How to Separate Logs by Level to Stdout and Stderr using uber-go/zap?

Logging to stdout and stderr using uber-go/zap

Separating the logging of different log levels to stdout and stderr using uber-go/zap can be achieved by employing a tee core. A tee core combines multiple cores, allowing logs to be written to different destinations based on their level.

How to create a tee core

To create a tee core, you need to construct two individual cores: one for stdout and another for stderr. Each core includes an encoder to format the log messages and a syncer to write the logs to the appropriate device.

Example tee core

The following snippet demonstrates how to create a tee core that separates info logs to stdout and error/fatal logs to stderr:

<code class="go">package main

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

func main() {
    // Syncers for stdout and stderr
    stdoutSyncer := zapcore.Lock(os.Stdout)
    stderrSyncer := zapcore.Lock(os.Stderr)

    // Create two level enablers
    infoLevel := zap.LevelEnablerFunc(
        func(level zapcore.Level) bool {
            return level == zapcore.InfoLevel
        },
    )

    errorFatalLevel := zap.LevelEnablerFunc(
        func(level zapcore.Level) bool {
            return level == zapcore.ErrorLevel || level == zapcore.FatalLevel
        },
    )

    // Create two cores
    infoCore := zapcore.NewCore(
        zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
        stdoutSyncer,
        infoLevel,
    )

    errorFatalCore := zapcore.NewCore(
        zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
        stderrSyncer,
        errorFatalLevel,
    )

    // Combine the cores into a tee core
    teeCore := zapcore.NewTee(
        infoCore,
        errorFatalCore,
    )

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

    // Log messages at different levels
    logger.Info("Info log")
    logger.Error("Error log")
}</code>

Output redirection

To verify the separation of log output, you can redirect stdout or stderr to /dev/null and observation the remaining output.

<code class="bash">$ go build main.go

$ ./main 2>/dev/null # Shows only stdout
{
    "level": "info",
    "ts": 1626900981.520349,
    "msg": "info log"
}

$ ./main 1>/dev/null # Shows only stderr
{
    "level": "error",
   "ts": 1626901025.056065,
    "msg": "error log"
}</code>

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