A Guide to Writing `slog` Handlers

This document is maintained by Jonathan Amsterdam jba@google.com.

Introduction

The standard library’s log/slog package has a two-part design. A "frontend," implemented by the Logger type, gathers structured log information like a message, level, and attributes, and passes them to a "backend," an implementation of the Handler interface. The package comes with two built-in handlers that should usually be adequate. But you may need to write your own handler, and that is not always straightforward. This guide is here to help.

Loggers and their handlers

Writing a handler requires an understanding of how the Logger and Handler types work together.

Each logger contains a handler. Certain Logger methods do some preliminary work, such as gathering key-value pairs into Attrs, and then call one or more Handler methods. These Logger methods are With, WithGroup, and the output methods like Info, Error and so on.

An output method fulfills the main role of a logger: producing log output. Here is a call to an output method:

logger.Info("hello", "key", value)

There are two general output methods, Log, and LogAttrs. For convenience, there is an output method for each of four common levels (Debug, Info, Warn and Error), and corresponding methods that take a context (DebugContext, InfoContext, WarnContext and ErrorContext).

Each Logger output method first calls its handler's Enabled method. If that call returns true, the method constructs a Record from its arguments and calls the handler's Handle method.

As a convenience and an optimization, attributes can be added to Logger by calling the With method:

logger = logger.With("k", v)

This call creates a new Logger value with the argument attributes; the original remains unchanged. All subsequent output from logger will include those attributes. A logger's With method calls its handler's WithAttrs method.

The WithGroup method is used to avoid key collisions in large programs by establishing separate namespaces. This call creates a new Logger value with a group named "g":

logger = logger.WithGroup("g")

All subsequent keys for logger will be qualified by the group name "g". Exactly what "qualified" means depends on how the logger's handler formats the output. The built-in TextHandler treats the group as a prefix to the key, separated by a dot: g.k for a key k, for example. The built-in JSONHandler uses the group as a key for a nested JSON object:

{"g": {"k": v}}

A logger's WithGroup method calls its handler's WithGroup method.

Implementing `Handler` methods

We can now talk about the four Handler methods in detail. Along the way, we will write a handler that formats logs using a format reminiscent of YAML. It will display this log output call:

logger.Info("hello", "key", 23)

something like this:

time: 2023-05-15T16:29:00
level: INFO
message: "hello"
key: 23
---

Although this particular output is valid YAML, our implementation doesn't consider the subtleties of YAML syntax, so it will sometimes produce invalid YAML. For example, it doesn't quote keys that have colons in them. We'll call it IndentHandler to forestall disappointment.

A brief aside before we start: it is tempting to embed slog.Handler in your custom handler and implement only the methods that you need. Loggers and handlers are too tightly coupled for that to work. You should implement all four handler methods.

We begin with the IndentHandler type and the New function that constructs it from an io.Writer and options:

type IndentHandler struct {
	opts Options
	// TODO: state for WithGroup and WithAttrs
	mu  *sync.Mutex
	out io.Writer
}

type Options struct {
	// Level reports the minimum level to log.
	// Levels with lower levels are discarded.
	// If nil, the Handler uses [slog.LevelInfo].
	Level slog.Leveler
}

func New(out io.Writer, opts *Options) *IndentHandler {
	h := &IndentHandler{out: out, mu: &sync.Mutex{}}
	if opts != nil {
		h.opts = *opts
	}
	if h.opts.Level == nil {
		h.opts.Level = slog.LevelInfo
	}
	return h
}

We'll support only one option, the ability to set a minimum level in order to suppress detailed log output. Handlers should always declare this option to be a slog.Leveler. The slog.Leveler interface is implemented by both Level and LevelVar. A Level value is easy for the user to provide, but changing the level of multiple handlers requires tracking them all. If the user instead passes a LevelVar, then a single change to that LevelVar will change the behavior of all handlers that contain it. Changes to LevelVars are goroutine-safe.

