tessl/golang-zap

Blazing fast, structured, leveled logging in Go.

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Zap

Name: tessl/golang-zap
Author: tessl

Zap is a blazingly fast, structured logging library for Go, providing both high-performance strongly-typed logging (Logger) and ergonomic loosely-typed logging (SugaredLogger). It offers zero-allocation JSON encoding, flexible log levels, and extensive customization through its modular architecture.

Package Information

Package Name: go.uber.org/zap
Package Type: Go module
Language: Go
Installation: go get -u go.uber.org/zap

Core Imports

import (
    "go.uber.org/zap"
    "go.uber.org/zap/zapcore"
)

For testing utilities:

import (
    "go.uber.org/zap/zaptest"
    "go.uber.org/zap/zaptest/observer"
)

For integrations:

import (
    "go.uber.org/zap/zapgrpc"  // gRPC integration
    "go.uber.org/zap/zapio"    // io.Writer integration
)

Basic Usage

import "go.uber.org/zap"

// Create a production logger with sensible defaults
logger, _ := zap.NewProduction()
defer logger.Sync()

// Structured logging with strongly-typed fields
logger.Info("user logged in",
    zap.String("username", "alice"),
    zap.Int("user_id", 42),
    zap.Duration("latency", 125*time.Millisecond),
)

// For more ergonomic API, use SugaredLogger
sugar := logger.Sugar()
sugar.Infow("user logged in",
    "username", "alice",
    "user_id", 42,
)

Architecture

Zap's architecture separates concerns for maximum flexibility and performance:

Logger vs SugaredLogger

Logger: Strongly-typed API that avoids reflection and allocations. Every field must be explicitly typed using field constructors like zap.String(), zap.Int(), etc. Optimal for hot paths where every microsecond matters.
SugaredLogger: Loosely-typed API that accepts interface{} values and uses reflection. More ergonomic with Printf-style and structured logging methods. 4-10x slower than Logger but still faster than most alternatives.

Core Components

Zap is built on the zapcore package, which defines low-level interfaces:

Core: The fundamental logging interface that determines what gets logged and where
Encoder: Transforms log entries into bytes (JSON, console, or custom formats)
WriteSyncer: Thread-safe destination for log output
Field: Efficient key-value pair representation for structured data

Configuration Approaches

Preset constructors: NewProduction(), NewDevelopment() for common scenarios
Config struct: Declarative configuration with JSON/YAML support
Low-level construction: Direct use of New() with custom Core for maximum control

Capabilities

Core Logging

Provides Logger and SugaredLogger types with methods for all log levels, dynamic level changes, and configuration options.

// Logger provides strongly-typed, high-performance logging
type Logger struct { /* ... */ }

func NewProduction(options ...Option) (*Logger, error)
func NewDevelopment(options ...Option) (*Logger, error)

func (log *Logger) Info(msg string, fields ...Field)
func (log *Logger) Error(msg string, fields ...Field)
func (log *Logger) With(fields ...Field) *Logger
func (log *Logger) Sugar() *SugaredLogger

// SugaredLogger provides ergonomic, loosely-typed logging
type SugaredLogger struct { /* ... */ }

func (s *SugaredLogger) Infow(msg string, keysAndValues ...interface{})
func (s *SugaredLogger) Infof(template string, args ...interface{})
func (s *SugaredLogger) Info(args ...interface{})

// AtomicLevel enables runtime level changes
type AtomicLevel struct { /* ... */ }

func NewAtomicLevel() AtomicLevel
func (lvl AtomicLevel) SetLevel(l zapcore.Level)
func (lvl AtomicLevel) ServeHTTP(w http.ResponseWriter, r *http.Request)

Core Logging API

Structured Fields

Over 100 field constructor functions for efficient, strongly-typed structured logging. Fields avoid allocations and reflection for maximum performance.

// Primitive types
func String(key string, val string) Field
func Int(key string, val int) Field
func Bool(key string, val bool) Field
func Float64(key string, val float64) Field

// Time types
func Time(key string, val time.Time) Field
func Duration(key string, val time.Duration) Field

// Error types
func Error(err error) Field
func NamedError(key string, err error) Field

// Advanced types
func Any(key string, value interface{}) Field
func Object(key string, val zapcore.ObjectMarshaler) Field
func Array(key string, val zapcore.ArrayMarshaler) Field
func Stringer(key string, val fmt.Stringer) Field
func Stringers[T fmt.Stringer](key string, values []T) Field
func Skip() Field

Field Constructors API

Core Abstractions

The zapcore package provides low-level interfaces and implementations for extending zap's capabilities with custom encoders, cores, and write destinations.

