name: Observability description: This skill should be used when the user asks about "Effect logging", "Effect.log", "Effect metrics", "Effect tracing", "spans", "telemetry", "Metric.counter", "Metric.gauge", "Metric.histogram", "OpenTelemetry", "structured logging", "log levels", "Effect.logDebug", "Effect.logInfo", "Effect.logWarning", "Effect.logError", or needs to understand how Effect handles logging, metrics, and distributed tracing. version: 1.0.0

Observability in Effect

Overview

Effect provides built-in observability:

• Logging - Structured, leveled logging
• Metrics - Counters, gauges, histograms
• Tracing - Distributed tracing with spans

All three integrate seamlessly with Effect's execution model.

Logging

Basic Logging

import { Effect } from "effect";

const program = Effect.gen(function* () {
  yield* Effect.log("Starting process");
  yield* Effect.logDebug("Debug information");
  yield* Effect.logInfo("Processing item");
  yield* Effect.logWarning("Resource running low");
  yield* Effect.logError("Failed to connect");
  yield* Effect.logFatal("Critical system failure");
});

Log Levels

import { LogLevel, Logger } from "effect";

// Set minimum log level
const filtered = program.pipe(Logger.withMinimumLogLevel(LogLevel.Info));

// Available levels (lowest to highest):
// Trace, Debug, Info, Warning, Error, Fatal, None

Structured Logging

// Log with structured data
yield *
  Effect.log("User action").pipe(
    Effect.annotateLogs({
      userId: "123",
      action: "login",
      ip: "192.168.1.1",
    }),
  );

// Annotations apply to all logs in scope
const program = Effect.gen(function* () {
  yield* Effect.log("First log"); // Has userId annotation
  yield* Effect.log("Second log"); // Has userId annotation
}).pipe(Effect.annotateLogs({ userId: "123" }));

Log Spans

// Add timing/context spans
const program = Effect.gen(function* () {
  yield* Effect.log("Processing");
  yield* processItems();
  yield* Effect.log("Complete");
}).pipe(Effect.withLogSpan("request-handler"));
// Logs include: [request-handler 45ms] Processing

Custom Logger

import { Logger } from "effect";

const JsonLogger = Logger.make(({ logLevel, message, annotations, date }) => {
  console.log(
    JSON.stringify({
      level: logLevel.label,
      message: String(message),
      timestamp: date.toISOString(),
      ...annotations,
    }),
  );
});

const program = Effect.gen(function* () {
  yield* Effect.log("Hello");
}).pipe(Effect.provide(Logger.replace(Logger.defaultLogger, JsonLogger)));

Metrics

Counter - Track Occurrences

import { Metric } from "effect";

const requestCount = Metric.counter("http_requests_total", {
  description: "Total HTTP requests",
});

const program = Effect.gen(function* () {
  yield* Metric.increment(requestCount);
  yield* Metric.incrementBy(requestCount, 5);
});

const tracked = handleRequest.pipe(Metric.trackAll(requestCount));

Gauge - Track Current Value

const activeConnections = Metric.gauge("active_connections", {
  description: "Current active connections",
});

const program = Effect.gen(function* () {
  yield* Metric.set(activeConnections, 10);
  yield* Metric.incrementBy(activeConnections, 1);
  yield* Metric.decrementBy(activeConnections, 1);
});

Histogram - Track Distributions

const requestDuration = Metric.histogram("http_request_duration_ms", {
  description: "Request duration in milliseconds",
  boundaries: [10, 50, 100, 250, 500, 1000],
});

yield * Metric.observe(requestDuration, 125);

const tracked = handleRequest.pipe(Metric.trackDuration(requestDuration));

Summary - Statistical Summary

const responseSizes = Metric.summary("response_size_bytes", {
  description: "Response payload sizes",
  maxAge: "1 minute",
  maxSize: 100,
  quantiles: [0.5, 0.9, 0.99],
});

yield * Metric.observe(responseSizes, 1024);

Frequency - Count by Tag

const statusCodes = Metric.frequency("http_status_codes");

yield * Metric.observe(statusCodes, "200");
yield * Metric.observe(statusCodes, "404");
yield * Metric.observe(statusCodes, "500");

