Wraps client operations so cross-cutting concerns can observe and extend them.

An interceptor sees the operation name and its options, may change the options, and calls next to continue the chain. Register them on a client:

{Temporalex.Client,
 name: MyApp.Temporal,
 backend: Temporalex.Backend.TemporalCore,
 target: "http://127.0.0.1:7233",
 namespace: "default",
 interceptors: [MyApp.Tracing, MyApp.Auth]}

They run outside-in: the first in the list wraps the second, and so on, with the real operation innermost. This is the hook for trace-context injection, logging, metrics, and auth headers.

Interceptors do not apply to Temporalex.Testing, which is an in-process harness rather than a client.

Trace context, and why this is client-side only

Temporal propagates context in workflow headers, so an interceptor injects it by adding to :headers:

defmodule MyApp.Tracing do
  @behaviour Temporalex.Interceptor

  alias Temporalex.Interceptor.Context

  @impl true
  def intercept(%Context{operation: :start_workflow}, opts, next) do
    headers = Map.put(Keyword.get(opts, :headers, %{}), "traceparent", current_traceparent())
    next.(Keyword.put(opts, :headers, headers))
  end

  def intercept(%Context{}, opts, next), do: next.(opts)
end

Client operations are never replayed, so an interceptor here may do whatever it likes — read the clock, mint an id, call out to a service.

That is not true inside a workflow. Workflow code is re-executed on replay and must emit the same commands every time, so a value minted per call (a fresh span id, say) would land in a command and differ on replay — nondeterminism. Propagating context onward from a workflow therefore means copying values that came from history, never generating new ones. See docs/scheduler_and_replay.md. Workflow-side interception is deliberately not part of this module.

Failure

An interceptor is ordinary code in the caller's process. If it raises, the operation raises — it is not isolated, and it must not be used for anything the caller's correctness depends on being optional.