How can you use Uber Go Zap library to separate logs of different levels to stdout and stderr?

Using uber-go/zap to Log Based on Log Level to stdout and stderr

Introduction

The uber-go/zap library offers robust logging capabilities. To meet specific requirements, it can be beneficial to direct logs of different levels to separate destinations, such as stdout and stderr. This article explores how to achieve this separation, ensuring logs are written to the appropriate devices based on their severity.

Custom Logging Configuration

To specify custom logging behavior in uber-go/zap, we can utilize the zap.Config struct. By modifying the OutputPaths and ErrorOutputPaths fields, we can control the destinations for different log levels. However, if only one path is set in either OutputPaths or ErrorOutputPaths, all logs will be written to that single destination, regardless of their level.

Solution: Zapcore.NewTee

To achieve the desired separation, we employ zapcore.NewTee, which creates a tee core that combines multiple cores. Each core can independently handle logs of specific levels and direct them to their respective destinations. Here's how we implement this solution:

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

func main() {
    stdoutSyncer := zapcore.Lock(os.Stdout)
    stderrSyncer := zapcore.Lock(os.Stderr)

    infoCore := zapcore.NewCore(
        zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
        stdoutSyncer,
        zap.InfoLevel,
    )

    errorCore := zapcore.NewCore(
        zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
        stderrSyncer,
        zap.ErrorLevel,
    )

    core := zapcore.NewTee(infoCore, errorCore)
    logger := zap.New(core)

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

In this example, we create two cores: one for info-level logs (written to stdout) and another for error-level logs (written to stderr). The zapcore.NewTee function combines these cores, allowing logs of different levels to be directed to separate destinations.

By redirecting stdout or stderr to /dev/null, we can further verify that each log message is indeed written to the correct device, depending on its level.

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