runa/docs/runtime.md

# Phase 1 actor prototype

Start the interactive, in-memory runtime session with:

```sh
cargo run -p runa
```

The executable loads [counter.rn](../runa-std/actors/counter.rn) as the default
behavior. Commands include `spawn`, `send`, `tick`, `inspect`, `actors`,
`events`, `stop`, and `restart`; enter `help` to see usage. All commands operate
on the same process-local runtime. This CLI is for local development, not a
versioned external API.

Example:

```text
runa> spawn 0
spawned ActorId(1)
runa> spawn 10
spawned ActorId(2)
runa> send 1 1 2
message queued
runa> tick 100
runa> inspect 1
runa> inspect 2
```

The sender commits state 1 and sends a structured message; the receiver commits
state 15. `tick <count> [instruction-budget]` explicitly advances the scheduler.
The default per-actor slice budget is 32 instructions.

## Current contracts

- `runa-actor` owns actor, mailbox and implementation IDs, structured messages,
  snapshots, statuses and lifecycle events.
- `runa-core` owns stable source and implementation identity types.
- `runa-code` preserves original source, derives identities, caches compiled
  artifacts, and provides an in-memory named behavior registry with revisions.
- `runa-rune` owns compilation and typed bounded invocation execution.
- `runa-runtime` owns implementation registration, queues, private committed
  state, scheduling, message effects and lifecycle coordination.
- `runa` composes the experiment and demonstration.

An implementation is published once under an ID. Actors reference that exact
implementation. Implementation IDs use source content and the compile
fingerprint; actor and mailbox IDs remain process-local counters. Actor IDs and
incarnation counters use checked increments.

`CodeStore::publish_behavior` compiles source before appending a behavior
revision. Creation expects no existing revision; updates require the caller's
expected current revision, so stale edits fail without changing the behavior
reference. Failed compilation also leaves the reference and history unchanged
(the exact source remains available for diagnostics). Rollback points the
behavior at an earlier immutable implementation and appends a new monotonically
increasing revision; it does not rewrite history. Behavior names are
case-sensitive paths limited to ASCII letters, digits, `_`, `-`, `.`, and `/`,
with empty, `.` and `..` path components rejected.

The runtime's `install` operation switches an actor's implementation only
when it is idle, has no active invocation, and has no reserved input. Queued
messages may remain; they will use the newly installed implementation. Active
or awaiting handlers keep their original implementation, and installation
attempts during execution fail explicitly so the caller can retry at the next
handler boundary. This is an in-memory library capability; the interactive CLI
does not yet expose behavior publishing, history, rollback, or installation.

The source identity hashes the exact original UTF-8 bytes with the versioned
`runa-source:v1` domain. Whitespace/comment canonicalization is intentionally
not attempted; textually different source has a different identity. The
implementation identity includes the source ID, pinned Rune version, compiler
settings/features, registered native-module manifest, runtime ABI version, and
Cargo target triple. The native-module manifest and runtime ABI identifier must
be updated when those compile inputs change.

The cache is in-memory only. Invalid source is retained in the current
`CodeStore` for diagnostics, but does not produce a compiled artifact. Cache
identity is not a claim that Rune bytecode is portable or safe to persist.
`CodeStore::compile_with_module` requires an explicit stable module identity;
callers must change it when registered native functions or their contracts
change. The CLI currently creates a fresh `CodeStore` when it starts.

Prototype state is an `i64`; messages contain integer `kind` and `value`
fields. Rune handlers have this deliberately restricted interface:

```text
handle(state, kind, value) -> (new_state, [(target_actor_id, kind, value), ...])
```

The returned effect vector describes sends; handlers cannot access the runtime
or directly mutate committed state. General schema/versioned values and
capability-bearing contexts are future work.

## Scheduling and lifecycle

`tick(nonzero_budget)` gives each runnable actor one bounded slice in ID order.
FIFO queues are bounded, including the reserved message in capacity accounting.
A queued message is reserved when its invocation starts. Subsequent messages
stay queued, including while the actor awaits. A wake marks an awaiting actor
runnable; the caller still needs to drive ticks.

Successful handlers validate every send target and its aggregate queue
capacity before updating state, acknowledging the reserved message, and
enqueueing effects. Invalid effects fail the actor without partial sends or a
state commit. This is a **logical, single-worker in-memory atomicity guarantee**:
observers using the runtime see either the transition before commit or after
commit, not an intermediate state/effect set. It is not crash-safe or durable.
External side effects performed by native callbacks are outside this boundary
and cannot be rolled back.

Runtime errors retain the last committed state and reserved message, set an
observable failed status, and emit a failure event. The failed message is not
silently acknowledged or automatically retried. External sends return errors
for unknown, unavailable, or full actors.

Stopping drops the invocation, retaining committed state, mailbox and reserved
message. Restarting a stopped or failed actor preserves its actor ID, mailbox
ID, implementation ID, committed state, and queued-message order; it increments
its incarnation and puts the reserved message at the front of the queue.
Restart is explicit. Repeated failure of the same reserved message can therefore
block later messages until an operator changes the implementation or otherwise
resolves it; there is no automatic poison-message policy or supervision.

### Transition and retry guarantees

| Situation | State | Reserved input | Effects | Status |
| --- | --- | --- | --- | --- |
| Handler returns a valid transition | New state commits | Acknowledged | All validated sends enqueue | `Idle` |
| Rune execution or transition decoding fails | Last committed state retained | Retained | None from the returned transition | `Failed` |
| Any target is unknown, stopped, failed, or over capacity | Last committed state retained | Retained | No sends from the transition enqueue | `Failed` |
| Stop during an invocation | Last committed state retained | Retained | None from the incomplete transition | `Stopped` |
| Restart after stop or failure | Retained until retried | Moved to queue front | None until retry completes | `Idle` |

Mailbox capacity counts queued plus reserved messages. Before applying a
transition, the runtime validates all effects, including aggregate sends to a
single target. For a self-send, the completing input is acknowledged as part
of the same transition, freeing its reserved slot before the new effects are
enqueued. An invalid target or over-capacity effect rejects the entire returned
transition.

These are process-local guarantees only. A host crash loses state, mailboxes,
reservations and events. External/native callback effects may already have
occurred even when the Rune invocation later fails or is stopped.

Lifecycle tests verify that a failed reservation retries from the last
committed state, that stop/restart preserves reserved-first FIFO order, and
that a running actor's queued messages are not processed until the reservation
finishes. Transition tests cover malformed results, runtime failures, invalid
targets, aggregate capacity, and successful message effects.

## Limits

This is not yet a full image: no persistent behavior registry or mailboxes,
transactional outbox, supervisors, API server, GUI, or automatic image
bootstrap. The initial implementation installation API is process-local and
requires an idle handler boundary; it does not upgrade active execution.
The example default script is bundled through `include_str!`.
Messages are not accepted by stopped/failed actors.

Native functions must be nonblocking. Dropping a future proves cleanup only
for the controlled test operation, not rollback of arbitrary external effects.
Instruction slices cannot interrupt a native callback.

The [execution decision](decisions/0001-worker-local-execution.md) records the
scheduling trade-offs. Heap profiling and source-level debugging remain open.