You might also consider adding a ReplaceAttr option to your handler, like the one for the built-in handlers. Although ReplaceAttr will complicate your implementation, it will also make your handler more generally useful.

The mutex will be used to ensure that writes to the io.Writer happen atomically. Unusually, IndentHandler holds a pointer to a sync.Mutex rather than holding a sync.Mutex directly. There is a good reason for that, which we'll explain later.

Our handler will need additional state to track calls to WithGroup and WithAttrs. We will describe that state when we get to those methods.

The `Enabled` method

The Enabled method is an optimization that can avoid unnecessary work. A Logger output method will call Enabled before it processes any of its arguments, to see if it should proceed.

The signature is

Enabled(context.Context, Level) bool

The context is available to allow decisions based on contextual information. For example, a custom HTTP request header could specify a minimum level, which the server adds to the context used for processing that request. A handler's Enabled method could report whether the argument level is greater than or equal to the context value, allowing the verbosity of the work done by each request to be controlled independently.

Our IndentHandler doesn't use the context. It just compares the argument level with its configured minimum level:

func (h *IndentHandler) Enabled(ctx context.Context, level slog.Level) bool {
	return level >= h.opts.Level.Level()
}

The `Handle` method

The Handle method is passed a Record containing all the information to be logged for a single call to a Logger output method. The Handle method should deal with it in some way. One way is to output the Record in some format, as TextHandler and JSONHandler do. But other options are to modify the Record and pass it on to another handler, enqueue the Record for later processing, or ignore it.

The signature of Handle is

Handle(context.Context, Record) error

The context is provided to support applications that provide logging information along the call chain. In a break with usual Go practice, the Handle method should not treat a canceled context as a signal to stop work.

If Handle processes its Record, it should follow the rules in the documentation. For example, a zero Time field should be ignored, as should zero Attrs.

A Handle method that is going to produce output should carry out the following steps:

Allocate a buffer, typically a []byte, to hold the output. It's best to construct the output in memory first, then write it with a single call to io.Writer.Write, to minimize interleaving with other goroutines using the same writer.
Format the special fields: time, level, message, and source location (PC). As a general rule, these fields should appear first and are not nested in groups established by WithGroup.
Format the result of WithGroup and WithAttrs calls.
Format the attributes in the Record.
Output the buffer.

That is how IndentHandler.Handle is structured:

func (h *IndentHandler) Handle(ctx context.Context, r slog.Record) error {
	buf := make([]byte, 0, 1024)
	if !r.Time.IsZero() {
		buf = h.appendAttr(buf, slog.Time(slog.TimeKey, r.Time), 0)
	}
	buf = h.appendAttr(buf, slog.Any(slog.LevelKey, r.Level), 0)
	if r.PC != 0 {
		fs := runtime.CallersFrames([]uintptr{r.PC})
		f, _ := fs.Next()
		buf = h.appendAttr(buf, slog.String(slog.SourceKey, fmt.Sprintf("%s:%d", f.File, f.Line)), 0)
	}
	buf = h.appendAttr(buf, slog.String(slog.MessageKey, r.Message), 0)
	indentLevel := 0
	// TODO: output the Attrs and groups from WithAttrs and WithGroup.
	r.Attrs(func(a slog.Attr) bool {
		buf = h.appendAttr(buf, a, indentLevel)
		return true
	})
	buf = append(buf, "---\n"...)
	h.mu.Lock()
	defer h.mu.Unlock()
	_, err := h.out.Write(buf)
	return err
}

The first line allocates a []byte that should be large enough for most log output. Allocating a buffer with some initial, fairly large capacity is a simple but significant optimization: it avoids the repeated copying and allocation that happen when the initial slice is empty or small. We'll return to this line in the section on speed and show how we can do even better.

The next part of our Handle method formats the special attributes, observing the rules to ignore a zero time and a zero PC.

Next, the method processes the result of WithAttrs and WithGroup calls. We'll skip that for now.

