---
name: parse-and-build-library-async-logger
group: api
category: facade
update-time: 20260614
description: Parse JSON async build config text and build the library-facing async logger facade through the sync-first async builder path.
key-word:
    - library
    - async
    - parse
    - public
---

## Parse-and-build-library-async-logger

Parse raw JSON async build config text and build a `LibraryAsyncLogger[RuntimeSink]` in one step. This facade is the text-driven library counterpart to `parse_async_logger_build_config_text(...)` plus `build_library_async_logger(...)`.

### Interface

```moonbit
pub fn parse_and_build_library_async_logger(
  input : String,
) -> LibraryAsyncLogger[RuntimeSink] raise {
```

#### input

- `input : String` - Raw JSON async logger build config text.

#### output

- `LibraryAsyncLogger[RuntimeSink]` - Library-facing async runtime logger wrapper.

### Explanation

Detailed rules explaining key parameters and behaviors

- This API parses async build config text, validates it, builds the async runtime logger through `build_library_async_logger(...)`, and narrows it to the library facade.
- Both the embedded sync logger config and async queue/runtime config are validated by the parser layer before any facade value is returned.
- The embedded `LoggerConfig` still goes through the normal synchronous config path first, so sink shape and any optional synchronous queue layer are already applied before the outer async layer is wrapped and then narrowed.
- That also means the parsed `logger.sink.kind` had already selected the concrete `RuntimeSink` variant before the library facade narrowed the helper surface.
- The returned facade wraps the same underlying `AsyncLogger[@bitlogger.RuntimeSink]` value that `parse_async_logger_build_config_text(...)` plus `build_async_logger(...)` would produce directly.
- The resulting facade keeps library-facing async operations such as `run()` and `shutdown()` while exposing a smaller public surface.
- The broader async helper surface is still preserved rather than rebuilt, but it is intentionally not directly exposed on the returned facade. Queue counters, lifecycle state, idle-wait helpers, and file-backed runtime helpers stay behind `to_async_logger()` instead of disappearing.
- The narrower facade does not change the underlying runtime-sink queue counters, failure state, sink shape, or runtime-dependent post-close behavior; it only hides the broader helper surface until `to_async_logger()` is used.
- The narrower facade also preserves the underlying target rules on its exposed write methods: `log(..., target=...)` can override the target for one call, while `info(...)`, `warn(...)`, and `error(...)` continue using the stored logger target unless the facade first derived another logger with `with_target(...)` or `child(...)`.
- In the current parsed-builder coverage, unwrapping this facade yields the same async state snapshot, runtime-sink variant, queue counters, lifecycle flags, and failure fields as `build_async_logger(parse_async_logger_build_config_text(input))`.
- Parsed file-backed runtime helpers also remain aligned after unwrap instead of being rebuilt into a different facade-specific sink state.
- Async state helpers such as `pending_count()`, `dropped_count()`, `state()`, `wait_idle()`, and failure-status inspection stay on the underlying `AsyncLogger`, not on the returned facade itself.
- `to_async_logger()` can recover the underlying async logger when a wider API is required.
- Use this parse/build facade when text-driven construction should preserve the runtime-sink build path but still hide broader async inspection helpers at the package boundary.

### How to Use

Here are some specific examples provided.

#### When Need Text-driven Async Library Bootstrapping

When a package accepts raw config text but wants a narrower async facade output:
```moonbit
let logger = parse_and_build_library_async_logger(
  "{\"logger\":{\"target\":\"lib.async\",\"sink\":{\"kind\":\"console\"}},\"async_config\":{\"max_pending\":4,\"overflow\":\"DropNewest\",\"max_batch\":1,\"linger_ms\":0,\"flush\":\"Never\"}}",
)
```

In this example, parsing and library-facade construction happen together.

And any configured synchronous runtime sink controls remain active under the returned `RuntimeSink`-backed async logger.

#### When Need Async State Helpers After Text-driven Library Bootstrapping

When JSON-driven construction should still allow internal async state inspection later:
```moonbit
let logger = parse_and_build_library_async_logger(raw) catch {
  err => return
}
let full = logger.to_async_logger()
ignore(full.pending_count())
```

In this example, the caller unwraps the library async facade before using async state helpers.

And unlike `ApplicationAsyncLogger`, the narrower parse/build result does not expose those broader async helpers directly on the facade surface.

#### When Need A Per-call Target Override Through The Library Facade

When parsed library configuration should still allow a one-call target override without unwrapping first:
```moonbit
let logger = parse_and_build_library_async_logger(raw) catch {
  err => return
}
logger.log(@bitlogger.Level::Error, "boom", target="lib.audit")
```

In this example, the emitted record uses `lib.audit` for that call.

And later `info(...)`, `warn(...)`, or `error(...)` calls still use the facade's stored target unless code derives another facade first with `with_target(...)` or `child(...)`.

### Error Case

e.g.:
- If the JSON text is malformed, parsing raises an error.

- If the embedded config is invalid, parsing raises before the library facade is returned.

- If callers assume async state or idle-wait helpers are available directly on the returned facade, they must unwrap first with `to_async_logger()`.

- If callers need file-backed runtime helpers after parse/build, they must unwrap first, but the resulting helper surface still matches the parsed direct builder path.

- If callers depend on queued or file-backed runtime behavior, they should read this facade as preserving the already-built `RuntimeSink` result from the parsed sync-first builder path, not as recomputing sink selection after narrowing.

### Notes

1. This is the narrow async library parse-and-build facade.

2. Use `build_library_async_logger(...)` when the config is already typed.

3. Use `build_library_async_text_logger(...)` instead when callers want the narrower `FormattedConsoleSink` text-console async shape rather than `RuntimeSink`.