// Core is the minimal interface for logging
type Core interface {
    LevelEnabler
    With([]Field) Core
    Check(Entry, *CheckedEntry) *CheckedEntry
    Write(Entry, []Field) error
    Sync() error
}

// Encoder transforms entries to bytes
type Encoder interface {
    ObjectEncoder
    Clone() Encoder
    EncodeEntry(Entry, []Field) (*buffer.Buffer, error)
}

// WriteSyncer is a thread-safe write destination
type WriteSyncer interface {
    io.Writer
    Sync() error
}

Core Abstractions API

Third-Party Integrations

Integration packages for gRPC logging and standard library io.Writer compatibility.

// zapgrpc provides gRPC-compatible logger
type Logger struct { /* ... */ }

func NewLogger(l *zap.Logger, options ...Option) *Logger
func (l *Logger) Info(args ...interface{})
func (l *Logger) Warning(args ...interface{})
func (l *Logger) Error(args ...interface{})

// zapio provides io.Writer that logs to zap
type Writer struct {
    Log   *zap.Logger
    Level zapcore.Level
}

func (w *Writer) Write(bs []byte) (n int, err error)

Integration APIs

Testing Utilities

Tools for testing log output including test loggers, in-memory observation, and assertion utilities.

// zaptest provides test logger
func NewLogger(t TestingT, opts ...LoggerOption) *zap.Logger

// observer captures logs in memory for assertions
type ObservedLogs struct { /* ... */ }

func New(enab zapcore.LevelEnabler) (zapcore.Core, *ObservedLogs)
func (o *ObservedLogs) All() []LoggedEntry
func (o *ObservedLogs) FilterMessage(msg string) *ObservedLogs

Testing Utilities API

Global Logger Functions

Zap provides global logger functions for convenience:

// Access global loggers
func L() *Logger
func S() *SugaredLogger

// Replace global loggers (returns restore function)
func ReplaceGlobals(logger *Logger) func()

// Redirect standard library log package
func RedirectStdLog(l *Logger) func()
func RedirectStdLogAt(l *Logger, level zapcore.Level) (func(), error)

Configuration Types

// Config provides declarative logger configuration
type Config struct {
    Level             AtomicLevel           `json:"level" yaml:"level"`
    Development       bool                  `json:"development" yaml:"development"`
    DisableCaller     bool                  `json:"disableCaller" yaml:"disableCaller"`
    DisableStacktrace bool                  `json:"disableStacktrace" yaml:"disableStacktrace"`
    Sampling          *SamplingConfig       `json:"sampling" yaml:"sampling"`
    Encoding          string                `json:"encoding" yaml:"encoding"`
    EncoderConfig     zapcore.EncoderConfig `json:"encoderConfig" yaml:"encoderConfig"`
    OutputPaths       []string              `json:"outputPaths" yaml:"outputPaths"`
    ErrorOutputPaths  []string              `json:"errorOutputPaths" yaml:"errorOutputPaths"`
    InitialFields     map[string]interface{} `json:"initialFields" yaml:"initialFields"`
}

func NewProductionConfig() Config
func NewDevelopmentConfig() Config
func (cfg Config) Build(opts ...Option) (*Logger, error)

// SamplingConfig controls log sampling for rate limiting
type SamplingConfig struct {
    Initial    int                                           `json:"initial" yaml:"initial"`
    Thereafter int                                           `json:"thereafter" yaml:"thereafter"`
    Hook       func(zapcore.Entry, zapcore.SamplingDecision) `json:"-" yaml:"-"`
}

Log Levels

const (
    DebugLevel  = zapcore.DebugLevel   // -1: Voluminous debug information
    InfoLevel   = zapcore.InfoLevel    //  0: General informational messages
    WarnLevel   = zapcore.WarnLevel    //  1: Warning messages
    ErrorLevel  = zapcore.ErrorLevel   //  2: Error messages
    DPanicLevel = zapcore.DPanicLevel  //  3: Panic in development mode
    PanicLevel  = zapcore.PanicLevel   //  4: Logs then panics
    FatalLevel  = zapcore.FatalLevel   //  5: Logs then calls os.Exit(1)
)