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In my previous post, I used the JournaledGrain to create an Event Sourced grain. This enabled us to raise events from within our grain which would be applied to our grain state. Next up, which I’m covering in this post is how to use EventStore for Orleans Grain Persistence.
This means when we raise events, they will also be persisted to EventStore. When our grain is activated, we can re-hydrate it by retrieving prior events from an EventStore stream and re-running them in our Grain to get back to current state.
Blog Post Series:
- Part 1 – Practical Orleans
- Part 2 – Grains and Silos
- Part 3 – Smart Cache Pattern
- Part 4 – Event Sourced Grain
- Part 5 – EventStore for Grain Persistence
EventStore
If you are unfamiliar with EventStore:The open-source, functional database with Complex Event Processing in JavaScript.If you don’t have a running instance, the easiest way is probably with Docker. Pull down the latest image from docker hub and as single node running. docker pull eventstore/eventstore docker run –name eventstore-node -it -p 2113:2113 -p 1113:1113 eventstore/eventstore There is a .NETStandard 2.0 compatible client package that I will be using in our Grain Project. Install-Package EventStore.ClientAPI.NetCore
Writing to EventStore
Anytime our grain was calling theJournaledGrain.RaiseEvent, we want to actually persist that to an EventStore Stream. For my demo, we will have one EventStore stream per instance of an Orleans grain. Meaning each bank account will have one event stream.
I’m going to create a new RaiseEvent method that will call the base.RaiseEvent and once confirmed they were applied, append them to our EventStore Stream. The additional private static methods are really just helpers for (de)serializing our events from/to json.
Re-hydrating
When our Grain activates withOnActivateAsync, this is when we will fetch all the events from our event stream and apply them to our grain. Basically this will be replaying all the events to build our grain state back up to current state.