Hi!
In my current project we are using akka-persistence with LevelDB. Our
application is completely event sourced using akka-persistence, i.e there
are no other mechanism for persisting business state in use.
However, we have identified the need for being able to convert persisted
messages, when deploying a new version of the application with changes
effecting the persisted data structure (.ie new attributes, deleted
attributes, changed datatypes). As of now, we must completely discard our
business data every time we deploy a new version, in order to avoid
exceptions when replay of persisted messages upon startup.
I would believe that many other projects face the same problem, and believe
that this could be solved with a general mechanism in akka persistence.
I have done a poc on "daisy-chaining" persistence plugins, by creating my
own plugin ("proxy-plugin") and have this plugin forward all messages to
the actual persistence plugin (leveldb or cassandra). In addition we are
using our own serializer, serializing all messages to json strings prior to
persisting (to avoid class version issues).
In my own proxy-plugin I can now register mapper classes (these
implementing a common apply() trait, intercept messages and modify the
persisted json-based messages from previous versions to the current
deployed. This way we can keep our persisted business messages across
versions of the message classes, without the need for discarding old data.
The poc now is working ok for journals, but not yet for snapshots.
There are also other cross-cutting concerns related to persistence where
such a feature could be useful, for instance auditing
Is this something that could be implemented in akka-persistence?
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