@kestack/effect-tracer

Effect tracing tools for in-process recording and OTLP export

Big picture

Effect programs already emit spans and logs through Effect.withSpan and Effect.log. This package gives that output two destinations. The recorder keeps spans and logs in memory inside the process, in emission order, so a test, a Story, or an in-app panel can read them back as plain data. The telemetry layers export them over OTLP/HTTP to a collector such as the @kestack/devtools DevTools Server, which stores them with its Lotel Tool.

The full telemetry layer uses the OpenTelemetry SDK and also exports metrics. The dev-telemetry layer does the same job for local development with effect/observability and fetch, batching every 100 ms and exporting provisional spans while they run.

laymos Stories use the recorder to attach traces to reports, and apps/docs uses it to drive a live trace panel.

Install

pnpm add @kestack/effect-tracer

Peer dependencies:

• effect (^4.0.0): the recorder installs an Effect Tracer and Logger, and the layers are Effect Layers.

Exports

@kestack/effect-tracer/recorder

Runs anywhere Effect runs, including the browser.

| Export | What it does | | ----------------------- | --------------------------------------------------------------------------------------------------- | | makeTraceRecorder | Creates a recorder with instrument, layer, and snapshot; options cap spans and stream events. | | sequenceAttribute | The span attribute name (tracer.sequence) that carries emission order. | | tracerAttributePrefix | The prefix (tracer.) of attributes this package adds to spans. | | sequenceOrder | Pads a sequence number so string sorting matches numeric order. | | readSequence | Parses a sequence attribute value back to a number, or null. |

@kestack/effect-tracer/telemetry

Node. Uses the OpenTelemetry SDK.

| Export | What it does | | -------------------- | ------------------------------------------------------------------------------------------------ | | makeTelemetryLayer | Builds one Layer exporting traces, logs, and metrics over OTLP/HTTP; each signal can be toggled. |

@kestack/effect-tracer/telemetry/dev-telemetry

Runs anywhere fetch exists.

| Export | What it does | | ----------------------- | ------------------------------------------------------------------------------------------------ | | makeDevTelemetryLayer | Builds a batching OTLP/HTTP Layer for local development; stamps deployment.environment: local. |

Usage

Record a program and read its spans back

Wrap an Effect with instrument, run it, then read the snapshot. Spans and logs come back in the order they were emitted, even inside one millisecond.

import { Effect } from 'effect';
import { makeTraceRecorder } from '@kestack/effect-tracer/recorder';

const recorder = makeTraceRecorder();

await Effect.runPromise(
  recorder.instrument(
    Effect.gen(function* () {
      yield* Effect.log('First');
      yield* Effect.void.pipe(Effect.withSpan('child-a'));
      yield* Effect.void.pipe(Effect.withSpan('child-b'));
    }).pipe(Effect.withSpan('parent')),
  ),
);

const { spans, logs, truncated } = recorder.snapshot();
spans.map(({ name }) => name); // ['parent', 'child-a', 'child-b']
spans[1]?.parentSpanId === spans[0]?.spanId; // true
logs[0]?.message; // 'First'

How it works:

• instrument installs the recorder's Tracer and Logger for that Effect only; nothing outside it is recorded.
• Every span and log gets a sequence so ties on the clock keep order.
• maxSpans (default 2000) stops recording and sets truncated; use onSpanEnd and onLog to stream instead of snapshotting.

Record every Effect a runtime runs

For a long-lived runtime such as a browser demo, provide recorder.layer once instead of wrapping each Effect.

import { ManagedRuntime } from 'effect';
import { makeTraceRecorder } from '@kestack/effect-tracer/recorder';

const recorder = makeTraceRecorder();

const runtime = ManagedRuntime.make(recorder.layer);

// Later, from a panel:
recorder.snapshot().spans;

How it works:

• recorder.layer is Layer<never>; it sets the Tracer and Logger references in the runtime.
• snapshot() is safe mid-run; running spans report status: 'running'.

Export to a local DevTools Server

Point the dev layer at a running devtools and every span and log in the runtime shows up in the Lotel Tool.

import { Effect, Layer, ManagedRuntime } from 'effect';
import { makeDevTelemetryLayer } from '@kestack/effect-tracer/telemetry/dev-telemetry';

const runtime = ManagedRuntime.make(
  makeDevTelemetryLayer({
    endpoint: 'http://127.0.0.1:14400',
    serviceName: 'server:api-1',
  }),
);

await runtime.runPromise(
  Effect.log('order accepted').pipe(Effect.withSpan('handle-order')),
);

// Disposing drains pending batches within shutdownTimeout.
await runtime.dispose();

How it works:

• Traces post to /v1/traces and logs to /v1/logs as OTLP/HTTP JSON.
• Batches flush every batchInterval (100 ms) or at maxBatchSize (100).
• A span that ends within a batch window replaces its provisional record; longer spans export both running and completed states.
• makeTelemetryLayer is the production counterpart with the OpenTelemetry SDK and metrics.