Orleans.FSharp.EventSourcing 2.0.0-alpha.1

Orleans.FSharp.EventSourcing

Declarative event sourcing for Orleans grains in F#.

What it does

Provides the eventSourcedGrain {} computation expression for defining event-sourced grains with pure apply and handle functions. Bridges to Orleans JournaledGrain for durable event storage.

Quick example

open Orleans.FSharp.EventSourcing

type State   = { Balance: decimal }
type Event   = Deposited of decimal | Withdrawn of decimal
type Command = Credit of decimal    | Debit of decimal

let bankAccount = eventSourcedGrain {
    defaultState { Balance = 0m }
    apply (fun state event ->
        match event with
        | Deposited amount -> { state with Balance = state.Balance + amount }
        | Withdrawn amount -> { state with Balance = state.Balance - amount })
    handle (fun state cmd ->
        match cmd with
        | Credit amount -> [ Deposited amount ]
        | Debit amount when state.Balance >= amount -> [ Withdrawn amount ]
        | Debit _ -> [])
    logConsistencyProvider "LogStorage"
}

CE keywords

Custom storage (ICustomStorageInterface)

Use customStorage to plug in your own event store (e.g. EventStoreDB, Marten in append-only mode, or any database). Pair it with logConsistencyProvider "CustomStorage".

let myGrain = eventSourcedGrain {
    defaultState MyState.empty
    apply applyEvent
    handle handleCommand
    logConsistencyProvider "CustomStorage"
    customStorage
        (fun () -> task {
            // Load current version and state from your store
            let! version, state = myStore.ReadAsync(grainId)
            return version, state
        })
        (fun events expectedVersion -> task {
            // Append events with optimistic concurrency
            return! myStore.AppendAsync(events, expectedVersion)
            // return true on success, false on concurrency conflict
        })
}

The customStorage operation takes two functions:

• read unit -> Task<int * 'State> — loads the current (version, state)
• write 'Event list -> int -> Task<bool> — appends events; returns true on success, false on version conflict

The generated C# stub automatically implements ICustomStorageInterface<WrappedEventSourcedState, WrappedEventSourcedEvent> and the [LogConsistencyProvider(ProviderName = "CustomStorage")] attribute is emitted on the class.

Key modules

Dependencies

• Microsoft.Orleans.EventSourcing
• Orleans.FSharp
• Orleans.FSharp.Runtime

License

MIT