> ## Documentation Index
> Fetch the complete documentation index at: https://developers.telnyx.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Execution Model

Four rules govern how code runs on an instance. They are the stateful actor's rules, and a class that respects them needs no locks and gets its retries from the scheduler; a task handler must therefore be safe to run twice, and an outward effect such as sending a text needs its own guard.

**One call at a time per instance.** An instance runs each method to completion before the next starts. A second text while a reply is being produced waits; `messages.add` followed by `queue` can never race another call on the same conversation. Different instances run in parallel, on whatever containers the platform placed them.

**Two budgets.** An inbound call, an RPC or a webhook, runs under a 30-second wall-clock budget by default: enough to record a message, not enough for a model round-trip with tools. Queued and scheduled tasks run in the instance's alarm handler with a budget of 15 minutes, and retry on failure, up to 5 times by default. The turn therefore lives in a task:

```ts theme={null}
async receive(text: string, from: string): Promise<void> {
  await this.messages.add("user", text);
  await this.queue("process");          // returns at once; the budget is met
}

async process(): Promise<void> {        // minutes, retried
  // the model, the tools, the reply
}
```

**Writes land before the caller hears back.** When a method returns, the runtime holds the response until every write it issued, to the message log, the state, the schedule or the SQL database, has been flushed. A caller never observes a result built on an unflushed write.

**Only what was written survives.** An instance can be evicted, restarted or relocated between any two calls; its fields vanish, its storage does not. [Memory Model](/docs/agents/model/memory-model) is that rule in full.

Under `dev`, calls and tasks run in one process the moment they are due; the budgets, the retry backoff and the restart recovery are only approximated there. [Execution Model](/docs/edge-compute/stateful-actors/concepts/execution-model) on Stateful Actors states the four rules for any actor.