A KillSwitch
allows the completion of graphs of FlowShape
from the outside. It consists of a flow element that
can be linked to a graph of FlowShape
needing completion control.
The KillSwitch
@scala[trait] @java[interface] allows to:
- complete the graph(s) via
shutdown()
- fail the graph(s) via
abort(Throwable error)
Scala : @@snip KillSwitch.scala { #kill-switch }
After the first call to either shutdown
or abort
, all subsequent calls to any of these methods will be ignored.
Graph completion is performed by both
- completing its downstream
- cancelling (in case of
shutdown
) or failing (in case ofabort
) its upstream.
A KillSwitch
can control the completion of one or multiple streams, and therefore comes in two different flavours.
UniqueKillSwitch
allows to control the completion of one materialized Graph
of FlowShape
. Refer to the
below for usage examples.
- Shutdown
Scala : @@snip KillSwitchDocSpec.scala { #unique-shutdown }
Java : @@snip KillSwitchDocTest.java { #unique-shutdown }
- Abort
Scala : @@snip KillSwitchDocSpec.scala { #unique-abort }
Java : @@snip KillSwitchDocTest.java { #unique-abort }
A SharedKillSwitch
allows to control the completion of an arbitrary number graphs of FlowShape
. It can be
materialized multiple times via its flow
method, and all materialized graphs linked to it are controlled by the switch.
Refer to the below for usage examples.
- Shutdown
Scala : @@snip KillSwitchDocSpec.scala { #shared-shutdown }
Java : @@snip KillSwitchDocTest.java { #shared-shutdown }
- Abort
Scala : @@snip KillSwitchDocSpec.scala { #shared-abort }
Java : @@snip KillSwitchDocTest.java { #shared-abort }
@@@ note
A UniqueKillSwitch
is always a result of a materialization, whilst SharedKillSwitch
needs to be constructed
before any materialization takes place.
@@@
There are many cases when consumers or producers of a certain service (represented as a Sink, Source, or possibly Flow)
are dynamic and not known in advance. The Graph DSL does not allow to represent this, all connections of the graph
must be known in advance and must be connected upfront. To allow dynamic fan-in and fan-out streaming, the Hubs
should be used. They provide means to construct Sink
and Source
pairs that are "attached" to each
other, but one of them can be materialized multiple times to implement dynamic fan-in or fan-out.
A MergeHub
allows to implement a dynamic fan-in junction point in a graph where elements coming from
different producers are emitted in a First-Comes-First-Served fashion. If the consumer cannot keep up then all of the
producers are backpressured. The hub itself comes as a Source
to which the single consumer can be attached.
It is not possible to attach any producers until this Source
has been materialized (started). This is ensured
by the fact that we only get the corresponding Sink
as a materialized value. Usage might look like this:
Scala : @@snip HubsDocSpec.scala { #merge-hub }
Java : @@snip HubDocTest.java { #merge-hub }
This sequence, while might look odd at first, ensures proper startup order. Once we get the Sink
,
we can use it as many times as wanted. Everything that is fed to it will be delivered to the consumer we attached
previously until it cancels.
A BroadcastHub
can be used to consume elements from a common producer by a dynamic set of consumers. The
rate of the producer will be automatically adapted to the slowest consumer. In this case, the hub is a Sink
to which the single producer must be attached first. Consumers can only be attached once the Sink
has
been materialized (i.e. the producer has been started). One example of using the BroadcastHub
:
Scala : @@snip HubsDocSpec.scala { #broadcast-hub }
Java : @@snip HubDocTest.java { #broadcast-hub }
The resulting Source
can be materialized any number of times, each materialization effectively attaching
a new subscriber. If there are no subscribers attached to this hub then it will not drop any elements but instead
backpressure the upstream producer until subscribers arrive. This behavior can be tweaked by using the combinators
.buffer
for example with a drop strategy, or just attaching a subscriber that drops all messages. If there
are no other subscribers, this will ensure that the producer is kept drained (dropping all elements) and once a new
subscriber arrives it will adaptively slow down, ensuring no more messages are dropped.
There is also another case when you could use an infinitive stream with BroadcastHub
. It could change policy for
the consumed records. Instead of skipping old elements for new sink you could set how many elements (upper bound)
should be remained to be resent to a new sink.
Scala : @@snip HubsDocSpec.scala { #broadcast-hub-retry }
Java : @@snip HubDocTest.java { #broadcast-hub-retry }
@@@ note
If an upper stream will be closed and there is no any attached sink to the source it also will be closed. You could duplicate last N messages only with indefinite streams.
@@@
The features provided by the Hub implementations are limited by default. This is by design, as various combinations
can be used to express additional features like unsubscribing producers or consumers externally. We show here
an example that builds a Flow
representing a publish-subscribe channel. The input of the Flow
is
published to all subscribers while the output streams all the elements published.
