How to Separate Info Logs to STDOUT and Error/Fatal Logs to STDERR Using Uber-Go/Zap?

How to Log to STDOUT and STDERR Based on Log Level using Uber-Go/Zap?

Zap, a popular Go logging library, allows you to control where log messages are written based on their level. Here's how to configure Zap to write:

• Info logs to STDOUT
• Error and Fatal logs to STDERR

Using ZapCore.NewTee with Separate Cores:

To achieve this separation, create a tee core that combines two cores:

• One core dedicated to the InfoLevel, writing to STDOUT.
• Another core dedicated to ErrorLevel and FatalLevel, writing to STDERR.

Here's an example program demonstrating this technique:

<code class="go">package main

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

func main() {
    // 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
    })

    // Write syncers
    stdoutSyncer := zapcore.Lock(os.Stdout)
    stderrSyncer := zapcore.Lock(os.Stderr)

    // Tee core for InfoLevel and ErrorLevel/FatalLevel separation
    core := zapcore.NewTee(
        zapcore.NewCore(
            zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
            stdoutSyncer,
            infoLevel,
        ),
        zapcore.NewCore(
            zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
            stderrSyncer,
            errorFatalLevel,
        ),
    )

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

    logger.Info("Info log")
    logger.Error("Error log")
}</code>

In this example, the program logs both an Info message to STDOUT and an Error message to STDERR. You can verify this by redirecting the respective outputs to /dev/null:

<code class="bash"># stdout only
$ go build main.go && ./main 2>/dev/null
{
  "level": "info",
  "ts": <timestamp>,
  "msg": "Info log"
}

# stderr only
$ go build main.go && ./main 1>/dev/null
{
  "level": "error",
  "ts": <timestamp>,
  "msg": "Error log"
}</code>

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