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Package logr

import "github.com/go-logr/logr"
Overview
Index
Examples
Subdirectories

Overview ▾

Package logr defines a general-purpose logging API and abstract interfaces to back that API. Packages in the Go ecosystem can depend on this package, while callers can implement logging with whatever backend is appropriate.

Usage

Logging is done using a Logger instance. Logger is a concrete type with methods, which defers the actual logging to a LogSink interface. The main methods of Logger are Info() and Error(). Arguments to Info() and Error() are key/value pairs rather than printf-style formatted strings, emphasizing "structured logging".

With Go's standard log package, we might write: 

log.Printf("setting target value %s", targetValue)

With logr's structured logging, we'd write: 

logger.Info("setting target", "value", targetValue)

Errors are much the same. Instead of: 

log.Printf("failed to open the pod bay door for user %s: %v", user, err)

We'd write: 

logger.Error(err, "failed to open the pod bay door", "user", user)

Info() and Error() are very similar, but they are separate methods so that LogSink implementations can choose to do things like attach additional information (such as stack traces) on calls to Error(). Error() messages are always logged, regardless of the current verbosity. If there is no error instance available, passing nil is valid.

Verbosity

Often we want to log information only when the application in "verbose mode". To write log lines that are more verbose, Logger has a V() method. The higher the V-level of a log line, the less critical it is considered. Log-lines with V-levels that are not enabled (as per the LogSink) will not be written. Level V(0) is the default, and logger.V(0).Info() has the same meaning as logger.Info(). Negative V-levels have the same meaning as V(0). Error messages do not have a verbosity level and are always logged.

Where we might have written: 

if flVerbose >= 2 {
    log.Printf("an unusual thing happened")
}

We can write: 

logger.V(2).Info("an unusual thing happened")

Logger Names

Logger instances can have name strings so that all messages logged through that instance have additional context. For example, you might want to add a subsystem name: 

logger.WithName("compactor").Info("started", "time", time.Now())

The WithName() method returns a new Logger, which can be passed to constructors or other functions for further use. Repeated use of WithName() will accumulate name "segments". These name segments will be joined in some way by the LogSink implementation. It is strongly recommended that name segments contain simple identifiers (letters, digits, and hyphen), and do not contain characters that could muddle the log output or confuse the joining operation (e.g. whitespace, commas, periods, slashes, brackets, quotes, etc).

Saved Values

Logger instances can store any number of key/value pairs, which will be logged alongside all messages logged through that instance. For example, you might want to create a Logger instance per managed object:

With the standard log package, we might write: 

log.Printf("decided to set field foo to value %q for object %s/%s",
    targetValue, object.Namespace, object.Name)

With logr we'd write: 

// Elsewhere: set up the logger to log the object name.
obj.logger = mainLogger.WithValues(
    "name", obj.name, "namespace", obj.namespace)

// later on...
obj.logger.Info("setting foo", "value", targetValue)

Best Practices

Logger has very few hard rules, with the goal that LogSink implementations might have a lot of freedom to differentiate. There are, however, some things to consider.

The log message consists of a constant message attached to the log line. This should generally be a simple description of what's occurring, and should never be a format string. Variable information can then be attached using named values.

Keys are arbitrary strings, but should generally be constant values. Values may be any Go value, but how the value is formatted is determined by the LogSink implementation.

Logger instances are meant to be passed around by value. Code that receives such a value can call its methods without having to check whether the instance is ready for use.

The zero logger (= Logger{}) is identical to Discard() and discards all log entries. Code that receives a Logger by value can simply call it, the methods will never crash. For cases where passing a logger is optional, a pointer to Logger should be used.

Key Naming Conventions

Keys are not strictly required to conform to any specification or regex, but it is recommended that they:

  • be human-readable and meaningful (not auto-generated or simple ordinals)
  • be constant (not dependent on input data)
  • contain only printable characters
  • not contain whitespace or punctuation
  • use lower case for simple keys and lowerCamelCase for more complex ones

These guidelines help ensure that log data is processed properly regardless of the log implementation. For example, log implementations will try to output JSON data or will store data for later database (e.g. SQL) queries.

While users are generally free to use key names of their choice, it's generally best to avoid using the following keys, as they're frequently used by implementations:

  • "caller": the calling information (file/line) of a particular log line
  • "error": the underlying error value in the `Error` method
  • "level": the log level
  • "logger": the name of the associated logger
  • "msg": the log message
  • "stacktrace": the stack trace associated with a particular log line or error (often from the `Error` message)
  • "ts": the timestamp for a log line

Implementations are encouraged to make use of these keys to represent the above concepts, when necessary (for example, in a pure-JSON output form, it would be necessary to represent at least message and timestamp as ordinary named values).

