Concept — message passing across isolated memories

Processes share nothing, so they communicate by copying bytes through the host. Each process has a mailbox (an async channel). Sending is fire-and-forget; receiving suspends the process until a message arrives.

The flow

1. Sender builds a message in its own linear memory and calls rusm::message::send(pid).
2. The host copies the message bytes out of the sender's memory and pushes them onto the target's mailbox (a Tokio channel). No memory is shared.
3. The target calls rusm::message::receive(), which awaits the mailbox; the host copies the bytes into the target's memory.

Why copy, not share

Sharing memory between instances would break isolation — the whole point of the model. Copying keeps each crash and each permission boundary local. Messages are ordinary serialized data (the rusm-rs and rusm-ts guest crates send serde JSON — one wire shared across Rust and TS guests).

Receive suspends, it doesn't spin

receive() is an async host call: an empty mailbox parks the Tokio task (see fibers & blocking→async), so a million waiting processes cost almost nothing.

Shipped in Phase 2 (opt-in mailbox depth surfaces in the observer snapshot).