Then it's time to process the attributes in the argument record. We use the Record.Attrs method to iterate over the attributes in the order the user passed them to the Logger output method. Handlers are free to reorder or de-duplicate the attributes, but ours does not.

Lastly, after adding the line "---" to the output to separate log records, our handler makes a single call to h.out.Write with the buffer we've accumulated. We hold the lock for this write to make it atomic with respect to other goroutines that may be calling Handle at the same time.

At the heart of the handler is the appendAttr method, responsible for formatting a single attribute:

func (h *IndentHandler) appendAttr(buf []byte, a slog.Attr, indentLevel int) []byte {
	// Resolve the Attr's value before doing anything else.
	a.Value = a.Value.Resolve()
	// Ignore empty Attrs.
	if a.Equal(slog.Attr{}) {
		return buf
	}
	// Indent 4 spaces per level.
	buf = fmt.Appendf(buf, "%*s", indentLevel*4, "")
	switch a.Value.Kind() {
	case slog.KindString:
		// Quote string values, to make them easy to parse.
		buf = fmt.Appendf(buf, "%s: %q\n", a.Key, a.Value.String())
	case slog.KindTime:
		// Write times in a standard way, without the monotonic time.
		buf = fmt.Appendf(buf, "%s: %s\n", a.Key, a.Value.Time().Format(time.RFC3339Nano))
	case slog.KindGroup:
		attrs := a.Value.Group()
		// Ignore empty groups.
		if len(attrs) == 0 {
			return buf
		}
		// If the key is non-empty, write it out and indent the rest of the attrs.
		// Otherwise, inline the attrs.
		if a.Key != "" {
			buf = fmt.Appendf(buf, "%s:\n", a.Key)
			indentLevel++
		}
		for _, ga := range attrs {
			buf = h.appendAttr(buf, ga, indentLevel)
		}
	default:
		buf = fmt.Appendf(buf, "%s: %s\n", a.Key, a.Value)
	}
	return buf
}

It begins by resolving the attribute, to run the LogValuer.LogValue method of the value if it has one. All handlers should resolve every attribute they process.

Next, it follows the handler rule that says that empty attributes should be ignored.

Then it switches on the attribute kind to determine what format to use. For most kinds (the default case of the switch), it relies on slog.Value's String method to produce something reasonable. It handles strings and times specially: strings by quoting them, and times by formatting them in a standard way.

When appendAttr sees a Group, it calls itself recursively on the group's attributes, after applying two more handler rules. First, a group with no attributes is ignored—not even its key is displayed. Second, a group with an empty key is inlined: the group boundary isn't marked in any way. In our case, that means the group's attributes aren't indented.

The `WithAttrs` method

One of slog's performance optimizations is support for pre-formatting attributes. The Logger.With method converts key-value pairs into Attrs and then calls Handler.WithAttrs. The handler may store the attributes for later consumption by the Handle method, or it may take the opportunity to format the attributes now, once, rather than doing so repeatedly on each call to Handle.

The signature of the WithAttrs method is

WithAttrs(attrs []Attr) Handler

The attributes are the processed key-value pairs passed to Logger.With. The return value should be a new instance of your handler that contains the attributes, possibly pre-formatted.

WithAttrs must return a new handler with the additional attributes, leaving the original handler (its receiver) unchanged. For example, this call:

logger2 := logger1.With("k", v)

creates a new logger, logger2, with an additional attribute, but has no effect on logger1.

We will show example implementations of WithAttrs below, when we discuss WithGroup.

The `WithGroup` method

Logger.WithGroup calls Handler.WithGroup directly, with the same argument, the group name. A handler should remember the name so it can use it to qualify all subsequent attributes.

The signature of WithGroup is:

WithGroup(name string) Handler

Like WithAttrs, the WithGroup method should return a new handler, not modify the receiver.

The implementations of WithGroup and WithAttrs are intertwined. Consider this statement:

logger = logger.WithGroup("g1").With("k1", 1).WithGroup("g2").With("k2", 2)

Subsequent logger output should qualify key "k1" with group "g1", and key "k2" with groups "g1" and "g2". The order of the Logger.WithGroup and Logger.With calls must be respected by the implementations of Handler.WithGroup and Handler.WithAttrs.

