---
name: async-logger-config
group: api
category: async
update-time: 20260512
description: Create the queue, batching, linger, and flush policy config used by async loggers.
key-word:
    - async
    - config
    - queue
    - public
---

## Async-logger-config

Create an `AsyncLoggerConfig` value describing queue capacity, overflow behavior, batching size, linger timing, and flush policy for async logger construction.

### Interface

```moonbit
pub fn AsyncLoggerConfig::new(
  max_pending~ : Int = 0,
  overflow~ : AsyncOverflowPolicy = AsyncOverflowPolicy::Blocking,
  max_batch~ : Int = 1,
  linger_ms~ : Int = 0,
  flush~ : AsyncFlushPolicy = AsyncFlushPolicy::Never,
) -> AsyncLoggerConfig {}
```

#### input

- `max_pending : Int` - Maximum queued records.
- `overflow : AsyncOverflowPolicy` - Queue overflow strategy.
- `max_batch : Int` - Maximum records drained per batch.
- `linger_ms : Int` - Optional wait window for batch accumulation.
- `flush : AsyncFlushPolicy` - Flush behavior for batch/shutdown phases.

#### output

- `AsyncLoggerConfig` - Async runtime config object used by `async_logger(...)` or async build helpers.

### Explanation

Detailed rules explaining key parameters and behaviors

- `max_batch <= 1` is normalized to `1`.
- `linger_ms < 0` is normalized to `0`.
- `overflow` and `flush` define the most important queue/runtime behavior tradeoffs.
- This type is used directly by `async_logger(...)` and embedded in `AsyncLoggerBuildConfig`.

### How to Use

Here are some specific examples provided.

#### When Tune Async Queue Backlog And Batch Size

When async behavior should be explicit in code:
```moonbit
let config = AsyncLoggerConfig::new(
  max_pending=128,
  overflow=AsyncOverflowPolicy::DropOldest,
  max_batch=8,
  linger_ms=10,
  flush=AsyncFlushPolicy::Batch,
)
```

In this example, queue pressure, batch size, and flush timing are configured together.

#### When Export Async Config

When async policy should be serialized or logged:
```moonbit
println(stringify_async_logger_config(AsyncLoggerConfig::new(max_pending=8)))
```

In this example, the config becomes a stable JSON payload.

### Error Case

e.g.:
- If `max_batch` is set to `0` or below, runtime config normalizes it to `1`.

- If `linger_ms` is negative, it is normalized to `0`.

### Notes

1. This type controls async runtime behavior, not synchronous queue wrapping.

2. Prefer explicit values for production services so overflow and flush semantics are visible.