How to Separate Logs by Severity Level Using uber-go/zap?

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

To separate logs based on severity level in uber-go/zap, utilize the concept of "tee-ing" zapcore cores. The approach leverages a tee core that combines two independent cores, each with specific level enablers.

Implementation:

1. Define Level Enablers:

   <code class="go">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
   })</code>

2. Set up Write Syncers:

   <code class="go">stdoutSyncer := zapcore.Lock(os.Stdout)
   stderrSyncer := zapcore.Lock(os.Stderr)</code>

3. Construct Tee Core:

   <code class="go">core := zapcore.NewTee(
    zapcore.NewCore(
        zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
        stdoutSyncer,
        infoLevel,
    ),
    zapcore.NewCore(
        zapcore.NewJSONEncoder(zap.NewProductionEncoderConfig()),
        stderrSyncer,
        errorFatalLevel,
    ),
   )</code>

4. Create Logger:

   <code class="go">logger := zap.New(core)</code>

Demonstration:

<code class="go">logger.Info("info log")
logger.Error("error log")</code>

Testing:
Redirect stdout or stderr to /dev/null to verify that info logs are printed to stdout and error logs to stderr:

$ go build main.go

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

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

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