Cloudflare Workers Observability

Production-grade observability for Cloudflare Workers: logging, metrics, tracing, and alerting.

Quick Start

// Structured logging with context
export default {
  async fetch(request: Request, env: Env, ctx: ExecutionContext): Promise<Response> {
    const requestId = crypto.randomUUID();
    const logger = createLogger(requestId, env);

    try {
      logger.info('Request received', { method: request.method, url: request.url });

      const result = await handleRequest(request, env);

      logger.info('Request completed', { status: result.status });
      return result;
    } catch (error) {
      logger.error('Request failed', { error: error.message, stack: error.stack });
      throw error;
    }
  }
};

// Simple logger factory
function createLogger(requestId: string, env: Env) {
  return {
    info: (msg: string, data?: object) => console.log(JSON.stringify({ level: 'info', requestId, msg, ...data, timestamp: Date.now() })),
    error: (msg: string, data?: object) => console.error(JSON.stringify({ level: 'error', requestId, msg, ...data, timestamp: Date.now() })),
    warn: (msg: string, data?: object) => console.warn(JSON.stringify({ level: 'warn', requestId, msg, ...data, timestamp: Date.now() })),
  };
}

Critical Rules

1. Always use structured JSON logging - Plain text logs are hard to parse and aggregate
2. Include request context - Request ID, method, path in every log entry
3. Never log sensitive data - Redact tokens, passwords, PII from logs
4. Use appropriate log levels - ERROR for failures, WARN for recoverable issues, INFO for operations
5. Sample high-volume logs - Use 1-10% sampling for request logs in production

Observability Components

Component	Purpose	When to Use
console.log	Basic logging	Development, debugging
Tail Workers	Real-time log streaming	Production log aggregation
Analytics Engine	Custom metrics/analytics	Business metrics, performance tracking
Logpush	Log export to external services	Long-term storage, compliance
Workers Trace Events	Distributed tracing	Request flow debugging

Top 8 Errors Prevented

Error	Symptom	Prevention
Logs not appearing	No output in dashboard	Enable "Standard" logging in wrangler.jsonc
Log truncation	Messages cut off at 128KB	Chunk large payloads, use sampling
Tail Worker not receiving	No events processed	Check binding name matches wrangler.jsonc
Analytics Engine write fails	Data not recorded	Verify AE binding, check blobs format
PII in logs	Security/compliance violation	Implement redaction middleware
Missing request context	Can't correlate logs	Add requestId to all log entries
Log volume explosion	High costs, noise	Implement sampling for high-frequency events
Alerting gaps	Incidents not detected	Configure monitors for error rate thresholds

Logging Configuration

wrangler.jsonc:

{
  "name": "my-worker",
  "observability": {
    "enabled": true,
    "head_sampling_rate": 1 // 0-1, 1 = 100% of requests
  },
  "tail_consumers": [
    {
      "service": "log-aggregator", // Tail Worker name
      "environment": "production"
    }
  ],
  "analytics_engine_datasets": [
    {
      "binding": "ANALYTICS",
      "dataset": "my_worker_metrics"
    }
  ]
}

Structured Logging Pattern

interface LogEntry {
  level: 'debug' | 'info' | 'warn' | 'error';
  message: string;
  requestId: string;
  timestamp: number;
  // Contextual data
  method?: string;
  path?: string;
  status?: number;
  duration?: number;
  // Error details
  error?: {
    name: string;
    message: string;
    stack?: string;
  };
  // Custom fields
  [key: string]: unknown;
}

class Logger {
  constructor(private requestId: string, private baseContext: object = {}) {}

  private log(level: LogEntry['level'], message: string, data?: object) {
    const entry: LogEntry = {
      level,
      message,
      requestId: this.requestId,
      timestamp: Date.now(),
      ...this.baseContext,
      ...data,
    };