We will look at two implementations of WithGroup and WithAttrs, one that pre-formats and one that doesn't.

Without pre-formatting

Our first implementation will collect the information from WithGroup and WithAttrs calls to build up a slice of group names and attribute lists, and loop over that slice in Handle. We start with a struct that can hold either a group name or some attributes:

// groupOrAttrs holds either a group name or a list of slog.Attrs.
type groupOrAttrs struct {
	group string      // group name if non-empty
	attrs []slog.Attr // attrs if non-empty
}

Then we add a slice of groupOrAttrs to our handler:

type IndentHandler struct {
	opts Options
	goas []groupOrAttrs
	mu   *sync.Mutex
	out  io.Writer
}

As stated above, The WithGroup and WithAttrs methods should not modify their receiver. To that end, we define a method that will copy our handler struct and append one groupOrAttrs to the copy:

func (h *IndentHandler) withGroupOrAttrs(goa groupOrAttrs) *IndentHandler {
	h2 := *h
	h2.goas = make([]groupOrAttrs, len(h.goas)+1)
	copy(h2.goas, h.goas)
	h2.goas[len(h2.goas)-1] = goa
	return &h2
}

Most of the fields of IndentHandler can be copied shallowly, but the slice of groupOrAttrs requires a deep copy, or the clone and the original will point to the same underlying array. If we used append instead of making an explicit copy, we would introduce that subtle aliasing bug.

The With methods are easy to write using withGroupOrAttrs:

func (h *IndentHandler) WithGroup(name string) slog.Handler {
	if name == "" {
		return h
	}
	return h.withGroupOrAttrs(groupOrAttrs{group: name})
}

func (h *IndentHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	if len(attrs) == 0 {
		return h
	}
	return h.withGroupOrAttrs(groupOrAttrs{attrs: attrs})
}

The Handle method can now process the groupOrAttrs slice after the built-in attributes and before the ones in the record:

func (h *IndentHandler) Handle(ctx context.Context, r slog.Record) error {
	buf := make([]byte, 0, 1024)
	if !r.Time.IsZero() {
		buf = h.appendAttr(buf, slog.Time(slog.TimeKey, r.Time), 0)
	}
	buf = h.appendAttr(buf, slog.Any(slog.LevelKey, r.Level), 0)
	if r.PC != 0 {
		fs := runtime.CallersFrames([]uintptr{r.PC})
		f, _ := fs.Next()
		buf = h.appendAttr(buf, slog.String(slog.SourceKey, fmt.Sprintf("%s:%d", f.File, f.Line)), 0)
	}
	buf = h.appendAttr(buf, slog.String(slog.MessageKey, r.Message), 0)
	indentLevel := 0
	// Handle state from WithGroup and WithAttrs.
	goas := h.goas
	if r.NumAttrs() == 0 {
		// If the record has no Attrs, remove groups at the end of the list; they are empty.
		for len(goas) > 0 && goas[len(goas)-1].group != "" {
			goas = goas[:len(goas)-1]
		}
	}
	for _, goa := range goas {
		if goa.group != "" {
			buf = fmt.Appendf(buf, "%*s%s:\n", indentLevel*4, "", goa.group)
			indentLevel++
		} else {
			for _, a := range goa.attrs {
				buf = h.appendAttr(buf, a, indentLevel)
			}
		}
	}
	r.Attrs(func(a slog.Attr) bool {
		buf = h.appendAttr(buf, a, indentLevel)
		return true
	})
	buf = append(buf, "---\n"...)
	h.mu.Lock()
	defer h.mu.Unlock()
	_, err := h.out.Write(buf)
	return err
}

You may have noticed that our algorithm for recording WithGroup and WithAttrs information is quadratic in the number of calls to those methods, because of the repeated copying. That is unlikely to matter in practice, but if it bothers you, you can use a linked list instead, which Handle will have to reverse or visit recursively. See the github.com/jba/slog/withsupport package for an implementation.

