go-logx

go-logx is a leveled Go logger with JSON and key/value text formatters. It also provides adapters for the standard library's log/slog package.

Contents

• Requirements
• Installation
• Quick start
• Configuration
• Formatters
• slog adapters
• Default logger
• Development
• Contributing
• License

Requirements

• Go 1.26 or later.

Installation

go get go.osspkg.com/logx

Quick start

package main

import (
	"os"

	"go.osspkg.com/logx"
)

func main() {
	logger := logx.New(
		logx.WithOutput(os.Stdout),
		logx.WithLevel(logx.LevelInfo),
	)

	logger.Info("server started", "address", ":8080", "port", 8080)
}

The default formatter is JSON. This call writes one JSON record followed by a newline:

{"time":"2026-10-10T12:00:00Z","level":"INFO","msg":"server started","ctx":{"address":":8080","port":"8080"}}

Configuration

New accepts options for the minimum log level, output writer, and formatter. The default level is LevelError, and the default writer is standard output.

Available levels, from the least restrictive to the most restrictive, are LevelDebug, LevelInfo, LevelWarn, and LevelError. A record is written when its level meets the configured threshold. Fatal writes a fatal record and then exits the process with status 1.

Context is supplied as alternating key/value arguments:

logger.Info("request handled", "method", "GET", "status", 200)

For the JSON formatter, context values are represented as strings. If there are no context pairs, the ctx field is omitted. Context keys are JSON object fields, so their order is unspecified.

Loggers serialize writes to their configured writer. To use a different formatter, pass it at construction time:

logger := logx.New(
	logx.WithLevel(logx.LevelDebug),
	logx.WithFormatter(logx.NewFormatString()),
)

Formatters

The built-in formatters are:

• NewFormatJSON() writes JSON records.
• NewFormatString() writes quoted key=value fields separated by tabs.
• NewFormatString(logx.WithDelimiter('|')) selects another single-byte delimiter.

Applications can provide a custom formatter by implementing logx.Formatter:

import (
	"io"

	"go.osspkg.com/logx"
)

type MessageOnlyFormatter struct{}

func (MessageOnlyFormatter) Encode(w io.Writer, m *logx.Message) error {
	_, err := io.WriteString(w, m.Message+"\n")
	return err
}

slog adapters

Use the standard library's JSON or text handler with the same logger options:

package main

import (
	"os"

	"go.osspkg.com/logx"
)

func main() {
	logger := logx.NewSLogJsonAdapter(
		logx.WithOutput(os.Stdout),
		logx.WithLevel(logx.LevelInfo),
	)
	logger.Info("worker ready", "worker", 3)
}

NewSLogStringAdapter selects slog text output. Passing WithFormatter instead routes the adapter through the supplied logx.Formatter.

Default logger

The package-level functions use a default logger initialized with JSON output and the LevelError threshold. Select another logger during startup with SetDefault:

logx.SetDefault(logx.New(logx.WithLevel(logx.LevelInfo)))
logx.Info("application started", "version", "1.2.3")

Call SetDefault before starting concurrent logging.

Development

Run the package tests from the repository root:

go test ./...

Run the full project checks with:

make ci

make ci installs the repository's goppy tooling and runs license, lint, test, and build checks. GitHub Actions runs this command on Linux and runs the Go test suite natively on macOS and Windows.

Contributing

Report bugs or request changes through GitHub Issues. For code changes, open a pull request and include tests for changed behavior.

License

BSD 3-Clause. See LICENSE.