Break Glass

Implementations may choose to give callers access to the underlying logging implementation. The recommended pattern for this is: 

// Underlier exposes access to the underlying logging implementation.
// Since callers only have a logr.Logger, they have to know which
// implementation is in use, so this interface is less of an abstraction
// and more of way to test type conversion.
type Underlier interface {
    GetUnderlying() <underlying-type>
}

Logger grants access to the sink to enable type assertions like this: 

func DoSomethingWithImpl(log logr.Logger) {
    if underlier, ok := log.GetSink().(impl.Underlier); ok {
       implLogger := underlier.GetUnderlying()
       ...
    }
}

Custom `With*` functions can be implemented by copying the complete Logger struct and replacing the sink in the copy: 

// WithFooBar changes the foobar parameter in the log sink and returns a
// new logger with that modified sink.  It does nothing for loggers where
// the sink doesn't support that parameter.
func WithFoobar(log logr.Logger, foobar int) logr.Logger {
   if foobarLogSink, ok := log.GetSink().(FoobarSink); ok {
      log = log.WithSink(foobarLogSink.WithFooBar(foobar))
   }
   return log
}

Don't use New to construct a new Logger with a LogSink retrieved from an existing Logger. Source code attribution might not work correctly and unexported fields in Logger get lost.

Beware that the same LogSink instance may be shared by different logger instances. Calling functions that modify the LogSink will affect all of those.

▾ Example

Code:

l := NewStdoutLogger()
l.Info("default info log", "stringVal", "value", "intVal", 12345)
l.V(0).Info("V(0) info log", "stringVal", "value", "intVal", 12345)
l.Error(fmt.Errorf("an error"), "error log", "stringVal", "value", "intVal", 12345)

Output:

"level"=0 "msg"="default info log" "stringVal"="value" "intVal"=12345
"level"=0 "msg"="V(0) info log" "stringVal"="value" "intVal"=12345
"msg"="error log" "error"="an error" "stringVal"="value" "intVal"=12345

Index ▾

Package files

context.go context_slog.go discard.go logr.go sloghandler.go slogr.go slogsink.go

func FromContextAsSlogLogger 

func FromContextAsSlogLogger(ctx context.Context) *slog.Logger

FromContextAsSlogLogger returns a slog.Logger from ctx or nil if no such Logger is found.

func NewContext 

func NewContext(ctx context.Context, logger Logger) context.Context

NewContext returns a new Context, derived from ctx, which carries the provided Logger.

func NewContextWithSlogLogger 

func NewContextWithSlogLogger(ctx context.Context, logger *slog.Logger) context.Context

NewContextWithSlogLogger returns a new Context, derived from ctx, which carries the provided slog.Logger.

func ToSlogHandler 

func ToSlogHandler(logger Logger) slog.Handler

ToSlogHandler returns a slog.Handler which writes to the same sink as the Logger.

The returned logger writes all records with level >= slog.LevelError as error log entries with LogSink.Error, regardless of the verbosity level of the Logger: 

logger := <some Logger with 0 as verbosity level>
slog.New(ToSlogHandler(logger.V(10))).Error(...) -> logSink.Error(...)

The level of all other records gets reduced by the verbosity level of the Logger and the result is negated. If it happens to be negative, then it gets replaced by zero because a LogSink is not expected to handled negative levels: 

slog.New(ToSlogHandler(logger)).Debug(...) -> logger.GetSink().Info(level=4, ...)
slog.New(ToSlogHandler(logger)).Warning(...) -> logger.GetSink().Info(level=0, ...)
slog.New(ToSlogHandler(logger)).Info(...) -> logger.GetSink().Info(level=0, ...)
slog.New(ToSlogHandler(logger.V(4))).Info(...) -> logger.GetSink().Info(level=4, ...)

