Context

A context is a class meant to be event-sourced, i.e. associated with a stream of events. To buld the state of a context using stored events we use the evolve function.

events are associated to members of the cluster that end up in Update/Delete/Remove of some entity

Static members that are mandatory for any cluster of entities are:

• Zero: the initial state (no events yet).

• StorageName and Version: this combination uniquely identifies the cluster and lets the storage know in which stream to store events and snapshots. Whatever will be the storage (memory, Postgres, EventstoreDb, etc.) the cluster will be stored in a stream named as the concatenation of the StorageName and the Version (i.e. "_todo_01")

• SnapshotsInterval: the number of events that can be stored after a snapshot before creating a new snapshot (i.e. the number of events between snapshots).

The Command handler, by the runCommand function, applies a command, then stores the related events and returns the EventStore (database) IDs of those stored events and the KafkaDeliveryResult (if Kafka broker is enabled).

Example of a cluster of entities handling the todos and the categories:

    type TodosCluster =
        {
            todos: Todos
            categories: Categories
        }
        static member Zero =
            {
                todos = Todos.Zero
                categories = Categories.Zero
            }
        static member StorageName =
            "_todo"
        static member Version =
            "_01"
        static member SnapshotsInterval =
            15

In the following example, the TodosContext can check the validity of the categories referenced by any todo before adding it (to preserve the invariant rule that you can add only todo with valid category ID references). It uses the "result" computational expression included in the FsToolkit.ErrorHandling library which supports the railway-oriented programming pattern of handling errors.

Example:

    member this.AddTodo (t: Todo) =
        let checkCategoryExists (c: Guid ) =
            this.categories.GetCategories() 
            |> List.exists (fun x -> x.Id = c) 
            |> boolToResult (sprintf "A category with id '%A' does not exist" c)

        result
            {
                let! categoriesMustExist = t.CategoryIds |> catchErrors checkCategoryExists
                let! todos = this.todos.AddTodo t
                return 
                    {
                        this with
                            todos = todos
                    }
            }

Todo: Context.fs