BitLogger/docs/api/logger-with-patch.md

name, group, category, update-time, description, key-word

name	group	category	update-time	description	key-word
logger-with-patch	api	logging	20260512	Wrap a logger with record transformation logic before the record reaches the sink.	logger patch transform public

Logger-with-patch

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.

Interface

pub fn[S] Logger::with_patch(self : Logger[S], patch : RecordPatch) -> Logger[PatchSink[S]] {}

input

• self : Logger[S] - Base logger to wrap.
• patch : RecordPatch - Record-to-record transformer applied before writing.

output

• Logger[PatchSink[S]] - A new logger that rewrites emitted records.

Explanation

Detailed rules explaining key parameters and behaviors

• Patches can rewrite target, message, and fields because they receive the full Record.
• This API does not change logging level gating; level checks still happen at the logger level.
• compose_patches(...) can be used to build ordered patch pipelines.
• This is the correct place for redaction, enrichment, and message prefixing.

How to Use

Here are some specific examples provided.

When Need Redaction Before Output

When sensitive data must be rewritten before reaching any sink:

let logger = Logger::new(console_sink(), target="auth")
  .with_patch(redact_fields(["token", "password"]))

In this example, matching field values are replaced before the sink sees them.

And caller code does not need custom redaction logic per log call.

When Need Combined Enrichment And Message Rewrite

When you want several transformations in a stable order:

let logger = Logger::new(console_sink(), target="svc")
  .with_patch(compose_patches([
    prefix_message("[safe] "),
    append_fields([field("service", "svc")]),
  ]))

In this example, patch order is explicit and deterministic.

Error Case

e.g.:

• If a patch returns the original record unchanged, the wrapper behaves like a pass-through transformer.

• If multiple patches rewrite the same field or property, the later patch wins according to composition order.

Notes

Notes are here.

1. Use patches for transformation, not filtering decisions.

2. Prefer helper patches such as prefix_message(...), append_fields(...), and redact_fields(...) for common cases.

3. Patch composition order matters.

4. Combine with with_filter(...) when both selection and rewriting are required.