slog

Overview ¶

Package slog provides structured logging, in which log records include a message, a severity level, and various other attributes expressed as key-value pairs.

It defines a type, Logger, which provides several methods (such as Logger.Info and Logger.Error) for reporting events of interest.

Each Logger is associated with a Handler. A Logger output method creates a Record from the method arguments and passes it to the Handler, which decides how to handle it. There is a default Logger accessible through top-level functions (such as Info and Error) that call the corresponding Logger methods.

A log record consists of a time, a level, a message, and a set of key-value pairs, where the keys are strings and the values may be of any type. As an example,

slog.Info("hello", "count", 3)

creates a record containing the time of the call, a level of Info, the message "hello", and a single pair with key "count" and value 3.

The Info top-level function calls the Logger.Info method on the default Logger. In addition to Logger.Info, there are methods for Debug, Warn and Error levels. Besides these convenience methods for common levels, there is also a Logger.Log method which takes the level as an argument. Each of these methods has a corresponding top-level function that uses the default logger.

The default handler formats the log record's message, time, level, and attributes as a string and passes it to the log package.

2022/11/08 15:28:26 INFO hello count=3

For more control over the output format, create a logger with a different handler. This statement uses New to create a new logger with a TextHandler that writes structured records in text form to standard error:

logger := slog.New(slog.NewTextHandler(os.Stderr, nil))

TextHandler output is a sequence of key=value pairs, easily and unambiguously parsed by machine. This statement:

logger.Info("hello", "count", 3)

produces this output:

time=2022-11-08T15:28:26.000-05:00 level=INFO msg=hello count=3

The package also provides JSONHandler, whose output is line-delimited JSON:

logger := slog.New(slog.NewJSONHandler(os.Stdout, nil))
logger.Info("hello", "count", 3)

produces this output:

{"time":"2022-11-08T15:28:26.000000000-05:00","level":"INFO","msg":"hello","count":3}

Both TextHandler and JSONHandler can be configured with HandlerOptions. There are options for setting the minimum level (see Levels, below), displaying the source file and line of the log call, and modifying attributes before they are logged.

Setting a logger as the default with

slog.SetDefault(logger)

will cause the top-level functions like Info to use it. SetDefault also updates the default logger used by the log package, so that existing applications that use log.Printf and related functions will send log records to the logger's handler without needing to be rewritten.

Some attributes are common to many log calls. For example, you may wish to include the URL or trace identifier of a server request with all log events arising from the request. Rather than repeat the attribute with every log call, you can use Logger.With to construct a new Logger containing the attributes:

logger2 := logger.With("url", r.URL)

The arguments to With are the same key-value pairs used in Logger.Info. The result is a new Logger with the same handler as the original, but additional attributes that will appear in the output of every call.

Levels ¶

A Level is an integer representing the importance or severity of a log event. The higher the level, the more severe the event. This package defines constants for the most common levels, but any int can be used as a level.

In an application, you may wish to log messages only at a certain level or greater. One common configuration is to log messages at Info or higher levels, suppressing debug logging until it is needed. The built-in handlers can be configured with the minimum level to output by setting HandlerOptions.Level. The program's `main` function typically does this. The default value is LevelInfo.

Setting the HandlerOptions.Level field to a Level value fixes the handler's minimum level throughout its lifetime. Setting it to a LevelVar allows the level to be varied dynamically. A LevelVar holds a Level and is safe to read or write from multiple goroutines. To vary the level dynamically for an entire program, first initialize a global LevelVar:

var programLevel = new(slog.LevelVar) // Info by default

Then use the LevelVar to construct a handler, and make it the default:

h := slog.NewJSONHandler(os.Stderr, &slog.HandlerOptions{Level: programLevel})
slog.SetDefault(slog.New(h))

Now the program can change its logging level with a single statement:

programLevel.Set(slog.LevelDebug)

Groups ¶

Attributes can be collected into groups. A group has a name that is used to qualify the names of its attributes. How this qualification is displayed depends on the handler. TextHandler separates the group and attribute names with a dot. JSONHandler treats each group as a separate JSON object, with the group name as the key.

