Refs across nodes

In a single-node system, an ActorRef is a direct in-memory handle. In a clustered system, an ActorRef can point at an actor on any node — and the same tell works the same way:

const remote: ActorRef<Msg> = /* ref to an actor on a different node */;
remote.tell({ kind: 'do-it' });
// → serialized, sent over the cluster transport, delivered to the remote mailbox

The framework hides the network layer. But understanding how helps when something goes wrong (messages disappearing, latency spiking, refs failing to resolve).

The path-with-host format

A local-only ref’s path looks like:

actor-ts://my-app/user/api/sessions/user-42

A cluster-aware ref includes the host:port of its owning node:

actor-ts://my-app@10.0.0.5:2552/user/api/sessions/user-42
            │       └────────┘
            │       node's address (assigned at Cluster.join time)
            └── system name

The host fragment tells the local runtime which node hosts this actor. When you tell a remote ref, the framework:

1. Reads the host:port from the ref’s path.
2. Looks up the cluster transport’s connection to that node.
3. Serializes the message + the ref’s local-path segments.
4. Writes a wire frame.
5. Receiving node deserializes; locally resolves the path segments to a real actor; tells the local ref.

How refs cross the wire

Inside a message, an ActorRef field serializes as the path-with-host string. On the receiving node, the deserializer reconstructs a RemoteActorRef pointing back at the original node:

type GetMsg = {
  kind: 'get';
  replyTo: ActorRef<number>;   // ← will serialize as a path string
};

remoteRegistry.tell({
  kind: 'get',
  replyTo: this.context.self,   // → "actor-ts://my-app@10.0.0.3:2552/user/asker"
});

The receiving node sees replyTo as a RemoteActorRef pointing at 10.0.0.3:2552/.../asker. Calling tell on it goes through the same encoding-and-transport machinery in reverse.

This means request/response between actors on different nodes just works — the reply travels back to the original asker over the cluster transport.

The cluster transport

The transport (TCP by default, in-memory in tests) carries:

• Cluster control traffic — gossip, heartbeats, downing signals.
• Envelope traffic — your tells wrapped in a routing envelope.

Both share the same TCP connection per peer pair. The framework multiplexes them; you don’t see this distinction.

See Transports for the TCP and in-memory implementations.

Resolving a remote ref by path

import { ActorSelection } from 'actor-ts';

const remote = await system.actorSelection(
  'actor-ts://my-app@10.0.0.5:2552/user/api/sessions/user-42',
).resolveOne(5_000);

remote.tell({ kind: 'do-it' });

actorSelection parses the path-with-host; resolveOne returns a RemoteActorRef you can tell. Useful when:

• The target’s location is known by convention (a well-known sharding path or singleton proxy).
• A message contains a path string (e.g., from an HTTP request).

For routine cluster work, you’d usually hold a ref obtained from the cluster’s own machinery (sharding region, singleton proxy, event-stream subscriber) — actorSelection is the lookup escape hatch.

What happens when the remote node disappears

const remote: ActorRef = /* on a node that just left the cluster */;

remote.tell({ kind: 'do-it' });
// → message routes to dead letters; sender doesn't see the failure

tell to a remote ref on a stopped/unreachable node:

1. The framework tries to write to the transport.
2. The transport sees no live connection (or one in half-closed state).
3. The message is dropped to dead letters.

Two ways to detect this:

• context.watch(remoteRef) — receive a Terminated notification when the remote actor stops or the remote node goes unreachable+downed. The Terminated.addressTerminated flag distinguishes “actor stopped” from “node lost.”
• Subscribe to cluster events — MemberRemoved / UnreachableMember for the host’s address tells you the whole node is gone.

See Death watch for the per-actor variant.

Serialization

Messages crossing the wire need to be serializable. The framework’s SerializationExtension handles this — JSON by default, CBOR when configured.

{
  kind: 'place',
  order: { items: ['book-1'], total: 19.99 },
  replyTo: this.context.self,   // ← ref serializes as path string
}

Plain values (strings, numbers, arrays, plain objects) serialize trivially. Things that don’t survive:

• Functions / closures — can’t cross a process boundary.
• Class instances with methods — methods are lost; only data fields survive.
• Map / Set — JSON serializes them as {} (no entries). Use plain objects or arrays.
• Symbols, BigInts (without serializer registration).

See Messages for the immutability

• wire-format conventions.

Performance

Rough numbers:

• Local tell (same actor system) — 50-200 ns.
• Cluster-local tell (loopback transport) — sub-millisecond.
• Cluster tell over LAN — 0.5-2 ms for small messages.
• Cluster tell over WAN — bounded by network RTT.

The framework batches envelopes opportunistically — many tells in quick succession share TCP writes when possible.

Don't assume a remote actor exists

const remote: ActorRef = ...;
remote.tell({ kind: 'do' });
// → did this arrive?  The local code can't tell.

tell is fire-and-forget locally; it’s even more “fire and forget” remotely. For confirmation, use ask with a sane timeout, watch the remote ref, or design the protocol with acknowledgments.

Latency is hard to measure from inside an actor

const t0 = Date.now();
remoteRegistry.tell({ kind: 'get', replyTo: ... });
// ↑ when does this clock stop? When the reply arrives — not when this returns.

tell returns immediately; latency is measured between the send and the receipt of a reply. Use ask if you want a Promise that captures the round-trip.

Sending huge payloads over the cluster transport

remoteActor.tell({ kind: 'upload', data: hugeBlob });   // 100 MB

The cluster transport’s max-frame-size default is 1 MB (configurable via actor-ts.remote.max-frame-size). Frames larger than that fail; the message dead-letters. For large payloads, use a separate side-channel (HTTP upload, S3 presigned URL) and pass a reference in the actor message.

Where to next

• Cluster overview — the bigger picture: how nodes find each other, gossip, membership.
• Transports — TCP and in-memory implementations.
• Serialization overview — the wire format for cross-node messages.
• Messages — the conventions that make messages serializable.
• Death watch — detecting that a remote actor stopped.

The RemoteActorRef API reference covers the wire ref implementation.