First, we connect a MergeHub
and a BroadcastHub
together to form a publish-subscribe channel. Once
we materialize this small stream, we get back a pair of Source
and Sink
that together define
the publish and subscribe sides of our channel.
Scala : @@snip HubsDocSpec.scala { #pub-sub-1 }
Java : @@snip HubDocTest.java { #pub-sub-1 }
We now use a few tricks to add more features. First of all, we attach a Sink.ignore
at the broadcast side of the channel to keep it drained when there are no subscribers. If this behavior is not the
desired one this line can be simply dropped.
Scala : @@snip HubsDocSpec.scala { #pub-sub-2 }
Java : @@snip HubDocTest.java { #pub-sub-2 }
We now wrap the Sink
and Source
in a Flow
using Flow.fromSinkAndSource
. This bundles
up the two sides of the channel into one and forces users of it to always define a publisher and subscriber side
(even if the subscriber side is just dropping). It also allows us to very simply attach a KillSwitch
as
a BidiStage
which in turn makes it possible to close both the original Sink
and Source
at the
same time.
Finally, we add backpressureTimeout
on the consumer side to ensure that subscribers that block the channel for more
than 3 seconds are forcefully removed (and their stream failed).
Scala : @@snip HubsDocSpec.scala { #pub-sub-3 }
Java : @@snip HubDocTest.java { #pub-sub-3 }
The resulting Flow now has a type of Flow[String, String, UniqueKillSwitch]
representing a publish-subscribe
channel which can be used any number of times to attach new producers or consumers. In addition, it materializes
to a UniqueKillSwitch
(see UniqueKillSwitch) that can be used to deregister a single user externally:
Scala : @@snip HubsDocSpec.scala { #pub-sub-4 }
Java : @@snip HubDocTest.java { #pub-sub-4 }
This is a @ref:may change feature*
A PartitionHub
can be used to route elements from a common producer to a dynamic set of consumers.
The selection of consumer is done with a function. Each element can be routed to only one consumer.
The rate of the producer will be automatically adapted to the slowest consumer. In this case, the hub is a Sink
to which the single producer must be attached first. Consumers can only be attached once the Sink
has
been materialized (i.e. the producer has been started). One example of using the PartitionHub
:
Scala : @@snip HubsDocSpec.scala { #partition-hub }
Java : @@snip HubDocTest.java { #partition-hub }
The partitioner
function takes two parameters; the first is the number of active consumers and the second
is the stream element. The function should return the index of the selected consumer for the given element,
i.e. int
greater than or equal to 0 and less than number of consumers.
The resulting Source
can be materialized any number of times, each materialization effectively attaching
a new consumer. If there are no consumers attached to this hub then it will not drop any elements but instead
backpressure the upstream producer until consumers arrive. This behavior can be tweaked by using the combinators
.buffer
for example with a drop strategy, or just attaching a consumer that drops all messages. If there
are no other consumers, this will ensure that the producer is kept drained (dropping all elements) and once a new
consumer arrives and messages are routed to the new consumer it will adaptively slow down, ensuring no more messages
are dropped.
It is possible to define how many initial consumers that are required before it starts emitting any messages to the attached consumers. While not enough consumers have been attached messages are buffered and when the buffer is full the upstream producer is backpressured. No messages are dropped.
The above example illustrate a stateless partition function. For more advanced stateful routing the @java[ofStateful
]
@scala[statefulSink
] can be used. Here is an example of a stateful round-robin function:
Scala : @@snip HubsDocSpec.scala { #partition-hub-stateful }
Java : @@snip HubDocTest.java { #partition-hub-stateful }
Note that it is a factory of a function to to be able to hold stateful variables that are
unique for each materialization. @java[In this example the partitioner
function is implemented as a class to
be able to hold the mutable variable. A new instance of RoundRobin
is created for each materialization of the hub.]
@@@ div { .group-java } @@snip HubDocTest.java { #partition-hub-stateful-function } @@@
The function takes two parameters; the first is information about active consumers, including an array of consumer identifiers and the second is the stream element. The function should return the selected consumer identifier for the given element. The function will never be called when there are no active consumers, i.e. there is always at least one element in the array of identifiers.
Another interesting type of routing is to prefer routing to the fastest consumers. The ConsumerInfo
has an accessor queueSize
that is approximate number of buffered elements for a consumer.
Larger value than other consumers could be an indication of that the consumer is slow.
Note that this is a moving target since the elements are consumed concurrently. Here is an example of
a hub that routes to the consumer with least buffered elements:
Scala : @@snip HubsDocSpec.scala { #partition-hub-fastest }
Java : @@snip HubDocTest.java { #partition-hub-fastest }