▾ Example

Code:

funcrLogger := funcr.New(func(prefix, args string) {
    if prefix != "" {
        fmt.Fprintln(os.Stdout, prefix, args)
    } else {
        fmt.Fprintln(os.Stdout, args)
    }
}, funcr.Options{
    Verbosity: 10,
})

slogLogger := slog.New(logr.ToSlogHandler(funcrLogger))
slogLogger.Info("hello world")
slogLogger.Error("ignore me", "err", errors.New("fake error"))
slogLogger.With("x", 1, "y", 2).WithGroup("group").With("str", "abc").Warn("with values and group")

slogLogger = slog.New(logr.ToSlogHandler(funcrLogger.V(int(-slog.LevelDebug))))
slogLogger.Info("info message reduced to debug level")

Output:

"level"=0 "msg"="hello world"
"msg"="ignore me" "error"=null "err"="fake error"
"level"=0 "msg"="with values and group" "x"=1 "y"=2 "group"={"str"="abc"}
"level"=4 "msg"="info message reduced to debug level"

type CallDepthLogSink

CallDepthLogSink represents a LogSink that knows how to climb the call stack to identify the original call site and can offset the depth by a specified number of frames. This is useful for users who have helper functions between the "real" call site and the actual calls to Logger methods. Implementations that log information about the call site (such as file, function, or line) would otherwise log information about the intermediate helper functions.

This is an optional interface and implementations are not required to support it. 

type CallDepthLogSink interface {
    // WithCallDepth returns a LogSink that will offset the call
    // stack by the specified number of frames when logging call
    // site information.
    //
    // If depth is 0, the LogSink should skip exactly the number
    // of call frames defined in RuntimeInfo.CallDepth when Info
    // or Error are called, i.e. the attribution should be to the
    // direct caller of Logger.Info or Logger.Error.
    //
    // If depth is 1 the attribution should skip 1 call frame, and so on.
    // Successive calls to this are additive.
    WithCallDepth(depth int) LogSink
}

type CallStackHelperLogSink

CallStackHelperLogSink represents a LogSink that knows how to climb the call stack to identify the original call site and can skip intermediate helper functions if they mark themselves as helper. Go's testing package uses that approach.

This is useful for users who have helper functions between the "real" call site and the actual calls to Logger methods. Implementations that log information about the call site (such as file, function, or line) would otherwise log information about the intermediate helper functions.

This is an optional interface and implementations are not required to support it. Implementations that choose to support this must not simply implement it as WithCallDepth(1), because Logger.WithCallStackHelper will call both methods if they are present. This should only be implemented for LogSinks that actually need it, as with testing.T. 

type CallStackHelperLogSink interface {
    // GetCallStackHelper returns a function that must be called
    // to mark the direct caller as helper function when logging
    // call site information.
    GetCallStackHelper() func()
}

type LogSink

LogSink represents a logging implementation. End-users will generally not interact with this type. 

type LogSink interface {
    // Init receives optional information about the logr library for LogSink
    // implementations that need it.
    Init(info RuntimeInfo)

    // Enabled tests whether this LogSink is enabled at the specified V-level.
    // For example, commandline flags might be used to set the logging
    // verbosity and disable some info logs.
    Enabled(level int) bool

    // Info logs a non-error message with the given key/value pairs as context.
    // The level argument is provided for optional logging.  This method will
    // only be called when Enabled(level) is true. See Logger.Info for more
    // details.
    Info(level int, msg string, keysAndValues ...any)

    // Error logs an error, with the given message and key/value pairs as
    // context.  See Logger.Error for more details.
    Error(err error, msg string, keysAndValues ...any)

    // WithValues returns a new LogSink with additional key/value pairs.  See
    // Logger.WithValues for more details.
    WithValues(keysAndValues ...any) LogSink

    // WithName returns a new LogSink with the specified name appended.  See
    // Logger.WithName for more details.
    WithName(name string) LogSink
}

type Logger

Logger is an interface to an abstract logging implementation. This is a concrete type for performance reasons, but all the real work is passed on to a LogSink. Implementations of LogSink should provide their own constructors that return Logger, not LogSink.

The underlying sink can be accessed through GetSink and be modified through WithSink. This enables the implementation of custom extensions (see "Break Glass" in the package documentation). Normally the sink should be used only indirectly. 

type Logger struct {
    // contains filtered or unexported fields
}

func Discard 

func Discard() Logger

Discard returns a Logger that discards all messages logged to it. It can be used whenever the caller is not interested in the logs. Logger instances produced by this function always compare as equal.

func FromContext 

func FromContext(ctx context.Context) (Logger, error)

FromContext returns a Logger from ctx or an error if no Logger is found.

func FromContextOrDiscard 

func FromContextOrDiscard(ctx context.Context) Logger

FromContextOrDiscard returns a Logger from ctx. If no Logger is found, this returns a Logger that discards all log messages.

func FromSlogHandler 

func FromSlogHandler(handler slog.Handler) Logger

FromSlogHandler returns a Logger which writes to the slog.Handler.

