com.lightbend.lagom.javadsl.api.broker

Interface Subscriber<Payload>

Type Parameters:

Payload - The message type.

Note: This class is not meant to be extended by client code.

public interface Subscriber<Payload>

A Subscriber for consuming messages from a message broker.

Nested Class Summary

Nested Classes

Modifier and Type	Interface and Description
static interface	Subscriber.GroupId Subscribers with the same group id belong to the same subscriber group.

Method Summary

Modifier and Type	Method and Description
CompletionStage<akka.Done>	atLeastOnce(akka.stream.javadsl.Flow<Payload,akka.Done,?> flow) Applies the passed flow to the messages processed by this subscriber.
akka.stream.javadsl.Source<Payload,?>	atMostOnceSource() Returns a stream of messages with at most once delivery semantic.
Subscriber<Payload>	withGroupId(String groupId) Returns a copy of this subscriber with the passed group id.
default Subscriber<Message<Payload>>	withMetadata() Returns this subscriber, but message payloads are wrapped in Message instances to allow accessing any metadata associated with the message.

Method Detail

withGroupId

Subscriber<Payload> withGroupId(String groupId)
                         throws IllegalArgumentException

Returns a copy of this subscriber with the passed group id.

Parameters:

groupId - The group id to assign to this subscriber.

Returns:

A copy of this subscriber with the passed group id.

Throws:

IllegalArgumentException - If the passed group id is illegal.

withMetadata

default Subscriber<Message<Payload>> withMetadata()

Returns this subscriber, but message payloads are wrapped in Message instances to allow accessing any metadata associated with the message.

Returns:

A copy of this subscriber.

atMostOnceSource

akka.stream.javadsl.Source<Payload,?> atMostOnceSource()

Returns a stream of messages with at most once delivery semantic.

If a failure occurs (e.g., an exception is thrown), the user is responsible of deciding how to recover from it (e.g., restarting the stream, aborting, ...).

atLeastOnce

CompletionStage<akka.Done> atLeastOnce(akka.stream.javadsl.Flow<Payload,akka.Done,?> flow)

Applies the passed flow to the messages processed by this subscriber. Messages are delivered to the passed flow at least once.

If a failure occurs (e.g., an exception is thrown), the stream may be automatically restarted starting with the message that caused the failure.

Whether the stream is automatically restarted depends on the Lagom message broker implementation in use. If the Kafka Lagom message broker module is being used, then by default the stream is automatically restarted when a failure occurs.

The flow may pull more elements from upstream but it must emit exactly one Done message for each message that it receives. It must also emit them in the same order that the messages were received. This means that the flow must not filter or collect a subset of the messages, instead it must split the messages into separate streams and map those that would have been dropped to Done.

Parameters:

flow - The flow to apply to each received message.

Returns:

A CompletionStage that may never complete if messages go through the passed flow flawlessly. However, the returned CompletionStage may complete with success if the passed flow signals cancellation upstream.

If the returned CompletionStage is completed with a failure, user-code is responsible of deciding what to do (e.g., it could retry to process the message that caused the failure, or it could report an application error).