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Tell vs Ask

PatternUse caseResponse
TellFire-and-forgetNone
AskRequest-responseFuture you can await
Use Tell when you don’t need a reply—logging, notifications, commands that trigger side effects. Use Ask when you need a result—queries, computations, or any flow that depends on the response.

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Message types (v4)

From v4.0.0, all message-passing APIs accept any. You can send:
  • Plain Go structs (with CBOR serialization when remote)
  • Protocol buffer messages (default ProtoSerializer)
  • Any type registered with the serializer
The framework handles serialization when the target is on another node. Locally, messages are passed by reference without serialization.

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Message ordering

Messages between a specific sender-receiver pair are delivered in the order they were sent (FIFO). This follows from the mailbox being a FIFO queue and the single-threaded processing guarantee per actor.

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Sender context

In Receive, you can access the sender via ctx.Sender(). It returns a *PID or nil (e.g., for scheduled messages or when the sender is unknown). Use ctx.Sender().Path() when you need the sender’s address for a reply.

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ReceiveContext messaging methods

Inside Receive, the ReceiveContext provides these messaging operations:
MethodPurpose
Tell(to *PID, message any)Fire-and-forget. Does not block.
Ask(to *PID, message any, timeout)Request-response. Blocks until reply or timeout.
Response(resp any)Reply to an Ask. Call exactly once per Ask message.
Request(to, message, opts...)Non-blocking Ask; use continuations for the reply.
PipeTo(to, task, opts...)Run task asynchronously; deliver result to to. See PipeTo.
PipeToName(actorName, task, opts...)Same, target by name.
Use Sender() to get the sender’s PID when replying. Use ActorSystem().ActorOf(ctx, name) to resolve an actor by name before sending.

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No shared state

Actors never share memory. All communication is through immutable (or effectively immutable) messages. This eliminates whole classes of concurrency bugs and makes reasoning about behavior easier.