    // Redact sensitive fields
    const sanitized = this.redact(entry);

    const output = JSON.stringify(sanitized);
    level === 'error' ? console.error(output) : console.log(output);
  }

  private redact(entry: LogEntry): LogEntry {
    const sensitiveKeys = ['password', 'token', 'secret', 'authorization', 'cookie'];
    const redacted = { ...entry };

    for (const key of Object.keys(redacted)) {
      if (sensitiveKeys.some(s => key.toLowerCase().includes(s))) {
        redacted[key] = '[REDACTED]';
      }
    }
    return redacted;
  }

  info(message: string, data?: object) { this.log('info', message, data); }
  warn(message: string, data?: object) { this.log('warn', message, data); }
  error(message: string, data?: object) { this.log('error', message, data); }
  debug(message: string, data?: object) { this.log('debug', message, data); }
}

Analytics Engine Usage

interface Env {
  ANALYTICS: AnalyticsEngineDataset;
}

export default {
  async fetch(request: Request, env: Env, ctx: ExecutionContext): Promise<Response> {
    const start = Date.now();
    const url = new URL(request.url);

    try {
      const response = await handleRequest(request, env);

      // Write success metric
      env.ANALYTICS.writeDataPoint({
        blobs: [request.method, url.pathname, String(response.status)],
        doubles: [Date.now() - start], // Response time in ms
        indexes: [url.pathname.split('/')[1] || 'root'], // Index for fast queries
      });

      return response;
    } catch (error) {
      // Write error metric
      env.ANALYTICS.writeDataPoint({
        blobs: [request.method, url.pathname, 'error', error.message],
        doubles: [Date.now() - start],
        indexes: ['error'],
      });
      throw error;
    }
  }
};

Tail Worker Pattern

// tail-worker.ts - Receives logs from other workers
interface TailEvent {
  scriptName: string;
  event: {
    request?: { method: string; url: string };
    response?: { status: number };
  };
  logs: Array<{
    level: string;
    message: unknown[];
    timestamp: number;
  }>;
  exceptions: Array<{
    name: string;
    message: string;
    timestamp: number;
  }>;
  outcome: 'ok' | 'exception' | 'exceededCpu' | 'exceededMemory' | 'canceled';
  eventTimestamp: number;
}

export default {
  async tail(events: TailEvent[], env: Env): Promise<void> {
    for (const event of events) {
      // Filter and forward logs
      const errorLogs = event.logs.filter(l => l.level === 'error');
      const exceptions = event.exceptions;

      if (errorLogs.length > 0 || exceptions.length > 0) {
        // Send to external logging service
        await fetch(env.LOGGING_ENDPOINT, {
          method: 'POST',
          headers: { 'Content-Type': 'application/json' },
          body: JSON.stringify({
            scriptName: event.scriptName,
            timestamp: event.eventTimestamp,
            errors: errorLogs,
            exceptions,
            outcome: event.outcome,
          }),
        });
      }
    }
  }
};

When to Load References

Load specific references based on the task:

• Setting up logging? → Load references/logging.md for structured logging patterns, log levels, redaction
• Building custom metrics? → Load references/analytics-engine.md for Analytics Engine SQL queries, data modeling
• Implementing log aggregation? → Load references/tail-workers.md for Tail Worker patterns, external service integration
• Creating dashboards/tracking? → Load references/custom-metrics.md for business metrics, performance tracking
• Setting up alerts? → Load references/alerting.md for error rate monitoring, PagerDuty/Slack integration

Templates

Template	Purpose	Use When
templates/logging-setup.ts	Production logging class	Setting up new worker with logging
templates/analytics-worker.ts	Analytics Engine integration	Adding custom metrics
templates/tail-worker.ts	Complete Tail Worker	Building log aggregation pipeline

Scripts

Script	Purpose	Command
scripts/setup-logging.sh	Configure logging settings	./setup-logging.sh
scripts/analyze-logs.sh	Query and analyze logs	./analyze-logs.sh --errors --last 1h

Resources

• Workers Observability: https://developers.cloudflare.com/workers/observability/
• Analytics Engine: https://developers.cloudflare.com/analytics/analytics-engine/
• Tail Workers: https://developers.cloudflare.com/workers/observability/tail-workers/
• Logpush: https://developers.cloudflare.com/logs/get-started/