Getting the mutex right

Let us revisit the last few lines of Handle:

h.mu.Lock()
defer h.mu.Unlock()
_, err := h.out.Write(buf)
return err

This code hasn't changed, but we can now appreciate why h.mu is a pointer to a sync.Mutex. Both WithGroup and WithAttrs copy the handler. Both copies point to the same mutex. If the copy and the original used different mutexes and were used concurrently, then their output could be interleaved, or some output could be lost. Code like this:

l2 := l1.With("a", 1)
go l1.Info("hello")
l2.Info("goodbye")

could produce output like this:

hegoollo a=dbye1

See this bug report for more detail.

With pre-formatting

Our second version of the WithGroup and WithAttrs methods provides pre-formatting. This implementation is more complicated than the previous one. Is the extra complexity worth it? That depends on your circumstances, but here is one circumstance where it might be. Say that you wanted your server to log a lot of information about an incoming request with every log message that happens during that request. A typical handler might look something like this:

func (s *Server) handleWidgets(w http.ResponseWriter, r *http.Request) {
    logger := s.logger.With(
        "url", r.URL,
        "traceID": r.Header.Get("X-Cloud-Trace-Context"),
        // many other attributes
        )
    // ...
}

A single handleWidgets request might generate hundreds of log lines. For instance, it might contain code like this:

for _, w := range widgets {
    logger.Info("processing widget", "name", w.Name)
    // ...
}

For every such line, the Handle method we wrote above will format all the attributes that were added using With above, in addition to the ones on the log line itself.

Maybe all that extra work doesn't slow down your server significantly, because it does so much other work that time spent logging is just noise. But perhaps your server is fast enough that all that extra formatting appears high up in your CPU profiles. That is when pre-formatting can make a big difference, by formatting the attributes in a call to With just once.

To pre-format the arguments to WithAttrs, we need to keep track of some additional state in the IndentHandler struct.

type IndentHandler struct {
	opts           Options
	preformatted   []byte   // data from WithGroup and WithAttrs
	unopenedGroups []string // groups from WithGroup that haven't been opened
	indentLevel    int      // same as number of opened groups so far
	mu             *sync.Mutex
	out            io.Writer
}

Mainly, we need a buffer to hold the pre-formatted data. But we also need to keep track of which groups we've seen but haven't output yet. We'll call those groups "unopened." We also need to track how many groups we've opened, which we can do with a simple counter, since an opened group's only effect is to change the indentation level.

This WithGroup is a lot like the previous one: it just remembers the new group, which is unopened initially.

func (h *IndentHandler) WithGroup(name string) slog.Handler {
	if name == "" {
		return h
	}
	h2 := *h
	// Add an unopened group to h2 without modifying h.
	h2.unopenedGroups = make([]string, len(h.unopenedGroups)+1)
	copy(h2.unopenedGroups, h.unopenedGroups)
	h2.unopenedGroups[len(h2.unopenedGroups)-1] = name
	return &h2
}

WithAttrs does all the pre-formatting:

func (h *IndentHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
	if len(attrs) == 0 {
		return h
	}
	h2 := *h
	// Force an append to copy the underlying array.
	pre := slices.Clip(h.preformatted)
	// Add all groups from WithGroup that haven't already been added.
	h2.preformatted = h2.appendUnopenedGroups(pre, h2.indentLevel)
	// Each of those groups increased the indent level by 1.
	h2.indentLevel += len(h2.unopenedGroups)
	// Now all groups have been opened.
	h2.unopenedGroups = nil
	// Pre-format the attributes.
	for _, a := range attrs {
		h2.preformatted = h2.appendAttr(h2.preformatted, a, h2.indentLevel)
	}
	return &h2
}

func (h *IndentHandler) appendUnopenedGroups(buf []byte, indentLevel int) []byte {
	for _, g := range h.unopenedGroups {
		buf = fmt.Appendf(buf, "%*s%s:\n", indentLevel*4, "", g)
		indentLevel++
	}
	return buf
}

It first opens any unopened groups. This handles calls like:

logger.WithGroup("g").WithGroup("h").With("a", 1)

Here, WithAttrs must output "g" and "h" before "a". Since a group established by WithGroup is in effect for the rest of the log line, WithAttrs increments the indentation level for each group it opens.

