# Segmenter observability

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## Purpose

The Segmenter formatter exports two OpenTelemetry instruments and a
small in-memory state ring so spec-conformance tests can assert exact
attribute payloads without standing up a full SDK exporter. The
instrument names match the canonical Wave-3 formatter contract —
every sibling formatter emits the same two metric names so dashboards
and alerts compose cleanly across the family.

## Features

- Histogram `i18n.format.segmenter.duration_ms` (unit `ms`) — per-segment wall-clock time
- Counter `i18n.format.cache_hit{formatter="segmenter"}` — increments on cache reuse only
- In-memory state ring (`getSegmenterMetricsState()`) for spec-conformance
- `resetSegmenterMetricsState()` test-only helper

## Usage: Basic Example

```ts
import { metrics } from "@opentelemetry/api";
import { MeterProvider } from "@opentelemetry/sdk-metrics";

const provider = new MeterProvider({ readers: [/* reader */] });
metrics.setGlobalMeterProvider(provider);

import { segmentText } from "@suaveplan/i18n-formatter-segmenter";
segmentText("hello", "en-GB", { granularity: "word" });
// emits via your reader
```

## Usage: Advanced Example

```ts
import { describe, expect, it } from "@suaveplan/testing/runner";
import {
    clearSegmenterCache,
    segmentText,
} from "@suaveplan/i18n-formatter-segmenter";
import {
    getSegmenterMetricsState,
    resetSegmenterMetricsState,
} from "@suaveplan/i18n-formatter-segmenter";

it("emits histogram + cache-hit counter", () => {
    clearSegmenterCache();
    resetSegmenterMetricsState();
    segmentText("hello", "en-GB", { granularity: "word" });
    segmentText("hello", "en-GB", { granularity: "word" });
    const state = getSegmenterMetricsState();
    expect(state.durationSamples).toHaveLength(2);
    expect(state.cacheHitSamples).toHaveLength(1);
    expect(state.durationSamples[0]?.attributes).toMatchObject({
        locale: "en-GB",
        granularity: "word",
    });
});
```

## API Reference

### `recordSegmenterFormat(locale, granularity, durationMs): void`

Records the duration histogram sample. Tags: `locale`, `granularity`.
Called internally by `segmentText` / `formatSegments`.

### `recordSegmenterCacheHit(): void`

Increments the cache-hit counter with the constant tag
`{ formatter: "segmenter" }`.

### `getSegmenterMetricsState(): SegmenterMetricsState`

Returns the in-memory ring of `durationSamples` and
`cacheHitSamples`. Used by spec-conformance tests.

### `resetSegmenterMetricsState(): void`

Clears both ring buffers. Test-only helper.

## Implementation Notes

- The OTel meter is named `suaveplan.i18n.formatter.segmenter`. Every
  formatter in the family has its own meter so OTel resource
  attributes can be tied per-package if a consumer chooses.
- `granularity` defaults to `"grapheme"` in the histogram tag when
  the caller omits it; `locale` is always required so it's never
  defaulted.
- Sample dashboard query (PromQL/Grafana over OTLP):
  `histogram_quantile(0.99, sum(rate(i18n_format_segmenter_duration_ms_bucket[5m])) by (le, locale, granularity))`
- The in-memory ring is intentionally unbounded — tests are short
  and call `resetSegmenterMetricsState()` between cases. A
  long-running test loop should call reset to avoid memory growth.