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Nanaloveyuki
80 lines
2.0 KiB
Markdown
80 lines
2.0 KiB
Markdown
---
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name: target-is
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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 a single exact target.
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key-word:
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- target
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- filter
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- predicate
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- public
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---
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## Target-is
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Create a `RecordPredicate` that returns `true` only when a record target exactly matches the expected string. Use it when one specific logical source should pass through a filter.
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### Interface
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```moonbit
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pub fn target_is(target : String) -> RecordPredicate {}
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```
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#### input
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- `target : String` - Exact target value expected from the record.
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#### output
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- `RecordPredicate` - Predicate that matches records whose `rec.target` equals `target`.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- Matching uses direct string equality.
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- This helper does not perform prefix or fuzzy matching.
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- It is useful for isolating one subsystem when targets are intentionally stable.
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- Combine it with level predicates when a single target needs stricter routing rules.
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### How to Use
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Here are some specific examples provided.
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#### When Keep One Exact Target
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When only one service target should pass:
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```moonbit
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let logger = Logger::new(console_sink())
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.with_filter(target_is("gateway.http"))
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```
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In this example, records from sibling targets such as `gateway.ws` are excluded.
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#### When Route A Single Target To A Sink
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When a split path needs an exact match:
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```moonbit
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let predicate = all_of([
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target_is("audit"),
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level_at_least(Level::Info),
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])
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```
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In this example, the filter remains predictable because it does not rely on naming prefixes.
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### Error Case
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e.g.:
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- If `target` is empty, only records whose target is also an empty string will match.
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- If record targets are generated dynamically, exact matching may be too strict and lead to no matches.
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### Notes
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1. Prefer `target_has_prefix(...)` when targets are hierarchical and you want subtree matching.
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2. Exact target filters are easiest to maintain when target naming is stable across the project.
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