Lastly, WithAttrs formats its argument attributes, using the same appendAttr method we saw above.

It's the Handle method's job to insert the pre-formatted material in the right place, which is after the built-in attributes and before the ones in the record:

func (h *IndentHandler) Handle(ctx context.Context, r slog.Record) error {
	buf := make([]byte, 0, 1024)
	if !r.Time.IsZero() {
		buf = h.appendAttr(buf, slog.Time(slog.TimeKey, r.Time), 0)
	}
	buf = h.appendAttr(buf, slog.Any(slog.LevelKey, r.Level), 0)
	if r.PC != 0 {
		fs := runtime.CallersFrames([]uintptr{r.PC})
		f, _ := fs.Next()
		buf = h.appendAttr(buf, slog.String(slog.SourceKey, fmt.Sprintf("%s:%d", f.File, f.Line)), 0)
	}
	buf = h.appendAttr(buf, slog.String(slog.MessageKey, r.Message), 0)
	// Insert preformatted attributes just after built-in ones.
	buf = append(buf, h.preformatted...)
	if r.NumAttrs() > 0 {
		buf = h.appendUnopenedGroups(buf, h.indentLevel)
		r.Attrs(func(a slog.Attr) bool {
			buf = h.appendAttr(buf, a, h.indentLevel+len(h.unopenedGroups))
			return true
		})
	}
	buf = append(buf, "---\n"...)
	h.mu.Lock()
	defer h.mu.Unlock()
	_, err := h.out.Write(buf)
	return err
}

It must also open any groups that haven't yet been opened. The logic covers log lines like this one:

logger.WithGroup("g").Info("msg", "a", 1)

where "g" is unopened before Handle is called and must be written to produce the correct output:

level: INFO
msg: "msg"
g:
    a: 1

The check for r.NumAttrs() > 0 handles this case:

logger.WithGroup("g").Info("msg")

Here there are no record attributes, so no group to open.

Testing

The Handler contract specifies several constraints on handlers. To verify that your handler follows these rules and generally produces proper output, use the testing/slogtest package.

That package's TestHandler function takes an instance of your handler and a function that returns its output formatted as a slice of maps. Here is the test function for our example handler:

func TestSlogtest(t *testing.T) {
	var buf bytes.Buffer
	err := slogtest.TestHandler(New(&buf, nil), func() []map[string]any {
		return parseLogEntries(t, buf.Bytes())
	})
	if err != nil {
		t.Error(err)
	}
}

Calling TestHandler is easy. The hard part is parsing your handler's output. TestHandler calls your handler multiple times, resulting in a sequence of log entries. It is your job to parse each entry into a map[string]any. A group in an entry should appear as a nested map.

If your handler outputs a standard format, you can use an existing parser. For example, if your handler outputs one JSON object per line, then you can split the output into lines and call encoding/json.Unmarshal on each. Parsers for other formats that can unmarshal into a map can be used out of the box. Our example output is enough like YAML so that we can use the gopkg.in/yaml.v3 package to parse it:

func parseLogEntries(t *testing.T, data []byte) []map[string]any {
	entries := bytes.Split(data, []byte("---\n"))
	entries = entries[:len(entries)-1] // last one is empty
	var ms []map[string]any
	for _, e := range entries {
		var m map[string]any
		if err := yaml.Unmarshal([]byte(e), &m); err != nil {
			t.Fatal(err)
		}
		ms = append(ms, m)
	}
	return ms
}

If you have to write your own parser, it can be far from perfect. The slogtest package uses only a handful of simple attributes. (It is testing handler conformance, not parsing.) Your parser can ignore edge cases like whitespace and newlines in keys and values. Before switching to a YAML parser, we wrote an adequate custom parser in 65 lines.

