com.lightbend.lagom.javadsl.persistence

Class ReadSideProcessor.ReadSideHandler<Event extends AggregateEvent<Event>>

Object

com.lightbend.lagom.javadsl.persistence.ReadSideProcessor.ReadSideHandler<Event>

Enclosing class:

ReadSideProcessor<Event extends AggregateEvent<Event>>

public abstract static class ReadSideProcessor.ReadSideHandler<Event extends AggregateEvent<Event>>
extends Object

An read side offset processor.

This is responsible for the actual read side handling, including handling offsets and the events themselves.

Constructor Summary

Constructors

Constructor and Description
ReadSideHandler()

Method Summary

Modifier and Type	Method and Description
CompletionStage<akka.Done>	globalPrepare() Prepare the database for all processors.
abstract akka.stream.javadsl.Flow<akka.japi.Pair<Event,Offset>,akka.Done,?>	handle() Flow to handle the events.
CompletionStage<Offset>	prepare(AggregateEventTag<Event> tag) Prepare this processor.

Methods inherited from class Object

equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

ReadSideHandler

public ReadSideHandler()

Method Detail

globalPrepare

public CompletionStage<akka.Done> globalPrepare()

Prepare the database for all processors.

This will be invoked at system startup. It is guaranteed to only be invoked once at a time across the entire cluster, and so is safe to be used to perform actions like creating tables, that could cause problems if done from multiple nodes.

It will be invoked again if it fails, and it may be invoked multiple times as nodes of the cluster go up or down. Unless the entire system is restarted, there is no way to guarantee that it will be invoked at a particular time - in particular, it should not be used for doing upgrades unless the entire system is restarted and a new cluster built from scratch.

Returns:

A completion stage that is redeemed when preparation is finished.

prepare

public CompletionStage<Offset> prepare(AggregateEventTag<Event> tag)

Prepare this processor.

The primary purpose of this method is to load the last offset that was processed, so that read side processing can continue from that offset.

This also provides an opportunity for processors to do any initialisation activities, such as creating or updating database tables, or migrating data.

This will be invoked at least once for each tag, and may be invoked multiple times, such as in the event of failure.

Parameters:

tag - The tag to get the offset for.

Returns:

A completion stage that is redeemed when preparation is finished.

handle

public abstract akka.stream.javadsl.Flow<akka.japi.Pair<Event,Offset>,akka.Done,?> handle()

Flow to handle the events.

If the handler does any blocking, this flow should be configured to use a dispatcher that is configured to allow for that blocking.