Tagged Metrics

const requestCount = Metric.counter("requests").pipe(Metric.tagged("service", "api"), Metric.tagged("version", "v1"));

// Dynamic tags
const taggedCount = Metric.counter("requests").pipe(Metric.taggedWithLabels(["method", "endpoint"]));

yield * Metric.increment(taggedCount).pipe(Metric.taggedWithLabels(["GET", "/users"]));

Reading Metrics

const program = Effect.gen(function* () {
  yield* Metric.increment(requestCount);
  yield* Metric.increment(requestCount);

  const snapshot = yield* Metric.value(requestCount);
  // snapshot.count === 2
});

Tracing

Creating Spans

import { Effect } from "effect";

const traced = handleRequest.pipe(Effect.withSpan("handle-request"));

const traced = handleRequest.pipe(
  Effect.withSpan("handle-request", {
    attributes: {
      "http.method": "GET",
      "http.url": "/api/users",
    },
  }),
);

Nested Spans

const program = Effect.gen(function* () {
  yield* fetchUser(id).pipe(Effect.withSpan("fetch-user"));
  yield* processData(data).pipe(Effect.withSpan("process-data"));
  yield* saveResult(result).pipe(Effect.withSpan("save-result"));
}).pipe(Effect.withSpan("main-operation"));
// Creates: main-operation
//            ├── fetch-user
//            ├── process-data
//            └── save-result

Adding Span Attributes

import { Tracer } from "effect";

const program = Effect.gen(function* () {
  yield* Effect.annotateCurrentSpan("user.id", userId);

  yield* Effect.annotateCurrentSpan("event", "user_validated");

  const result = yield* processUser(userId);

  yield* Effect.annotateCurrentSpan("result.status", result.status);
});

Span Status

const program = Effect.gen(function* () {
  try {
    return yield* riskyOperation;
  } catch (error) {
    yield* Effect.setSpanStatus({
      code: "error",
      message: error.message,
    });
    return yield* Effect.fail(error);
  }
}).pipe(Effect.withSpan("risky-operation"));

Custom Tracer

import { Tracer } from "effect";

const ConsoleTracer = Tracer.make({
  span: (name, parent, context, links, startTime) => ({
    attribute: (key, value) => console.log(`[${name}] ${key}=${value}`),
    end: (endTime, exit) => console.log(`[${name}] ended`),
    event: (name, startTime, attributes) => console.log(`[${name}] event: ${name}`),
    status: (status) => console.log(`[${name}] status: ${status.code}`),
  }),
});

const program = myEffect.pipe(Effect.provide(Tracer.layer(ConsoleTracer)));

OpenTelemetry Integration

import { NodeSdk } from "@effect/opentelemetry";
import { OTLPTraceExporter } from "@opentelemetry/exporter-trace-otlp-http";

const TracingLive = NodeSdk.layer(() => ({
  resource: { serviceName: "my-service" },
  spanProcessor: new BatchSpanProcessor(new OTLPTraceExporter()),
}));

const program = myEffect.pipe(Effect.provide(TracingLive));

Combining Observability

const handleRequest = (req: Request) =>
  Effect.gen(function* () {
    yield* Effect.log("Request received").pipe(Effect.annotateLogs({ path: req.path, method: req.method }));

    yield* Metric.increment(requestCount);

    const result = yield* processRequest(req);

    yield* Effect.annotateCurrentSpan("response.status", result.status);
    yield* Metric.observe(requestDuration, result.duration);

    return result;
  }).pipe(
    Effect.withSpan("handle-request", {
      attributes: {
        "http.method": req.method,
        "http.url": req.path,
      },
    }),
  );

Best Practices

1. Use structured logging - Add context via annotations
2. Name spans descriptively - Use verb-noun format
3. Add meaningful attributes - Enable debugging/analysis
4. Track key metrics - Request count, latency, errors
5. Use appropriate log levels - Debug in dev, Info in prod

Additional Resources

For comprehensive observability documentation, consult ${CLAUDE_PLUGIN_ROOT}/references/llms-full.txt.

Search for these sections:

• "Built-in Logging" for logging APIs
• "Metrics" for metric types
• "Tracing" for distributed tracing