---
name: async-logger-run
group: api
category: async
update-time: 20260707
description: Start the async logger worker loop so queued records are drained to the underlying sink while lifecycle and failure state are reset and updated around execution.
key-word:
    - async
    - logger
    - worker
    - public
---

## Async-logger-run

Start the async logger worker loop. This is the core runtime API that drains queued records to the underlying sink and updates worker lifecycle state around that drain loop.

### Interface

```moonbit
pub async fn[S : @bitlogger.Sink] AsyncLogger::run(self : AsyncLogger[S]) -> Unit {}
```

#### input

- `self : AsyncLogger[S]` - Async logger whose queue should be drained by the worker loop.

#### output

- `Unit` - No return value. The method runs until the queue is closed or a worker failure occurs.

### Explanation

Detailed rules explaining key parameters and behaviors

- `run()` sets `is_running` to `true` before worker execution begins.
- `run()` moves the logger lifecycle phase from `ready` or `failed` into `running`.
- `run()` now enforces a single-worker contract. If the same logger already has a running worker, the later call raises `AsyncLoggerAlreadyRunning` instead of silently starting another drain loop.
- If the logger is already closed, `run()` raises `AsyncLoggerClosed` instead of trying to restart a terminal logger.
- Every invocation clears previous failure state first by setting `has_failed=false` and `last_error()` to an empty string once that `run()` call has actually started executing.
- The method then keeps draining records until `queue.get()` stops with `AsyncLoggerClosed` or a worker error escapes.
- On a normal queue-close exit, `run()` leaves the logger in `closed` if shutdown/close had already started, otherwise it returns to `ready`.
- On failure, the logger records `last_error`, transitions to `failed` or `closed_failed`, and then raises the error back out of `run()`.
- A worker failure does not guarantee the async backlog was fully drained first. If the failure happens after some records were already written, later queued records can remain pending when `run()` exits.
- A later `run()` invocation can resume draining that retained backlog, but the stale failure flag and stale `last_error()` value are only cleared after the new worker call actually begins running.

### How to Use

Here are some specific examples provided.

#### When Need Background Queue Drain

When async logging should be processed by a worker task:
```moonbit
let logger = async_logger(console_sink())
@async.with_task_group(group => {
  group.spawn_bg(() => logger.run())
  logger.info("started")
  logger.shutdown()
})
```

In this example, `run()` is the worker loop that makes the async logger actually deliver queued records.

#### When Need Explicit Worker Lifetime Control

When worker execution should be started under application control:
```moonbit
group.spawn_bg(() => logger.run())
```

In this example, the application decides when the worker begins instead of hiding that lifecycle step.

### Error Case

e.g.:
- If the worker loop fails, `has_failed()` becomes `true`, `last_error()` stores the error text, and `run()` raises that failure to the caller.

- After a failed run, `pending_count()` can still be greater than zero until later shutdown or restart logic finishes handling the retained backlog.

- If `run()` is never started, accepted records may remain queued and not reach the sink.

- A later `run()` attempt starts from a fresh failure flag and empty `last_error()` string once that retrying worker has actually started, even if an earlier run failed.

- Starting more than one `run()` task for the same logger raises `AsyncLoggerAlreadyRunning` on the later attempt.

- Starting `run()` after a terminal close path raises `AsyncLoggerClosed`.

### Notes

1. `async_logger(...)` only constructs the logger; `run()` is what activates queue draining.

2. Pair this API with `shutdown()` for a complete worker lifecycle.

3. Pair it with `has_failed()`, `last_error()`, or `state()` when tests need to inspect how a worker exit affected logger health.

4. Start one deliberate worker task per logger. A concurrent second startup now fails explicitly with `AsyncLoggerAlreadyRunning`.