abstract class PushPullStage[In, Out] extends AbstractStage[In, Out, SyncDirective, SyncDirective, Context[Out], LifecycleContext]
PushPullStage
implementations participate in 1-bounded regions. For every external non-completion signal these
stages produce *exactly one* push or pull signal.
#onPush is called when an element from upstream is available and there is demand from downstream, i.e.
in onPush
you are allowed to call Context#push to emit one element downstream, or you can absorb the
element by calling Context#pull. Note that you can only emit zero or one element downstream from onPull
.
To emit more than one element you have to push the remaining elements from #onPull, one-by-one.
onPush
is not called again until onPull
has requested more elements with Context#pull.
StatefulStage has support for making it easy to emit more than one element from onPush
.
#onPull is called when there is demand from downstream, i.e. you are allowed to push one element
downstream with Context#push, or request elements from upstreams with Context#pull. If you
always perform transitive pull by calling ctx.pull
from onPull
you can use PushStage instead of
PushPullStage
.
Stages are allowed to do early completion of downstream and cancel of upstream. This is done with Context#finish, which is a combination of cancel/complete.
Since onComplete is not a backpressured signal it is sometimes preferable to push a final element and then immediately finish. This combination is exposed as Context#pushAndFinish which enables stages to propagate completion events without waiting for an extra round of pull.
Another peculiarity is how to convert termination events (complete/failure) into elements. The problem here is that the termination events are not backpressured while elements are. This means that simply calling Context#push as a response to #onUpstreamFinish or #onUpstreamFailure will very likely break boundedness and result in a buffer overflow somewhere. Therefore the only allowed command in this case is Context#absorbTermination which stops the propagation of the termination signal, and puts the stage in a akka.stream.stage.Context#isFinishing state. Depending on whether the stage has a pending pull signal it has not yet "consumed" by a push its #onPull handler might be called immediately or later. From #onPull final elements can be pushed before completing downstream with Context#finish or Context#pushAndFinish.
StatefulStage has support for making it easy to emit final elements.
All these rules are enforced by types and runtime checks where needed. Always return the Directive
from the call to the Context method, and do only call Context commands once per callback.
- Annotations
- @deprecated
- Deprecated
(Since version 2.4.2) Please use GraphStage instead.
- Source
- Stage.scala
- See also
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Instance Constructors
- new PushPullStage()
Abstract Value Members
-
abstract
def
onPull(ctx: Context[Out]): SyncDirective
onPull
is called when there is demand from downstream, i.e.onPull
is called when there is demand from downstream, i.e. you are allowed to push one element downstream with akka.stream.stage.Context#push, or request elements from upstreams with akka.stream.stage.Context#pull- Definition Classes
- AbstractStage
-
abstract
def
onPush(elem: In, ctx: Context[Out]): SyncDirective
onPush
is called when an element from upstream is available and there is demand from downstream, i.e.onPush
is called when an element from upstream is available and there is demand from downstream, i.e. inonPush
you are allowed to call akka.stream.stage.Context#push to emit one element downstream, or you can absorb the element by calling akka.stream.stage.Context#pull. Note that you can only emit zero or one element downstream fromonPull
.To emit more than one element you have to push the remaining elements from #onPull, one-by-one.
onPush
is not called again untilonPull
has requested more elements with akka.stream.stage.Context#pull.- Definition Classes
- AbstractStage
Concrete Value Members
-
final
def
!=(arg0: Any): Boolean
- Definition Classes
- AnyRef → Any
-
final
def
##(): Int
- Definition Classes
- AnyRef → Any
-
def
+(other: String): String
- Implicit
- This member is added by an implicit conversion from PushPullStage[In, Out] to any2stringadd[PushPullStage[In, Out]] performed by method any2stringadd in scala.Predef.
- Definition Classes
- any2stringadd
-
def
->[B](y: B): (PushPullStage[In, Out], B)
- Implicit
- This member is added by an implicit conversion from PushPullStage[In, Out] to ArrowAssoc[PushPullStage[In, Out]] performed by method ArrowAssoc in scala.Predef.
- Definition Classes
- ArrowAssoc
- Annotations
- @inline()
-
final
def
==(arg0: Any): Boolean
- Definition Classes
- AnyRef → Any
-
final
def
asInstanceOf[T0]: T0
- Definition Classes
- Any
-
def
clone(): AnyRef
- Attributes
- protected[java.lang]
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
def
decide(t: Throwable): Supervision.Directive
If an exception is thrown from #onPush this method is invoked to decide how to handle the exception.
If an exception is thrown from #onPush this method is invoked to decide how to handle the exception. By default this method returns Supervision.Stop.
If an exception is thrown from #onPull the stream will always be completed with failure, because it is not always possible to recover from that state. In concrete stages it is of course possible to use ordinary try-catch-recover inside
onPull
when it is know how to recover from such exceptions.- Definition Classes
- AbstractStage
-
def
ensuring(cond: (PushPullStage[In, Out]) ⇒ Boolean, msg: ⇒ Any): PushPullStage[In, Out]
- Implicit
- This member is added by an implicit conversion from PushPullStage[In, Out] to Ensuring[PushPullStage[In, Out]] performed by method Ensuring in scala.Predef.
