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Active active docs mention plugin support (#29418)

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* Mention specific support for active active is needed in plugins
* Align metadata name journal vs snapshot
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Johan Andrén 2020-07-22 16:22:12 +02:00 • committed by Christopher Batey
parent 7ec631d3c1
commit f63ca66e56
3 changed files with 18 additions and 7 deletions
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akka-docs/src/main/paradox/typed/persistence-active-active.md
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@ -33,7 +33,9 @@ there is no longer the single writer principle as there is with a normal `EventS
The state of an active-active `EventSourcedBehavior` is **eventually consistent**. Event replication may be delayed
due to network partitions and outages and the event handler and those reading the state must be designed to handle this.
## Relaxing the single writer p`rinciple for availability
To be able to use active active the journal and snapshot store used is required to have specific support for the metadata that active active needs (see @ref[Journal Support](#journal-support))
## Relaxing the single writer principle for availability
Taking the example of using active-active to run a replica per data center.
@ -283,4 +285,12 @@ and then enable direct replication through `withDirectReplication()` on @apidoc[
The "event publishing" feature publishes each event to the local system event bus as a side effect after it has been written,
the `ActiveActiveShardingDirectReplication` actor subscribes to these events and forwards them to the replicas allowing them
to fast forward the stream of events for the origin replica. (With additional potential future support in journals for fast forwarding [#29311](https://github.com/akka/akka/issues/29311)).
to fast forward the stream of events for the origin replica. (With additional potential future support in journals for fast forwarding [#29311](https://github.com/akka/akka/issues/29311)).
## Journal Support
For a journal plugin to support active active it needs to store and read metadata for each event if it is defined in the @apiref[PersistentRepr]
`metadata` field. To attach the metadata after writing it, `PersistentRepr.withMetadata` is used.
For a snapshot plugin to support active active it needs to store and read metadata for the snapshot if it is defined in the @apiref[akka.persistence.SnapshotMetadata] `metadata` field.
To attach the metadata when reading the snapshot the `akka.persistence.SnapshotMetadata.apply` factory overload taking a `metadata` parameter is used.