Fixes#28769
Use case for this is if you have a sequence of elements that has been
partitioned across multiple streams, and you want to merge them back
together in order. It will typically be used in combination with
`zipWithIndex` to define the index for the sequence, followed by a
`Partition`, followed by the processing of different substreams with
different flows (each flow emitting exactly one output for each input),
and then merging with this stage, using the index from `zipWithIndex`.
A more concrete use case is if you're consuming messages from a message
broker, and you have a flow that you wish to apply to some messages, but
not others, you can partition the message stream according to which
should be processed by the flow and which should bypass it, and then
bring the elements back together acknowledgement. If an ordinary merge
was used rather than this, the messages that bypass the processing flow
would likely overtake the messages going through the processing flow,
and the result would be out of order offset acknowledgement which would
lead to dropping messages on failure.
I've included a minimal version of the above example in the documentation.
RetryFlow wraps a flow with context and allows to individually retry elements. A decider function decides if another try should be made and gives the element to try with.
Retries are backed off exponentially.
Retries are limited by maxRetries.
Inspired by the work of Gilad Hoch <gilad.hoch@thomsonreuters.com> and Martynas Mickevičius <self@2m.lt>
* ⇒, →, ←
* because we don't want to show them in documentation snippets and
then it's complicated to avoid that when snippets are
located in src/test/scala in individual modules
* dont replace object `→` in FSM.scala and PersistentFSM.scala
* Add CopyrightHeader support for sbt-boilerplate plugin.
* Add CopyrightHeader support for `*.proto` files.
* Add regex match for both `–` and `-` for CopyrightHeader.
* Add CopyrightHeader support for sbt build files.
* Update copyright from 2018 to 2019.
* +str,typ introduce akka typed ask for akka stream
address feedback and add actor interop stages incl ask to docs
more compile tests and adjusted things
last docs
* document adding stages to docs in CONTRIBUTING
* address review comments
* rebase conflicts
