# OpenSpec Change: enterprise-events-otel

- **Change ID**: enterprise-events-otel
- **Package**: `@suaveplan/events-otel`
- **Category**: universal
- **Target path**: `universal/events/events-otel`
- **Status**: PROPOSED
- **Date**: 2026-04-24
- **Reference spec**: `/mnt/s/web/suaveplan/enterprise/events-otel.md`

---

## Summary

Deliver `@suaveplan/events-otel` on the genesis branch from scratch at enterprise quality. The package is an opt-in OpenTelemetry tracing companion for `@suaveplan/events`: it wraps each dispatch in a span, propagates W3C Trace Context across synchronous and async subscriber invocations, records event-level attributes on each span, and correctly marks listener exceptions as span errors. Attaches via the public `@suaveplan/events` interceptor API. Sampling-aware: honours the parent sampling decision and supports rule-based head sampling.

---

## Current-state audit (genesis)

The package does not exist on genesis yet — fresh build. No prior source, no test results, no coverage data.

---

## Available workspace deps on genesis

| Package | Role |
|---|---|
| `@suaveplan/events` (`workspace:^`) | Event bus and interceptor API |
| `@suaveplan/error` (`workspace:^`) | Base class for `EventsOtelError`; `defineCodes` for `EVENTS_OTEL.*` codes |
| `@suaveplan/types` (`workspace:^`) | `LoggerLike`, `Clock` injection interfaces |
| `zod` (`catalog:forms`) | Zod schemas for all public option types |
| `@opentelemetry/api` (`catalog:observability`) | `Tracer`, `Span`, `SpanStatusCode`, W3C context propagation |
| `tslib` (`catalog:build`) | Runtime helpers |

Dev deps: `vitest`, `@vitest/coverage-istanbul`, `fast-check` (all `catalog:testing`).

---

## Public API surface

### Setup

```ts
// Attaches OTel tracing companion to an existing EventBus
function attachEventTracing(bus: EventBusLike, options?: EventTracingOptions): EventTracingHandle

interface EventTracingOptions {
  tracer?: Tracer                    // default: global tracer from @opentelemetry/api
  propagator?: TextMapPropagator    // default: W3CTraceContextPropagator
  eventFilter?: (name: string) => boolean
  samplingRule?: SamplingRule
  clock?: Clock
  logger?: LoggerLike
}

interface EventTracingHandle {
  detach(): void
}
```

### Span attribute convention

```ts
// All span attribute keys as stable constants
const SPAN_ATTRIBUTES = {
  EVENT_NAME:     "suaveplan.event.name",
  EVENT_ID:       "suaveplan.event.id",
  EVENT_CATEGORY: "suaveplan.event.category",
  LISTENER_ID:    "suaveplan.listener.id",
  BUS_ID:         "suaveplan.bus.id",
} as const

// Span names
const SPAN_NAMES = {
  DISPATCH: "events.dispatch",
  LISTENER: "events.listener",
} as const
```

### Sampling rule

```ts
type SamplingRule =
  | { type: "always" }
  | { type: "never" }
  | { type: "rate"; rate: number }               // 0.0–1.0
  | { type: "event-name"; names: string[] }      // sample only named events
```

### Trace context envelope (for transport interop)

```ts
// Serialize current trace context into a plain object for transport
function serializeTraceContext(): Record<string, string>

// Restore trace context from a serialized envelope
function deserializeTraceContext(headers: Record<string, string>): Con```text

### Error handling

```ts
class EventsOtelError extends SuaveplanError<EventsOtelErrorCode>

type EventsOtelErrorCode =
  | "EVENTS_OTEL_ATTACH_FAILED"
  | "EVENTS_OTEL_ALREADY_ATTACHED"
  | "EVENTS_OTEL_PROPAGATION_FAILED"

function registerEventsOtelErrorCodes(): void
```

### Zod schemas

```ts
const EventTracingOptionsSchema: ZodSchema
const SamplingRuleSchema: ZodSchema
```

---

## New additions for genesis

### 1. W3C Trace Context propagation across async subscribers

The interceptor captures the active `Context` at dispatch time. Each listener invocation runs with that context restored as the active context, so any nested OTel operations inside listeners are correctly parented to the dispatch span.

### 2. Listener exception → span error

If a listener throws, the dispatch span's status is set to `SpanStatusCode.ERROR` with the exception recorded via `span.recordException(err)`. The error is re-thrown after recording.

### 3. Sampling-aware dispatch

When `samplingRule` is `"rate"` or `"event-name"`, spans are only created for the matching subset. The companion checks the parent sampling decision first; if the parent is sampled, the rule is bypassed to maintain trace completeness.

### 4. `serializeTraceContext` / `deserializeTraceContext`

These helpers allow `@suaveplan/events-transport` to inject and extract trace context from transport envelopes, enabling cross-context trace propagation without a direct package dependency on OTel internals.

### 5. Property-based tests (fast-check)

- Parent-child relationship: every listener span is a child of its dispatch span
- Context propagation under reentrancy: nested dispatches inside listeners produce correct span trees
- Sampling rate accuracy: at sampling rate 0.5, sampled span count is within 5% of 50% over 10k dispatches

---

## File layout

```text
universal/events/events-otel/
  package.json
  tsconfig.json
  vitest.config.ts
  src/
    index.ts
    error-codes.ts
    constants/
      index.ts
    core/
      attach-event-tracing/
        attach-event-tracing.ts
        attach-event-tracing.md
        attach-event-tracing.types.ts
        attach-event-tracing.test.ts
        index.ts
    propagation/
      trace-context/
        trace-context.ts
        trace-context.md
        trace-context.test.ts
        index.ts
    sampling/
      sampling-rule/
        sampling-rule.ts
        sampling-rule.md
        sampling-rule.types.ts
        sampling-rule.test.ts
        index.ts
  tests/
    integration.test.ts
    property.test.ts
```

---

## Impact

**Downstream packages**: `@suaveplan/events-transport` uses `serializeTraceContext`/`deserializeTraceContext` for cross-context propagation. Any application using `@suaveplan/events` with production tracing can add this companion without code changes to publishers or subscribers.

**Workspace dep prerequisites**: `@suaveplan/events`, `@suaveplan/error`, `@suaveplan/types` must be published to Verdaccio.

**Breaking changes**: none (new package).