tractor/docs/guide/rpc.rst

RPC: calling into other actors

Every spawn call from /guide/spawning hands you back a ~tractor.Portal: a live handle for calling into another actor's memory domain. The name is borrowed from trio's portal concept — an object you use to submit work into a separate concurrency domain — except here that domain is a whole other process.

There are no proxy objects and no special calling conventions: you pass a plain function reference plus keyword args, and Python's normal await-able semantics apply. The function just happens to run somewhere else; from the calling task it looks as though it was called locally. And since this is all structured concurrency (SC) under the hood, the remote task runs inside the callee's supervised task tree while its result — or its failure, as a boxed ~tractor.RemoteActorError — always comes back to you.

Portal.run(): pass the function, not a string

~tractor.Portal.run schedules an async function as a new task in the remote actor and waits on its result:

async with tractor.open_nursery() as an:
    portal = await an.start_actor(
        'service',
        enable_modules=[__name__],
    )
    answer = await portal.run(movie_theatre_question)

The rules of engagement:

• the target must be an async function and its defining module must be in the callee's enable_modules allowlist, else an ~tractor.ModuleNotExposed error is relayed back (see /guide/spawning for the capability-allowlist story).
• arguments are passed by keyword only; they ride the IPC layer as msgspec-encoded msgs, so keep them serializable.
• every call schedules a fresh task remotely — call it twice and the callee runs two tasks, each supervised in its own right.
• remote exceptions re-raise locally as ~tractor.RemoteActorError with the original type preserved via .boxed_type.

Note

Passing dotted-path strings to run() is an ancient, deprecated form; always pass the function reference. If you really need name-based addressing use run_from_ns() below.

Namespaced daemons: run_from_ns()

Sometimes the calling process can't (or shouldn't) import the target function — think a long-running rpc-daemon serving modules your client never loads. For that, ~tractor.Portal.run_from_ns takes the explicit namespace path:

await portal.run_from_ns('mypkg.service', 'ping')

This is literally how .run() works underneath: the pair is encoded as a 'mod.path:func' style msg and resolved against the callee's enabled modules.

One special namespace exists: 'self' resolves to the remote ~tractor.Actor instance, i.e. the runtime itself. It's how internal machinery (cancel requests, registry ops) travels; don't build your app on it.

One-shot subactors: to_actor.run()

When the call should own a fresh subactor whose entire job is one function call, tractor.to_actor.run spawns it, runs the task, returns its result and reaps the process — all in one blocking call:

from functools import partial

final = await tractor.to_actor.run(
    partial(fib, n=10),
    an=an,
)

Semantics worth knowing:

• it blocks until the remote task returns, re-raising any remote error in the usual boxed form right in the calling task.
• placement also determines process ownership: an= spawns and reaps a fresh child in an existing actor nursery, while passing neither does the same in a private call-scoped nursery (booting the runtime if needed). portal= instead runs one linked task in an existing actor; it neither spawns nor reaps that actor, so the portal's owner remains responsible for its lifetime.
• concurrency composes the plain trio way: schedule multiple run() calls into a local task nursery (see examples/parallelism/to_actor_one_shots.py).

A reused actor must expose both the target module and the to_actor context trampoline:

async with tractor.open_nursery() as an:
    portal = await an.start_actor(
        'worker',
        enable_modules=[
            __name__,
            tractor.to_actor.MODULE,
        ],
    )
    try:
        final = await tractor.to_actor.run(
            partial(fib, n=10),
            portal=portal,
        )
    finally:
        await portal.cancel_actor()

Pure RPC daemons: run_daemon()

When a process's only job is to sit at the root of its own tree and serve RPC, skip the boilerplate with tractor.run_daemon:

import tractor

tractor.run_daemon(
    ['mypkg.service'],
    name='service',
)

It's a blocking convenience (it calls trio.run() for you): boot a root actor with the given modules enabled for RPC, then sleep until cancelled. Pair it with the discovery system — tractor.find_actor / tractor.wait_for_actor from a separate program — and you've got a tiny service architecture with zero framework ceremony; see examples/service_daemon_discovery.py for the full pattern.

Fan-out: RPC through nested trees

Portals compose. An RPC task is just a trio task, so it can open its own ~tractor.ActorNursery and portal into its children — one inbound call fanning out into a whole sub-tree of work. The mid-tier function from the nested-tree example:

../../examples/nested_actor_tree.py

The root portals into the supervisor actor; the supervisor's RPC task spawns the leaf workers, portals into each, and returns the combined result back up. Failures at any depth relay hop-by-hop as boxed errors, and cancelling the root call tears down the entire sub-tree — SC, transitively.

When to graduate to Context

The ~tractor.Portal.run method is great for one-shot, request-response calls. Reach for ~tractor.Portal.open_context with an @tractor.context endpoint as soon as you want:

• a long-lived dialog with state held on both sides,
• bidirectional streaming via ctx.open_stream(),
• typed payload contracts (pld_spec) enforced at the msg layer,
• or task-scoped cancellation: Context.cancel() cancels just the linked remote task, whereas ~tractor.Portal.cancel_actor nukes the entire remote runtime and its process.

tractor.to_actor.run already enters the full ~tractor.Portal.open_context lifecycle. The older ~tractor.Portal.run path instead uses the Context returned by the lower-level Actor.start_remote_task() directly, avoiding a Started handshake but owning less lifecycle machinery. A follow-up should factor their shared linked-task lifecycle without requiring Portal.run() to delegate through the public context API or add another wire message. Take the full tour in the context guide </guide/context>.

• /guide/spawning — where portals come from and how their actors are supervised.
• /guide/context — the structured cross-actor task API: handshake, streaming, typed payloads.
• /guide/cancellation — what happens to in-flight RPC when trees get torn down.