How to Send Info Logs to stdout and Error/Fatal Logs to stderr Using uber-go/zap?

How to Log to stdout or stderr Based on Log Level Using uber-go/zap?

Problem:

Configure uber-go/zap to write info logs to stdout and error and fatal logs to stderr.

Answer:

To achieve this, use zapcore.NewTee with two cores, each set to handle specific log levels:

<code class="go">import (
    "go.uber.org/zap"
    "go.uber.org/zap/zapcore"
    "os"
)

func main() {
    // Create a level enabler for info logs
    infoLevel := zap.LevelEnablerFunc(func(level zapcore.Level) bool {
        return level == zapcore.InfoLevel
    })

    // Create a level enabler for error and fatal logs
    errorFatalLevel := zap.LevelEnablerFunc(func(level zapcore.Level) bool {
        return level == zapcore.ErrorLevel || level == zapcore.FatalLevel
    })

    // Create stdout and stderr writers
    stdoutSyncer := zapcore.Lock(os.Stdout)
    stderrSyncer := zapcore.Lock(os.Stderr)

    // Create a tee core that writes to both stdout and stderr
    core := zapcore.NewTee(
        // Core for info logs
        zapcore.NewCore(
            zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
            stdoutSyncer,
            infoLevel,
        ),
        // Core for error and fatal logs
        zapcore.NewCore(
            zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
            stderrSyncer,
            errorFatalLevel,
        ),
    )

    // Create a logger with the tee core
    logger := zap.New(core)

    // Log info and error messages
    logger.Info("info log")
    logger.Error("error log")
}</code>

This configuration allows you to control where different log levels are written to, providing greater flexibility in your logging strategy.

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