A composable transformation wrapped around an event store's adapter call.

Middleware is how Sourced.EventStore stays agnostic of concerns like domain event conversion and telemetry. Each middleware receives the Sourced.Operation flowing through the pipeline, a next value standing for the rest of the stack, and its own configured opts. It may transform the operation on the way in, must call next/2 to continue, and may transform the returned operation's result on the way out:

defmodule MyApp.Tracing do
  @behaviour Sourced.Middleware

  @impl true
  def init(opts), do: opts

  @impl true
  def call(op, opts, next) do
    op = %{op | opts: annotate(op.opts, opts)}
    Sourced.Middleware.next(next, op)
  end
end
end

Configuring a pipeline

Entries passed to Sourced.EventStore.new/1 via :middleware are either a bare module or a {module, opts} tuple (Plug-style). The first entry is the outermost wrapper; the adapter call is the innermost terminal:

Sourced.EventStore.new(
  adapter: Sourced.EventStore.Postgres,
  middleware: [
    {Sourced.Middleware.Domain, [OrderPlaced, OrderShipped]},
    {MyApp.Tracing, sample: 0.1}
  ]
)

Initialization

Each entry's opts are passed through the middleware's init/1 when the store is built, and it is init/1's return value that call/3 receives on every operation. This is where option validation and any expensive preprocessing belong, so the work happens once per store rather than on every operation — see Sourced.Middleware.Domain for an example. Implementing init/1 is optional; without it the opts are passed through untouched.