fastify-logging

Fastify Logging with Pino

You are configuring logging for a Fastify application. Fastify ships with Pino as its built-in logger. Follow these instructions precisely — they encode hard constraints and production-critical patterns.

Core Architecture

Pino is an asynchronous, zero-overhead, structured JSON logger. It maximizes throughput by offloading formatting and I/O to worker threads rather than performing them on the main event-loop thread. This is fundamental to how Fastify achieves its performance characteristics — never undermine it by adding synchronous formatting in-process.

Enable logging by passing logger: true in Fastify options for default behavior, or pass a custom Pino configuration object for fine-grained control. You can also pass a pre-configured Pino instance directly if you need to share the same logger across multiple parts of your application.

// Minimal — default Pino with info level
const fastify = require('fastify')({ logger: true })

// Custom configuration object
const fastify = require('fastify')({
  logger: {
    level: 'info',
    // additional Pino options here
  }
})

// Pre-configured Pino instance
const pino = require('pino')
const logger = pino({ level: 'debug' })
const fastify = require('fastify')({ logger })

Request Correlation

Fastify automatically generates and injects a reqId into every log message throughout the entire request lifecycle. This is a hard invariant — every log line produced within a request context carries this ID, enabling you to trace all activity for a single request across your log output.

Customize ID generation via the genReqId option when you need deterministic or externally-provided IDs (e.g., propagating a trace ID from an upstream load balancer):

const fastify = require('fastify')({
  logger: true,
  genReqId: (req) => req.headers['x-request-id'] || crypto.randomUUID()
})

Do not roll your own correlation mechanism. Use this built-in facility.

Child Loggers