General considerations

Copying records

Most handlers won't need to copy the slog.Record that is passed to the Handle method. Those that do must take special care in some cases.

A handler can make a single copy of a Record with an ordinary Go assignment, channel send or function call if it doesn't retain the original. But if its actions result in more than one copy, it should call Record.Clone to make the copies so that they don't share state. This Handle method passes the record to a single handler, so it doesn't require Clone:

type Handler1 struct {
    h slog.Handler
    // ...
}

func (h *Handler1) Handle(ctx context.Context, r slog.Record) error {
    return h.h.Handle(ctx, r)
}

This Handle method might pass the record to more than one handler, so it should use Clone:

type Handler2 struct {
    hs []slog.Handler
    // ...
}

func (h *Handler2) Handle(ctx context.Context, r slog.Record) error {
    for _, hh := range h.hs {
        if err := hh.Handle(ctx, r.Clone()); err != nil {
            return err
        }
    }
    return nil
}

Concurrency safety

A handler must work properly when a single Logger is shared among several goroutines. That means that mutable state must be protected with a lock or some other mechanism. In practice, this is not hard to achieve, because many handlers won't have any mutable state.

The Enabled method typically consults only its arguments and a configured level. The level is often either set once initially, or is held in a LevelVar, which is already concurrency-safe.
The WithAttrs and WithGroup methods should not modify the receiver, for reasons discussed above.
The Handle method typically works only with its arguments and stored fields.

Calls to output methods like io.Writer.Write should be synchronized unless it can be verified that no locking is needed. As we saw in our example, storing a pointer to a mutex enables a logger and all of its clones to synchronize with each other. Beware of facile claims like "Unix writes are atomic"; the situation is a lot more nuanced than that.

Some handlers have legitimate reasons for keeping state. For example, a handler might support a SetLevel method to change its configured level dynamically. Or it might output the time between successive calls to Handle, which requires a mutable field holding the last output time. Synchronize all accesses to such fields, both reads and writes.

The built-in handlers have no directly mutable state. They use a mutex only to sequence calls to their contained io.Writer.

Robustness

Logging is often the debugging technique of last resort. When it is difficult or impossible to inspect a system, as is typically the case with a production server, logs provide the most detailed way to understand its behavior. Therefore, your handler should be robust to bad input.

For example, the usual advice when a function discovers a problem, like an invalid argument, is to panic or return an error. The built-in handlers do not follow that advice. Few things are more frustrating than being unable to debug a problem that causes logging to fail; it is better to produce some output, however imperfect, than to produce none at all. That is why methods like Logger.Info convert programming bugs in their list of key-value pairs, like missing values or malformed keys, into Attrs that contain as much information as possible.

One place to avoid panics is in processing attribute values. A handler that wants to format a value will probably switch on the value's kind:

switch attr.Value.Kind() {
case KindString: ...
case KindTime: ...
// all other Kinds
default: ...
}

What should happen in the default case, when the handler encounters a Kind that it doesn't know about? The built-in handlers try to muddle through by using the result of the value's String method, as our example handler does. They do not panic or return an error. Your own handlers might in addition want to report the problem through your production monitoring or error-tracking telemetry system. The most likely explanation for the issue is that a newer version of the slog package added a new Kind—a backwards-compatible change under the Go 1 Compatibility Promise—and the handler wasn't updated. That is certainly a problem, but it shouldn't deprive readers from seeing the rest of the log output.

There is one circumstance where returning an error from Handler.Handle is appropriate. If the output operation itself fails, the best course of action is to report this failure by returning the error. For instance, the last two lines of the built-in Handle methods are

_, err := h.w.Write(*state.buf)
return err

Although the output methods of Logger ignore the error, one could write a handler that does something with it, perhaps falling back to writing to standard error.

