the persistent query APIs also are affected.
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Changed Operators
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=================
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``expand()`` is now based on an Iterator
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----------------------------------------
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Previously the ``expand`` combinator required two functions as input: the first
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one lifted incoming values into an extrapolation state and the second one
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extracted values from that, possibly evolving that state. This has been
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simplified into a single function that turns the incoming element into an
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Iterator.
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The most prominent use-case previously was to just repeat the previously received value::
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Flow[Int].expand(identity)(s => (s, s)) // This no longer works!
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In Akka 2.4.x this is simplified to:
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.. includecode:: ../code/docs/stream/MigrationsScala.scala#expand-continually
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If state needs to be be kept during the expansion process then this state will
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need to be managed by the Iterator. The example of counting the number of
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expansions might previously have looked like::
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// This no longer works!
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Flow[Int].expand((_, 0)){ case (in, count) => (in, count) -> (in, count + 1) }
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In Akka 2.4.x this is formulated like so:
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.. includecode:: ../code/docs/stream/MigrationsScala.scala#expand-state
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