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Table of Contents
What is a structured log? Why use slog?
How to create a basic structured log?
How to pass context information in a function?
What should I pay attention to when customizing the log level and format?
Home Backend Development Golang Go slog structured logging tutorial

Go slog structured logging tutorial

Jul 09, 2025 am 02:39 AM
go Structured logs

Go's slog library enables structured logging to be simple and efficient. 1. slog is a lightweight structured log package officially launched by Go. It does not require third-party dependencies and supports JSON and text format output; 2. Use slog.Info, slog.Error and other methods to directly enter key-value pairs to create structured logs; 3. Loggers of different formats can be initialized through slog.NewJSONHandler or slog.NewTextHandler; 4. Use With method to bind context information such as request ID and automatically append to each log; 5. Custom log levels need to explicitly set Handler Level parameters, such as slog.LevelDebug; 6. The official Handler has met the needs of most scenarios. The core value of structured logs is to facilitate machine analysis and problem investigation.

Go slog structured logging tutorial

Using Go's slog to make structured logs is actually simpler than you think. Go 1.21 introduces standard library slog, which supports structured logging, and can write clear and easy-to-parse logs without relying on third-party libraries.

Go slog structured logging tutorial

What is a structured log? Why use slog?

Structured logs are the organization of log information into key-value pairs, rather than a whole string. For example, when recording a request failure, in addition to writing "request failure", you can also bring status code, time-consuming, user ID and other information to facilitate subsequent analysis.

slog is a structured log package officially launched by Go. Its advantages are that it is lightweight, easy to use, and has built-in JSON and text format output, which is suitable for most scenarios. Unlike some third-party libraries that require additional configuration or middleware.

Go slog structured logging tutorial

How to create a basic structured log?

The easiest way to use slog is to directly call slog.Info , slog.Error and other methods and pass in multiple key-value pairs:

 package main

import (
    "log/slog"
)

func main() {
    slog.Info("User login", "user_id", 123, "status", "success")
}

The output (default text format) will look like this:

Go slog structured logging tutorial
 INFO User login user_id=123 status=success

If you want the output to be in JSON format, you can initialize a logger with options:

 handler := slog.NewJSONHandler(os.Stdout, nil)
logger := slog.New(handler)
logger.Info("User login", "user_id", 123, "status", "success")

The output becomes:

 {"time":"2025-04-05T12:00:00Z","level":"INFO","msg":"User login","user_id":123,"status":"success"}

How to pass context information in a function?

In actual development, we often need to record some context information, such as request ID, user identity, etc. At this time, you can use the With method of slog to bind these fields:

 logger := slog.Default().With("request_id", "abc123")

logger.Info("Processing request", "step", "start")

In this way, each log will be automatically brought with request_id field. You can also encapsulate it into middleware or tool functions, injecting these common fields at the beginning of an HTTP request.


What should I pay attention to when customizing the log level and format?

Although slog provides Info and Error levels by default, if you want to customize the levels, such as adding Debug or Trace, you can control which levels of log output by setting the Handler's Level:

 opts := &slog.HandlerOptions{
    Level: slog.LevelDebug,
}
handler := slog.NewTextHandler(os.Stdout, opts)
logger := slog.New(handler)

logger.Debug("This debug log will be shown")

Note: By default, slog does not output Debug-level content, and Level must be explicitly set to take effect.

In addition, if you want to change the output format, such as adding timestamps and adjusting the field order, you can inherit and implement your own Handler. But most of the time, the official JSON and Text Handler are enough.


Basically that's it. The core of structured logs is to record information in a structured manner to facilitate machine analysis and manual investigation. Although slog is not particularly rich in functions, it is more concise and standard, and is suitable for most Go projects to get started quickly.

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