Use Group to create a Group attribute from a name and a list of key-value pairs:

slog.Group("request",
    "method", r.Method,
    "url", r.URL)

TextHandler would display this group as

request.method=GET request.url=http://example.com

JSONHandler would display it as

"request":{"method":"GET","url":"http://example.com"}

Use Logger.WithGroup to qualify all of a Logger's output with a group name. Calling WithGroup on a Logger results in a new Logger with the same Handler as the original, but with all its attributes qualified by the group name.

This can help prevent duplicate attribute keys in large systems, where subsystems might use the same keys. Pass each subsystem a different Logger with its own group name so that potential duplicates are qualified:

logger := slog.Default().With("id", systemID)
parserLogger := logger.WithGroup("parser")
parseInput(input, parserLogger)

When parseInput logs with parserLogger, its keys will be qualified with "parser", so even if it uses the common key "id", the log line will have distinct keys.

Contexts ¶

Some handlers may wish to include information from the context.Context that is available at the call site. One example of such information is the identifier for the current span when tracing is enabled.

The Logger.Log and Logger.LogAttrs methods take a context as a first argument, as do their corresponding top-level functions.

Although the convenience methods on Logger (Info and so on) and the corresponding top-level functions do not take a context, the alternatives ending in "Context" do. For example,

slog.InfoContext(ctx, "message")

It is recommended to pass a context to an output method if one is available.

Attrs and Values ¶

An Attr is a key-value pair. The Logger output methods accept Attrs as well as alternating keys and values. The statement

slog.Info("hello", slog.Int("count", 3))

behaves the same as

slog.Info("hello", "count", 3)

There are convenience constructors for Attr such as Int, String, and Bool for common types, as well as the function Any for constructing Attrs of any type.

The value part of an Attr is a type called Value. Like an [any], a Value can hold any Go value, but it can represent typical values, including all numbers and strings, without an allocation.

For the most efficient log output, use Logger.LogAttrs. It is similar to Logger.Log but accepts only Attrs, not alternating keys and values; this allows it, too, to avoid allocation.

The call

logger.LogAttrs(ctx, slog.LevelInfo, "hello", slog.Int("count", 3))

is the most efficient way to achieve the same output as

slog.InfoContext(ctx, "hello", "count", 3)

Customizing a type's logging behavior ¶

If a type implements the LogValuer interface, the Value returned from its LogValue method is used for logging. You can use this to control how values of the type appear in logs. For example, you can redact secret information like passwords, or gather a struct's fields in a Group. See the examples under LogValuer for details.

A LogValue method may return a Value that itself implements LogValuer. The Value.Resolve method handles these cases carefully, avoiding infinite loops and unbounded recursion. Handler authors and others may wish to use Value.Resolve instead of calling LogValue directly.

Wrapping output methods ¶

The logger functions use reflection over the call stack to find the file name and line number of the logging call within the application. This can produce incorrect source information for functions that wrap slog. For instance, if you define this function in file mylog.go:

func Infof(logger *slog.Logger, format string, args ...any) {
    logger.Info(fmt.Sprintf(format, args...))
}

and you call it like this in main.go:

Infof(slog.Default(), "hello, %s", "world")

then slog will report the source file as mylog.go, not main.go.

A correct implementation of Infof will obtain the source location (pc) and pass it to NewRecord. The Infof function in the package-level example called "wrapping" demonstrates how to do this.

Working with Records ¶

Sometimes a Handler will need to modify a Record before passing it on to another Handler or backend. A Record contains a mixture of simple public fields (e.g. Time, Level, Message) and hidden fields that refer to state (such as attributes) indirectly. This means that modifying a simple copy of a Record (e.g. by calling Record.Add or Record.AddAttrs to add attributes) may have unexpected effects on the original. Before modifying a Record, use Record.Clone to create a copy that shares no state with the original, or create a new Record with NewRecord and build up its Attrs by traversing the old ones with Record.Attrs.

