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Nanaloveyuki
81 lines
2.1 KiB
Markdown
81 lines
2.1 KiB
Markdown
---
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name: field-equals
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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 field by exact key and value.
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key-word:
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- field
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- filter
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- predicate
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- public
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---
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## Field-equals
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Create a `RecordPredicate` that returns `true` when a record contains a field whose key and value both match exactly. Use it for stable attribute-based routing.
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### Interface
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```moonbit
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pub fn field_equals(key : String, value : String) -> RecordPredicate {}
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```
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#### input
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- `key : String` - Field key to inspect.
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- `value : String` - Exact value expected for that key.
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#### output
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- `RecordPredicate` - Predicate that matches records containing a field with the expected key and value.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- Matching requires both `field.key == key` and `field.value == value`.
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- The predicate returns `true` on the first matching field.
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- This helper is useful for routing records by environment, tenant, operation name, or fixed tags.
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- It is stricter than `has_field(...)` because presence alone is not enough.
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### How to Use
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Here are some specific examples provided.
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#### When Keep One Tenant Stream
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When log routing should isolate one tenant:
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```moonbit
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let logger = Logger::new(console_sink())
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.with_filter(field_equals("tenant", "acme"))
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```
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In this example, records for other tenants are excluded.
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#### When Combine With Exact Target Matching
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When field and target must both match:
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```moonbit
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let predicate = all_of([
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target_is("billing"),
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field_equals("region", "cn"),
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])
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```
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In this example, only billing records tagged for the `cn` region remain.
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### Error Case
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e.g.:
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- If `key` or `value` is empty, matching still uses exact equality and may produce no results unless records contain the same empty string.
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- If a record contains the key with multiple values, any one exact match is enough for the predicate to return `true`.
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### Notes
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1. Prefer exact field matching over message substring matching for long-term routing rules.
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2. Keep field naming stable across producers if this predicate is reused in shared configs.
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