---
name: runtime-sink-file-flush
group: api
category: runtime
update-time: 20260707
description: Flush the file sink behind a RuntimeSink when one is present, with queued-file success tied to both queue drain and file flush outcome.
key-word:
    - runtime
    - sink
    - file
    - public
---

## Runtime-sink-file-flush

Flush the file sink behind a `RuntimeSink`. This helper is the direct file-specific runtime flush surface for code that owns a sink value instead of a `ConfiguredLogger`.

### Interface

```moonbit
pub fn RuntimeSink::file_flush(self : RuntimeSink) -> Bool {
```

#### input

- `self : RuntimeSink` - Runtime sink whose file sink should be flushed.

#### output

- `Bool` - Whether the file-specific flush path succeeded.

### Explanation

Detailed rules explaining key parameters and behaviors

- Plain `File` runtime variants forward directly to `FileSink::flush()`.
- `QueuedFile` runtime variants first drain the queue, then flush the wrapped file sink.
- For `QueuedFile`, the returned `Bool` is `true` only when all queued records were consumed, the final file flush succeeded, and no new write, flush, or rotation failures were observed during that flush path.
- For `QueuedFile`, this step may also be the point where queued records finally reach the inner file sink, so failure counters can change here even if earlier `log(...)` calls only enqueued records.
- Non-file runtime variants return `false`.
- After a file-backed runtime sink has already cleared its handle, later `file_flush()` calls return `false`.

### How to Use

Here are some specific examples provided.

#### When Need Explicit Direct File Durability Steps

When code holds a file-backed `RuntimeSink` directly and should flush it explicitly:
```moonbit
ignore(sink.file_flush())
```

In this example, the runtime sink attempts to flush its file-backed output directly.

#### When Need A File-specific Success Flag

When code should branch on the outcome of a direct file flush:
```moonbit
let ok = sink.file_flush()
```

In this example, callers can distinguish file-specific flush success from a non-file sink shape.

### Error Case

e.g.:
- If the runtime sink is not file-backed, the method returns `false`.

- If the file handle was already closed earlier through this runtime sink or another facade sharing the same underlying file-backed state, the method returns `false`.

- If callers want generic queue or sink advancement instead of file-specific behavior, `flush()` is the broader API.

- On queued file sinks, a `false` result may mean queue consumption happened but file delivery still observed a new failure.

### Notes

1. Prefer this helper when direct runtime code specifically cares about file flush behavior.

2. Queued file sinks may perform both queue drain work and file flush work here, including surfacing delayed failures from previously queued records.

3. Library or application facades that share the same wrapped file-backed runtime sink still observe the same flush availability state.