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Nanaloveyuki
107 lines
2.8 KiB
Markdown
107 lines
2.8 KiB
Markdown
---
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name: logger-with-patch
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group: api
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category: logging
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update-time: 20260512
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description: Wrap a logger with record transformation logic before the record reaches the sink.
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key-word:
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- logger
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- patch
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- transform
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- public
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---
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## Logger-with-patch
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Attach a `RecordPatch` to a logger so each record is transformed before it is written to the sink. This API is the standard way to rewrite target, message, or fields without changing caller code.
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### Interface
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```moonbit
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pub fn[S] Logger::with_patch(self : Logger[S], patch : RecordPatch) -> Logger[PatchSink[S]] {}
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```
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#### input
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- `self : Logger[S]` - Base logger to wrap.
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- `patch : RecordPatch` - Record-to-record transformer applied before writing.
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#### output
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- `Logger[PatchSink[S]]` - A new logger that rewrites emitted records.
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---
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e.g.:
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```moonbit
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pub fn[S] Logger::with_patch(self : Logger[S], patch : RecordPatch) -> Logger[PatchSink[S]] {}
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```
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#### input
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- `self : Logger[S]` - Logger to transform.
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- `patch : RecordPatch` - Rewrite logic.
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#### output
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- `Logger[PatchSink[S]]` - Logger with transformation behavior.
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---
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- Patches can rewrite `target`, `message`, and `fields` because they receive the full `Record`.
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- This API does not change logging level gating; level checks still happen at the logger level.
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- `compose_patches(...)` can be used to build ordered patch pipelines.
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- This is the correct place for redaction, enrichment, and message prefixing.
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### How to Use
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Here are some specific examples provided.
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#### When Need Redaction Before Output
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When sensitive data must be rewritten before reaching any sink:
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```moonbit
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let logger = Logger::new(console_sink(), target="auth")
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.with_patch(redact_fields(["token", "password"]))
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```
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In this example, matching field values are replaced before the sink sees them.
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And caller code does not need custom redaction logic per log call.
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#### When Need Combined Enrichment And Message Rewrite
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When you want several transformations in a stable order:
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```moonbit
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let logger = Logger::new(console_sink(), target="svc")
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.with_patch(compose_patches([
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prefix_message("[safe] "),
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append_fields([field("service", "svc")]),
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]))
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```
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In this example, patch order is explicit and deterministic.
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### Error Case
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e.g.:
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- If a patch returns the original record unchanged, the wrapper behaves like a pass-through transformer.
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- If multiple patches rewrite the same field or property, the later patch wins according to composition order.
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### Notes
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Notes are here.
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1. Use patches for transformation, not filtering decisions.
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2. Prefer helper patches such as `prefix_message(...)`, `append_fields(...)`, and `redact_fields(...)` for common cases.
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3. Patch composition order matters.
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4. Combine with `with_filter(...)` when both selection and rewriting are required.
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