Logrus — Structured Logging Library for Go

Introduction

Logrus is a structured logger for Go that produces both human-readable text and machine-parsable JSON output. It serves as a drop-in replacement for the standard log package while adding structured fields, log levels, and a hook system for routing entries to external services.

What Logrus Does

• Emits structured log entries with arbitrary key-value fields
• Supports seven severity levels from Trace to Fatal
• Provides pluggable formatters including JSON and colored text
• Offers a hook interface for sending entries to Syslog, Logstash, Elasticsearch, or custom backends
• Works as a drop-in replacement for the Go standard library logger

Architecture Overview

Logrus centers on an Entry struct that carries a map of fields plus metadata (timestamp, level, message). Each Logger instance holds a formatter, output writer, and a slice of hooks. When a log method is called, the logger creates an Entry, runs all registered hooks for that level, formats the entry, and writes it to the output. Thread safety is achieved via a mutex on the logger.

Setup & Configuration

• Import with the alias log to replace the standard logger seamlessly
• Set the global level with log.SetLevel(log.DebugLevel)
• Switch to JSON output via log.SetFormatter(&log.JSONFormatter{})
• Register hooks with log.AddHook(myHook) for side-channel delivery
• Create multiple logger instances with logrus.New() for isolated configuration

Key Features

• Field-based structured logging with WithFields and WithField
• Fatal and Panic levels that call os.Exit(1) or panic() after logging
• Thread-safe by default with per-logger mutexes
• Caller reporting via log.SetReportCaller(true) for file and line info
• Large ecosystem of third-party hooks and formatters

Comparison with Similar Tools

• Zap — higher throughput with zero-allocation design but less ergonomic API
• Zerolog — even faster zero-allocation logger focused on JSON output
• slog (Go 1.21+) — standard library structured logging, fewer features but no dependency
• Apex/log — similar API with a handler-based architecture instead of hooks

FAQ

Q: Is Logrus still maintained? A: The author considers it feature-complete and in maintenance mode. Security patches and critical fixes are still applied, but new features are unlikely.

Q: How does Logrus compare to slog? A: slog is built into Go 1.21+ and has lower overhead. Logrus offers a richer hook ecosystem and more formatters but adds an external dependency.

Q: Can I use Logrus in performance-critical paths? A: Logrus allocates on each log call. For hot paths, consider Zap or Zerolog which offer zero-allocation modes.

Q: Does Logrus support context-based logging? A: Yes, use log.WithContext(ctx) to attach a context, and hooks can extract values from it.

Sources

• https://github.com/sirupsen/logrus
• https://pkg.go.dev/github.com/sirupsen/logrus