raboof/pekko
1
0
Fork 0
Code Issues Pull requests Projects Releases 6 Packages Wiki Activity Actions 10

Tagging for replicated event sourcing (#29442)

Browse source 
* Tagging for replicated event sourcing
* Docs improvements
* Support for currentEventsByTag in persistence testkit and test coverage for tags in replicated
This commit is contained in:
Johan Andrén 2020-07-31 14:07:37 +02:00 • committed by Christopher Batey
parent c945fbd7a1
commit 9fb76bbea4
16 changed files with 585 additions and 216 deletions
Download patch file Download diff file Expand all files Collapse all files

42
akka-docs/src/main/paradox/typed/replicated-eventsourcing.md
View file

@ -66,7 +66,7 @@ Scala
: @@snip [ReplicatedEventSourcingCompileOnlySpec.scala](/akka-persistence-typed-tests/src/test/scala/docs/akka/persistence/typed/ReplicatedEventSourcingCompileOnlySpec.scala) { #replicas }
Java
: @@snip [ReplicatedEventSourcingCompileOnlyTest.java](/akka-persistence-typed-tests/src/test/java/jdocs/akka/persistence/typed/ReplicatedEventSourcingCompileOnlyTest.java) { #replicas }
: @@snip [MyReplicatedBehavior.java](/akka-persistence-typed-tests/src/test/java/jdocs/akka/persistence/typed/MyReplicatedBehavior.java) { #replicas }
Then to enable replication create the event sourced behavior with the factory method:
@ -75,7 +75,7 @@ Scala
: @@snip [ReplicatedEventSourcingCompileOnlySpec.scala](/akka-persistence-typed-tests/src/test/scala/docs/akka/persistence/typed/ReplicatedEventSourcingCompileOnlySpec.scala) { #factory }
Java
: @@snip [ReplicatedEventSourcingCompileOnlyTest.java](/akka-persistence-typed-tests/src/test/java/jdocs/akka/persistence/typed/ReplicatedEventSourcingCompileOnlyTest.java) { #factory }
: @@snip [MyReplicatedBehavior.java](/akka-persistence-typed-tests/src/test/java/jdocs/akka/persistence/typed/MyReplicatedBehavior.java) { #factory }
The factory takes in:
@ -90,7 +90,7 @@ Scala
: @@snip [ReplicatedEventSourcingCompileOnlySpec.scala](/akka-persistence-typed-tests/src/test/scala/docs/akka/persistence/typed/ReplicatedEventSourcingCompileOnlySpec.scala) { #factory-shared}
Java
: @@snip [ReplicatedEventSourcingCompileOnlyTest.java](/akka-persistence-typed-tests/src/test/java/jdocs/akka/persistence/typed/ReplicatedEventSourcingCompileOnlyTest.java) { #factory-shared }
: @@snip [MyReplicatedBehavior.java](/akka-persistence-typed-tests/src/test/java/jdocs/akka/persistence/typed/MyReplicatedBehavior.java) { #factory-shared }
@@@ div { .group-scala }
@ -271,6 +271,42 @@ Java
More advanced routing among the replicas is currently left as an exercise for the reader (or may be covered in a future release [#29281](https://github.com/akka/akka/issues/29281), [#29319](https://github.com/akka/akka/issues/29319)).
## Tagging events and running projections
Just like for regular `EventSourcedBehavior`s it is possible to tag events along with persisting them.
This is useful for later retrival of events for a given tag. The same @ref[API for tagging provided for EventSourcedBehavior](persistence.md#tagging) can
be used for replicated event sourced behaviors as well.
Tagging is useful in practice to build queries that lead to other data representations or aggregations of the these event
streams that can more directly serve user queries – known as building the “read side” in @ref[CQRS](cqrs.md) based applications.
Creating read side projections is possible through [Akka Projection](https://doc.akka.io/docs/akka-projection/current/)
or through direct usage of the @ref[events by tag queries](../persistence-query.md#eventsbytag-and-currenteventsbytag).
The tagging is invoked in each replicas, which requires some special care in using tags, or else the same event will be
tagged one time for each replica and show up in the event by tag stream one time for each replica. In addition to this
the tags will be written in the respective journal of the replicas, which means that unless they all share a single journal
the tag streams will be local to the replica even if the same tag is used on multiple replicas.
One strategy for dealing with this is to include the replica id in the tag name, this means there will be a tagged stream of events
per replica that contains all replicated events, but since the events can arrive in different order, they can also come in different
order per replica tag.
Another strategy would be to tag only the events that are local to the replica and not events that are replicated. Either
using a tag that will be the same for all replicas, leading to a single stream of tagged events where the events from each
replica is present only once, or with a tag including the replica id meaning that there will be a stream of tagged events
with the events accepted locally for each replica.
Determining the replica id of the replicated actor itself and the origin replica id of an event is possible through the
@apidoc[ReplicationContext] when the tagger callback is invoked like this:
Scala
: @@snip [ReplicatedEventSourcingTaggingSpec.scala](/akka-persistence-typed-tests/src/test/scala/akka/persistence/typed/ReplicatedEventSourcingTaggingSpec.scala) { #tagging }
Java
: @@snip [ReplicatedStringSet.java](/akka-persistence-typed-tests/src/test/java/jdocs/akka/persistence/typed/ReplicatedStringSet.java) { #tagging }
In this sample we are using a shared journal, and single tag but only tagging local events to it and therefore ending up
with a single stream of tagged events from all replicas without duplicates.
## Direct Replication of Events