MultiWriter

multiwriter is a Go library that allows logging to multiple destinations that implement the io.Writer interface, with optional colorization based on log levels. It is designed to handle simultaneous output to multiple sources, such as files, consoles, or external logging systems like Kafka.

Features

• Log simultaneously to multiple destinations (io.Writer).
• Colorize output based on log levels (INFO, WARN, ERROR, DEBUG).
• Best-effort mode to ignore destination errors (ignoreErrors=true).
• Support for custom destinations, with opt-in color output capability detection (Colored() bool).

Behavior

• Write(p) returns n=len(p) on success (even when colorization adds ANSI bytes to some destinations).
• If ignoreErrors=false, the first destination error stops the write and is returned.
• If ignoreErrors=true, destination errors are ignored and Write(p) returns (len(p), nil).
• MultiWriter serializes concurrent Write calls (so log lines are less likely to interleave).

Installation

Install the package using go get:

go get github.com/rzaripov1990/multiwriter

Examples

Basic Example: Log to Console and File

The following example demonstrates how to log to both the console (os.Stdout) and a log file:

package main

import (
    "log"
    "os"
    "github.com/rzaripov1990/multiwriter"
)

func main() {
    file, err := os.Create("./custom-file.log")
    if err != nil {
        panic(err)
    }
    defer file.Close()

    // Create a MultiWriter instance for console and file output
    mw := multiwriter.New(false, false, os.Stdout, file)

    // Set the logger output to MultiWriter
    log.SetOutput(mw)

    // Log messages with different levels
    log.Println(`"INFO": informational message`)
    log.Println(`"WARN": warning message`)
    log.Println(`"ERROR": error message`)
    log.Println(`"DEBUG": debug message`)
}

Advanced Example: Log to Console and Kafka Topic

In this example, logs are sent to both the console (with colorization) and a Kafka topic:

package main

import (
    "context"
    "os"

    colored_logger "github.com/rzaripov1990/multiwriter/ext/colored"
    kafka_logger "github.com/rzaripov1990/multiwriter/ext/kafka"
    "github.com/rzaripov1990/multiwriter"
    "log/slog"

    kgo "github.com/segmentio/kafka-go"
)

func main() {
    // Enable colorization and ignore errors
    colorize := true
    ignoreErrors := true

    // Destination 1: Kafka logger setup
    kafka := kafka_logger.New(
        &kgo.Writer{
            Addr:     kgo.TCP("127.0.0.1:9092"),
            Balancer: &kgo.RoundRobin{},
            Topic:    "logs",
        },
    )
    defer kafka.Close()

    // Destination 2: Standard output with colorization
    stdout := colored_logger.New(true, os.Stdout)

    // Create logger with MultiWriter
    logger := slog.New(
        slog.NewJSONHandler(
            multiwriter.New(colorize, ignoreErrors, stdout, kafka),
            &slog.HandlerOptions{
                AddSource: false,
                Level:     slog.LevelDebug,
            },
        ),
    )

    ctx := context.Background()
    slogValues := []slog.Attr{
        slog.String("possession", "version"),
        slog.String("song", "concept"),
        slog.String("construction", "direction"),
        slog.String("reading", "quantity"),
        slog.String("historian", "efficiency"),
        slog.String("establishment", "courage"),
    }

    // Log messages with various log levels and attributes
    logger.LogAttrs(ctx, slog.LevelDebug, "Debugging log message", slogValues[:2]...)
    logger.LogAttrs(ctx, slog.LevelInfo, "Informational log message", slogValues[2:4]...)
    logger.LogAttrs(ctx, slog.LevelWarn, "Warning log message", slogValues[4:]...)
    logger.LogAttrs(ctx, slog.LevelError, "Error log message", nil)
}

Adding New Destinations

To add a new log destination, simply implement the io.Writer interface. Optionally, if the destination supports colorized output, implement the Colored() bool function.

type CustomWriter struct {}

func (cw *CustomWriter) Write(p []byte) (n int, err error) {
    // Custom implementation for writing logs
    return len(p), nil
}

func (cw *CustomWriter) Colored() bool {
    return true // Supports colorized output
}

Development

Run unit tests:

go test ./...

Run integration tests (Kafka/file demo) explicitly:

go test -tags=integration ./...

To bring up Kafka locally for integration tests, see example/docker-compose.yml.