Performance considerations ¶

If profiling your application demonstrates that logging is taking significant time, the following suggestions may help.

If many log lines have a common attribute, use Logger.With to create a Logger with that attribute. The built-in handlers will format that attribute only once, at the call to Logger.With. The Handler interface is designed to allow that optimization, and a well-written Handler should take advantage of it.

The arguments to a log call are always evaluated, even if the log event is discarded. If possible, defer computation so that it happens only if the value is actually logged. For example, consider the call

slog.Info("starting request", "url", r.URL.String())  // may compute String unnecessarily

The URL.String method will be called even if the logger discards Info-level events. Instead, pass the URL directly:

slog.Info("starting request", "url", &r.URL) // calls URL.String only if needed

The built-in TextHandler will call its String method, but only if the log event is enabled. Avoiding the call to String also preserves the structure of the underlying value. For example JSONHandler emits the components of the parsed URL as a JSON object. If you want to avoid eagerly paying the cost of the String call without causing the handler to potentially inspect the structure of the value, wrap the value in a fmt.Stringer implementation that hides its Marshal methods.

You can also use the LogValuer interface to avoid unnecessary work in disabled log calls. Say you need to log some expensive value:

slog.Debug("frobbing", "value", computeExpensiveValue(arg))

Even if this line is disabled, computeExpensiveValue will be called. To avoid that, define a type implementing LogValuer:

type expensive struct { arg int }

func (e expensive) LogValue() slog.Value {
    return slog.AnyValue(computeExpensiveValue(e.arg))
}

Then use a value of that type in log calls:

slog.Debug("frobbing", "value", expensive{arg})

Now computeExpensiveValue will only be called when the line is enabled.

The built-in handlers acquire a lock before calling io.Writer.Write to ensure that exactly one Record is written at a time in its entirety. Although each log record has a timestamp, the built-in handlers do not use that time to sort the written records. User-defined handlers are responsible for their own locking and sorting.

Writing a handler ¶

For a guide to writing a custom handler, see https://golang.org/s/slog-handler-guide.

package main

import (
	"log/slog"
	"log/slog/internal/slogtest"
	"os"
)

func main() {
	// A slog.TextHandler can output log messages.
	logger1 := slog.New(slog.NewTextHandler(
		os.Stdout,
		&slog.HandlerOptions{ReplaceAttr: slogtest.RemoveTime},
	))
	logger1.Info("message 1")

	// A slog.DiscardHandler will discard all messages.
	logger2 := slog.New(slog.DiscardHandler)
	logger2.Info("message 2")

}

Output:
level=INFO msg="message 1"

package main

import (
	"context"
	"fmt"
	"log/slog"
	"os"
	"path/filepath"
	"runtime"
	"time"
)

// Infof is an example of a user-defined logging function that wraps slog.
// The log record contains the source position of the caller of Infof.
func Infof(logger *slog.Logger, format string, args ...any) {
	if !logger.Enabled(context.Background(), slog.LevelInfo) {
		return
	}
	var pcs [1]uintptr
	runtime.Callers(2, pcs[:]) // skip [Callers, Infof]
	r := slog.NewRecord(time.Now(), slog.LevelInfo, fmt.Sprintf(format, args...), pcs[0])
	_ = logger.Handler().Handle(context.Background(), r)
}

func main() {
	replace := func(groups []string, a slog.Attr) slog.Attr {
		// Remove time.
		if a.Key == slog.TimeKey && len(groups) == 0 {
			return slog.Attr{}
		}
		// Remove the directory from the source's filename.
		if a.Key == slog.SourceKey {
			source := a.Value.Any().(*slog.Source)
			source.File = filepath.Base(source.File)
		}
		return a
	}
	logger := slog.New(slog.NewTextHandler(os.Stdout, &slog.HandlerOptions{AddSource: true, ReplaceAttr: replace}))
	Infof(logger, "message, %s", "formatted")

}

Output:
level=INFO source=example_wrap_test.go:43 msg="message, formatted"