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Nanaloveyuki
80 lines
2.1 KiB
Markdown
80 lines
2.1 KiB
Markdown
---
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name: message-contains
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group: api
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category: filtering
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update-time: 20260512
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description: Create a reusable record predicate that matches records by message fragment.
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key-word:
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- message
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- filter
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- predicate
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- public
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---
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## Message-contains
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Create a `RecordPredicate` that returns `true` when a log message contains the given fragment. This helper is mainly useful for diagnostics, temporary routing, and focused debugging.
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### Interface
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```moonbit
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pub fn message_contains(fragment : String) -> RecordPredicate {}
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```
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#### input
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- `fragment : String` - Substring expected to appear in `rec.message`.
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#### output
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- `RecordPredicate` - Predicate that matches records whose message contains the fragment.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- Matching uses `String::contains(...)` on the full message text.
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- The predicate is case-sensitive unless the message was normalized earlier.
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- This helper is convenient for ad hoc triage when targets or fields are not enough.
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- It should usually complement more stable filters rather than replace them in long-term configs.
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### How to Use
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Here are some specific examples provided.
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#### When Watch A Temporary Error Pattern
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When a debugging session focuses on one phrase:
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```moonbit
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let logger = Logger::new(console_sink())
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.with_filter(message_contains("retry failed"))
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```
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In this example, only messages carrying that fragment remain visible.
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#### When Combine With Target Filtering
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When the same message should be isolated inside one subsystem:
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```moonbit
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let predicate = all_of([
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target_has_prefix("worker.sync"),
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message_contains("timeout"),
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])
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```
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In this example, unrelated timeout messages from other targets are ignored.
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### Error Case
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e.g.:
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- If `fragment` is empty, the predicate effectively matches every message because every string contains an empty substring.
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- If messages are localized or reformatted frequently, fragment-based matching can become brittle.
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### Notes
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1. Prefer field- or target-based filtering for stable production rules.
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2. Message substring filters are most valuable during debugging and migration periods.
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