The logr verbosity level is mapped to slog levels such that V(0) becomes slog.LevelInfo and V(4) becomes slog.LevelDebug.

▾ Example

Code:

logrLogger := logr.FromSlogHandler(slog.NewTextHandler(os.Stdout, debugWithoutTime))

logrLogger.Info("hello world")
logrLogger.Error(errors.New("fake error"), "ignore me")
logrLogger.WithValues("x", 1, "y", 2).WithValues("str", "abc").WithName("foo").WithName("bar").V(4).Info("with values, verbosity and name")

Output:

level=INFO msg="hello world"
level=ERROR msg="ignore me" err="fake error"
level=DEBUG msg="with values, verbosity and name" x=1 y=2 str=abc logger=foo/bar

func New 

func New(sink LogSink) Logger

New returns a new Logger instance. This is primarily used by libraries implementing LogSink, rather than end users. Passing a nil sink will create a Logger which discards all log lines.

func (Logger) Enabled 

func (l Logger) Enabled() bool

Enabled tests whether this Logger is enabled. For example, commandline flags might be used to set the logging verbosity and disable some info logs.

▾ Example

Code:

l := NewStdoutLogger()
if loggerV := l.V(5); loggerV.Enabled() {
    // Do something expensive.
    loggerV.Info("this is an expensive log message")
}

func (Logger) Error 

func (l Logger) Error(err error, msg string, keysAndValues ...any)

Error logs an error, with the given message and key/value pairs as context. It functions similarly to Info, but may have unique behavior, and should be preferred for logging errors (see the package documentations for more information). The log message will always be emitted, regardless of verbosity level.

The msg argument should be used to add context to any underlying error, while the err argument should be used to attach the actual error that triggered this log line, if present. The err parameter is optional and nil may be passed instead of an error instance.

▾ Example

Code:

l := NewStdoutLogger()
l.Error(fmt.Errorf("the error"), "this is an error log", "stringVal", "value", "intVal", 12345)
l.Error(nil, "this is an error log with nil error", "stringVal", "value", "intVal", 12345)

Output:

"msg"="this is an error log" "error"="the error" "stringVal"="value" "intVal"=12345
"msg"="this is an error log with nil error" "error"=null "stringVal"="value" "intVal"=12345

func (Logger) GetSink 

func (l Logger) GetSink() LogSink

GetSink returns the stored sink.

func (Logger) GetV 

func (l Logger) GetV() int

GetV returns the verbosity level of the logger. If the logger's LogSink is nil as in the Discard logger, this will always return 0.

func (Logger) Info 

func (l Logger) Info(msg string, keysAndValues ...any)

Info logs a non-error message with the given key/value pairs as context.

The msg argument should be used to add some constant description to the log line. The key/value pairs can then be used to add additional variable information. The key/value pairs must alternate string keys and arbitrary values.

▾ Example

Code:

l := NewStdoutLogger()
l.Info("this is a V(0)-equivalent info log", "stringVal", "value", "intVal", 12345)

Output:

"level"=0 "msg"="this is a V(0)-equivalent info log" "stringVal"="value" "intVal"=12345

func (Logger) IsZero 

func (l Logger) IsZero() bool

IsZero returns true if this logger is an uninitialized zero value

func (Logger) V 

func (l Logger) V(level int) Logger

V returns a new Logger instance for a specific verbosity level, relative to this Logger. In other words, V-levels are additive. A higher verbosity level means a log message is less important. Negative V-levels are treated as 0.

▾ Example

Code:

l := NewStdoutLogger()
l.V(0).Info("V(0) info log")
l.V(1).Info("V(1) info log")
l.V(2).Info("V(2) info log")

Output:

"level"=0 "msg"="V(0) info log"

func (Logger) WithCallDepth 

func (l Logger) WithCallDepth(depth int) Logger

WithCallDepth returns a Logger instance that offsets the call stack by the specified number of frames when logging call site information, if possible. This is useful for users who have helper functions between the "real" call site and the actual calls to Logger methods. If depth is 0 the attribution should be to the direct caller of this function. If depth is 1 the attribution should skip 1 call frame, and so on. Successive calls to this are additive.

