pipeline: inputs: systemd: document FLUENT_BIT_PARSER per-unit parser

[nourdouf]

Summary

Documents the new in_systemd capability to select a parser at runtime, per journal entry, via the FLUENT_BIT_PARSER field. This can be advertised from a systemd unit with:

[Service]
LogExtraFields=FLUENT_BIT_PARSER=logfmt

When present, the plugin applies the named parser to the entry's MESSAGE field and emits structured fields instead of the raw message. The doc covers behavior (metadata field stripped, MESSAGE-only parsing, graceful fallback when the parser is missing or parsing fails, per-entry resolution) and includes a logfmt example.

Related

Documentation for code PR fluent/fluent-bit#11353 (adds FLUENT_BIT_PARSER support to the systemd input).

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Fluent Bit is licensed under Apache 2.0.

Summary by CodeRabbit

• Documentation
  • Added guidance for selecting parsers per Systemd journal entry at runtime.
  • Documented configuration examples for YAML and classic conf formats.
  • Clarified parser behavior, including fallback handling, metadata processing, and per-entry resolution.
  • Explained how parser selection can be configured through the FLUENT_BIT_PARSER journal field and advertised from a unit file.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between 0c302db and 8194feb.

📒 Files selected for processing (1)

• pipeline/inputs/systemd.md

🚧 Files skipped from review as they are similar to previous changes (1)

• pipeline/inputs/systemd.md

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📝 Walkthrough

Walkthrough

The Systemd input documentation describes per-entry parser selection through the FLUENT_BIT_PARSER journal field. It covers metadata handling, fallback behavior, per-entry resolution, and YAML and conf configuration examples.

Changes

Systemd parser documentation

Layer / File(s)	Summary
Document per-unit parser selection pipeline/inputs/systemd.md	Documents FLUENT_BIT_PARSER, LogExtraFields, parser behavior, fallback handling, and configuration examples.

Estimated code review effort: 1 (Trivial) | ~3 minutes

Merge Risk: ⚪ Minimal · up to 8194f

This change documents per-entry systemd parser selection and fallback behavior. No current merge-readiness risk is identified.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly identifies the Systemd input documentation change for the FLUENT_BIT_PARSER per-unit parser capability. It is concise and directly related to the main change.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.)

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[eschabell]

@nourdouf fixing a few vale issue, then will review.

[eschabell]

@nourdouf ready to go, just waiting on code PR to merge.