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Nanaloveyuki
83 lines
2.1 KiB
Markdown
83 lines
2.1 KiB
Markdown
---
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name: append-fields
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group: api
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category: patching
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update-time: 20260512
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description: Create a reusable record patch that appends extra fields to the record.
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key-word:
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- patch
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- fields
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- transform
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- public
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---
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## Append-fields
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Create a `RecordPatch` that appends extra fields to `rec.fields`. Use it when records should be enriched with stable metadata before reaching sinks.
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### Interface
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```moonbit
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pub fn append_fields(extra_fields : Array[Field]) -> RecordPatch {}
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```
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#### input
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- `extra_fields : Array[Field]` - Fields appended after the record's existing field list.
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#### output
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- `RecordPatch` - Patch that returns a record with appended fields.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- If `extra_fields` is empty, the patch returns the original record unchanged.
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- If the original record has no fields, the appended fields become the new field list.
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- Otherwise, the original fields stay first and `extra_fields` are appended afterward.
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- This helper is useful for environment tags, service metadata, and bridge-layer context.
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### How to Use
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Here are some specific examples provided.
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#### When Add Service Metadata
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When every record should carry shared context:
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```moonbit
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let logger = Logger::new(console_sink())
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.with_patch(append_fields([
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field("service", "billing"),
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field("region", "cn"),
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]))
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```
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In this example, the extra fields are added to every emitted record.
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#### When Compose With Message Rewriting
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When both visible and structured context are needed:
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```moonbit
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let patch = compose_patches([
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prefix_message("[api] "),
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append_fields([field("component", "gateway")]),
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])
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```
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In this example, the record gains both textual and structured enrichment.
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### Error Case
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e.g.:
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- If `extra_fields` is empty, the patch behaves like a no-op.
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- If appended field keys duplicate existing keys, both copies remain in the field list.
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### Notes
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1. This helper appends fields; it does not deduplicate or overwrite existing entries.
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2. Field order can matter for downstream formatting or inspection, so keep appended context intentional.
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