Speed

Most programs don't need fast logging. Before making your handler fast, gather data—preferably production data, not benchmark comparisons—that demonstrates that it needs to be fast. Avoid premature optimization.

If you need a fast handler, start with pre-formatting. It may provide dramatic speed-ups in cases where a single call to Logger.With is followed by many calls to the resulting logger.

If log output is the bottleneck, consider making your handler asynchronous. Do the minimal amount of processing in the handler, then send the record and other information over a channel. Another goroutine can collect the incoming log entries and write them in bulk and in the background. You might want to preserve the option to log synchronously so you can see all the log output to debug a crash.

Allocation is often a major cause of a slow system. The slog package already works hard at minimizing allocations. If your handler does its own allocation, and profiling shows it to be a problem, then see if you can minimize it.

One simple change you can make is to replace calls to fmt.Sprintf or fmt.Appendf with direct appends to the buffer. For example, our IndentHandler appends string attributes to the buffer like so:

buf = fmt.Appendf(buf, "%s: %q\n", a.Key, a.Value.String())

As of Go 1.21, that results in two allocations, one for each argument passed to an any parameter. We can get that down to zero by using append directly:

buf = append(buf, a.Key...)
buf = append(buf, ": "...)
buf = strconv.AppendQuote(buf, a.Value.String())
buf = append(buf, '\n')

Another worthwhile change is to use a sync.Pool to manage the one chunk of memory that most handlers need: the []byte buffer holding the formatted output.

Our example Handle method began with this line:

buf := make([]byte, 0, 1024)

As we said above, providing a large initial capacity avoids repeated copying and re-allocation of the slice as it grows, reducing the number of allocations to one. But we can get it down to zero in the steady state by keeping a global pool of buffers. Initially, the pool will be empty and new buffers will be allocated. But eventually, assuming the number of concurrent log calls reaches a steady maximum, there will be enough buffers in the pool to share among all the ongoing Handler calls. As long as no log entry grows past a buffer's capacity, there will be no allocations from the garbage collector's point of view.

We will hide our pool behind a pair of functions, allocBuf and freeBuf. The single line to get a buffer at the top of Handle becomes two lines:

bufp := allocBuf()
defer freeBuf(bufp)

One of the subtleties involved in making a sync.Pool of slices is suggested by the variable name bufp: your pool must deal in pointers to slices, not the slices themselves. Pooled values must always be pointers. If they aren't, then the any arguments and return values of the sync.Pool methods will themselves cause allocations, defeating the purpose of pooling.

There are two ways to proceed with our slice pointer: we can replace buf with *bufp throughout our function, or we can dereference it and remember to re-assign it before freeing:

bufp := allocBuf()
buf := *bufp
defer func() {
	*bufp = buf
	freeBuf(bufp)
}()

Here is our pool and its associated functions:

var bufPool = sync.Pool{
	New: func() any {
		b := make([]byte, 0, 1024)
		return &b
	},
}

func allocBuf() *[]byte {
	return bufPool.Get().(*[]byte)
}

func freeBuf(b *[]byte) {
	// To reduce peak allocation, return only smaller buffers to the pool.
	const maxBufferSize = 16 << 10
	if cap(*b) <= maxBufferSize {
		*b = (*b)[:0]
		bufPool.Put(b)
	}
}

The pool's New function does the same thing as the original code: create a byte slice with 0 length and plenty of capacity. The allocBuf function just type-asserts the result of the pool's Get method.

The freeBuf method truncates the buffer before putting it back in the pool, so that allocBuf always returns zero-length slices. It also implements an important optimization: it doesn't return large buffers to the pool. To see why this important, consider what would happen if there were a single, unusually large log entry—say one that was a megabyte when formatted. If that megabyte-sized buffer were put in the pool, it could remain there indefinitely, constantly being reused, but with most of its capacity wasted. The extra memory might never be used again by the handler, and since it was in the handler's pool, it might never be garbage-collected for reuse elsewhere. We can avoid that situation by excluding large buffers from the pool.

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