- Definition Classes
- Ensuring
-
def
ensuring(cond: (PushPullStage[In, Out]) ⇒ Boolean): PushPullStage[In, Out]
- Implicit
- This member is added by an implicit conversion from PushPullStage[In, Out] to Ensuring[PushPullStage[In, Out]] performed by method Ensuring in scala.Predef.
- Definition Classes
- Ensuring
-
def
ensuring(cond: Boolean, msg: ⇒ Any): PushPullStage[In, Out]
- Implicit
- This member is added by an implicit conversion from PushPullStage[In, Out] to Ensuring[PushPullStage[In, Out]] performed by method Ensuring in scala.Predef.
- Definition Classes
- Ensuring
-
def
ensuring(cond: Boolean): PushPullStage[In, Out]
- Implicit
- This member is added by an implicit conversion from PushPullStage[In, Out] to Ensuring[PushPullStage[In, Out]] performed by method Ensuring in scala.Predef.
- Definition Classes
- Ensuring
-
final
def
eq(arg0: AnyRef): Boolean
- Definition Classes
- AnyRef
-
def
equals(arg0: Any): Boolean
- Definition Classes
- AnyRef → Any
-
def
finalize(): Unit
- Attributes
- protected[java.lang]
- Definition Classes
- AnyRef
- Annotations
- @throws( classOf[java.lang.Throwable] )
-
def
formatted(fmtstr: String): String
- Implicit
- This member is added by an implicit conversion from PushPullStage[In, Out] to StringFormat[PushPullStage[In, Out]] performed by method StringFormat in scala.Predef.
- Definition Classes
- StringFormat
- Annotations
- @inline()
-
final
def
getClass(): Class[_]
- Definition Classes
- AnyRef → Any
-
def
hashCode(): Int
- Definition Classes
- AnyRef → Any
-
final
def
isInstanceOf[T0]: Boolean
- Definition Classes
- Any
-
final
def
ne(arg0: AnyRef): Boolean
- Definition Classes
- AnyRef
-
final
def
notify(): Unit
- Definition Classes
- AnyRef
-
final
def
notifyAll(): Unit
- Definition Classes
- AnyRef
-
def
onDownstreamFinish(ctx: Context[Out]): TerminationDirective
onDownstreamFinish
is called when downstream has canceled.onDownstreamFinish
is called when downstream has canceled.By default the cancel signal is immediately propagated with akka.stream.stage.Context#finish.
- Definition Classes
- AbstractStage
-
def
onUpstreamFailure(cause: Throwable, ctx: Context[Out]): TerminationDirective
onUpstreamFailure
is called when upstream has signaled that the stream is completed with failure.onUpstreamFailure
is called when upstream has signaled that the stream is completed with failure. It is not called if #onPull or #onPush of the stage itself throws an exception.Note that elements that were emitted by upstream before the failure happened might not have been received by this stage when
onUpstreamFailure
is called, i.e. failures are not backpressured and might be propagated as soon as possible.Here you cannot call akka.stream.stage.Context#push, because there might not be any demand from downstream. To emit additional elements before terminating you can use akka.stream.stage.Context#absorbTermination and push final elements from #onPull. The stage will then be in finishing state, which can be checked with akka.stream.stage.Context#isFinishing.
- Definition Classes
- AbstractStage
-
def
onUpstreamFinish(ctx: Context[Out]): TerminationDirective
onUpstreamFinish
is called when upstream has signaled that the stream is successfully completed.onUpstreamFinish
is called when upstream has signaled that the stream is successfully completed. Here you cannot call akka.stream.stage.Context#push, because there might not be any demand from downstream. To emit additional elements before terminating you can use akka.stream.stage.Context#absorbTermination and push final elements from #onPull. The stage will then be in finishing state, which can be checked with akka.stream.stage.Context#isFinishing.By default the finish signal is immediately propagated with akka.stream.stage.Context#finish.
*IMPORTANT NOTICE:* this signal is not back-pressured, it might arrive from upstream even though the last action by this stage was a “push”.
- Definition Classes
- AbstractStage
-
def
postStop(): Unit
User overridable callback.
User overridable callback.
Is called after the Stages final action is performed. // TODO need better wording here Empty default implementation.
- Definition Classes
- AbstractStage
- Annotations
- @throws( classOf[Exception] )
-
def
preStart(ctx: LifecycleContext): Unit
User overridable callback.
User overridable callback.
It is called before any other method defined on the
Stage
. Empty default implementation.- Definition Classes
- AbstractStage
- Annotations
- @throws( classOf[Exception] )
-
def
restart(): Stage[In, Out]
Used to create a fresh instance of the stage after an error resulting in a Supervision.Restart directive.
Used to create a fresh instance of the stage after an error resulting in a Supervision.Restart directive. By default it will return the same instance untouched, so you must override it if there are any state that should be cleared before restarting, e.g. by returning a new instance.
- Definition Classes
- AbstractStage
-
final
def
synchronized[T0](arg0: ⇒ T0): T0
- Definition Classes
- AnyRef
-
def
toString(): String
- Definition Classes
- AnyRef → Any
-
final
def
wait(): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
final
def
wait(arg0: Long, arg1: Int): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
final
def
wait(arg0: Long): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
def
→[B](y: B): (PushPullStage[In, Out], B)
- Implicit
- This member is added by an implicit conversion from PushPullStage[In, Out] to ArrowAssoc[PushPullStage[In, Out]] performed by method ArrowAssoc in scala.Predef.
- Definition Classes
- ArrowAssoc