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Nanaloveyuki
84 lines
3.1 KiB
Markdown
84 lines
3.1 KiB
Markdown
---
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name: async-logger-dropped-count
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group: api
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category: async
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update-time: 20260512
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description: Read the number of records dropped by the async logger because of overflow or queue clearing.
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key-word:
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- async
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- logger
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- queue
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- public
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---
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## Async-logger-dropped-count
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Read the cumulative number of records dropped by an async logger. This metric is useful when overflow policy or forced shutdown behavior may discard queued data.
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### Interface
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```moonbit
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pub fn[S] AsyncLogger::dropped_count(self : AsyncLogger[S]) -> Int {}
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```
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#### input
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- `self : AsyncLogger[S]` - Async logger whose dropped-record counter should be inspected.
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#### output
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- `Int` - Number of records dropped so far.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- The counter increases when overflow policy discards records.
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- The counter can also increase when `close(clear=true)` or `shutdown(clear=true)` abandons queued records.
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- It can also increase when shutdown fallback logic converts remaining pending records into dropped records during a clear-close path.
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- After a worker failure short-circuits `wait_idle()`, that shutdown-fallback increase is runtime-dependent in the current implementation: native-worker shutdown can convert leftover pending records into additional dropped records, while compatibility shutdown can leave that closed-queue remainder visible in `pending_count()` instead.
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- That also means `dropped_count()` can still stay unchanged even after `is_closed=true` and retained failure state on compatibility-style shutdown paths, because closure there does not necessarily convert the leftover failed backlog into dropped records.
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- Later log attempts against an already closed queue do not add to this counter by themselves.
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- This is a cumulative counter for the lifetime of the logger value.
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- Use this helper when you need a focused loss metric rather than a full `state()` snapshot.
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### How to Use
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Here are some specific examples provided.
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#### When Need To Detect Loss Under Pressure
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When queue overflow may discard records:
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```moonbit
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if logger.dropped_count() > 0 {
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println("async log loss detected")
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}
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```
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In this example, the code checks whether the async logger has already discarded records.
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#### When Validate Shutdown Behavior In Tests
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When a test intentionally clears the queue:
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```moonbit
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logger.close(clear=true)
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ignore(logger.dropped_count())
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```
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In this example, the drop counter helps confirm that abandoned backlog was tracked.
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### Error Case
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e.g.:
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- If no records have been dropped, the method simply returns `0`.
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- If callers need to know why records were lost, they must interpret this metric together with overflow policy and shutdown behavior.
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- If `wait_idle()` returned early because the worker failed, `dropped_count()` may still stay unchanged on compatibility shutdown paths even though one leftover closed-queue item remains visible in `pending_count()`.
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### Notes
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1. This helper reports record loss only; it does not explain the full logger state.
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2. Pair it with `pending_count()` or `state()` when investigating backlog pressure.
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