If the underlying log implementation supports a WithCallDepth(int) method, it will be called and the result returned. If the implementation does not support CallDepthLogSink, the original Logger will be returned.

To skip one level, WithCallStackHelper() should be used instead of WithCallDepth(1) because it works with implementions that support the CallDepthLogSink and/or CallStackHelperLogSink interfaces.

func (Logger) WithCallStackHelper 

func (l Logger) WithCallStackHelper() (func(), Logger)

WithCallStackHelper returns a new Logger instance that skips the direct caller when logging call site information, if possible. This is useful for users who have helper functions between the "real" call site and the actual calls to Logger methods and want to support loggers which depend on marking each individual helper function, like loggers based on testing.T.

In addition to using that new logger instance, callers also must call the returned function.

If the underlying log implementation supports a WithCallDepth(int) method, WithCallDepth(1) will be called to produce a new logger. If it supports a WithCallStackHelper() method, that will be also called. If the implementation does not support either of these, the original Logger will be returned.

func (Logger) WithName 

func (l Logger) WithName(name string) Logger

WithName returns a new Logger instance with the specified name element added to the Logger's name. Successive calls with WithName append additional suffixes to the Logger's name. It's strongly recommended that name segments contain only letters, digits, and hyphens (see the package documentation for more information).

▾ Example

Code:

l := NewStdoutLogger()
l = l.WithName("name1")
l.Info("this is an info log", "stringVal", "value", "intVal", 12345)
l = l.WithName("name2")
l.Info("this is an info log", "stringVal", "value", "intVal", 12345)

Output:

name1: "level"=0 "msg"="this is an info log" "stringVal"="value" "intVal"=12345
name1/name2: "level"=0 "msg"="this is an info log" "stringVal"="value" "intVal"=12345

func (Logger) WithSink 

func (l Logger) WithSink(sink LogSink) Logger

WithSink returns a copy of the logger with the new sink.

func (Logger) WithValues 

func (l Logger) WithValues(keysAndValues ...any) Logger

WithValues returns a new Logger instance with additional key/value pairs. See Info for documentation on how key/value pairs work.

▾ Example

Code:

l := NewStdoutLogger()
l = l.WithValues("stringVal", "value", "intVal", 12345)
l = l.WithValues("boolVal", true)
l.Info("this is an info log", "floatVal", 3.1415)

Output:

"level"=0 "msg"="this is an info log" "stringVal"="value" "intVal"=12345 "boolVal"=true "floatVal"=3.1415

type Marshaler

Marshaler is an optional interface that logged values may choose to implement. Loggers with structured output, such as JSON, should log the object return by the MarshalLog method instead of the original value. 

type Marshaler interface {
    // MarshalLog can be used to:
    //   - ensure that structs are not logged as strings when the original
    //     value has a String method: return a different type without a
    //     String method
    //   - select which fields of a complex type should get logged:
    //     return a simpler struct with fewer fields
    //   - log unexported fields: return a different struct
    //     with exported fields
    //
    // It may return any value of any type.
    MarshalLog() any
}

▾ Example

Code:

package logr_test

import (
    "github.com/go-logr/logr"
)

// ObjectRef references a Kubernetes object
type ObjectRef struct {
    Name      string `json:"name"`
    Namespace string `json:"namespace,omitempty"`
}

func (ref ObjectRef) String() string {
    if ref.Namespace != "" {
        return ref.Namespace + "/" + ref.Name
    }
    return ref.Name
}

func (ref ObjectRef) MarshalLog() any {
    // We implement fmt.Stringer for non-structured logging, but we want the
    // raw struct when using structured logs.  Some logr implementations call
    // String if it is present, so we want to convert this struct to something
    // that doesn't have that method.
    type forLog ObjectRef // methods do not survive type definitions
    return forLog(ref)
}

var _ logr.Marshaler = ObjectRef{}

func ExampleMarshaler() {
    l := NewStdoutLogger()
    pod := ObjectRef{Namespace: "kube-system", Name: "some-pod"}
    l.Info("as string", "pod", pod.String())
    l.Info("as struct", "pod", pod)
    // Output:
    // "level"=0 "msg"="as string" "pod"="kube-system/some-pod"
    // "level"=0 "msg"="as struct" "pod"={"name"="some-pod" "namespace"="kube-system"}
}

▾ Example (Secret)

Code:

package logr_test

import (
    "github.com/go-logr/logr"
)

// ComplexObjectRef contains more fields than it wants to get logged.
type ComplexObjectRef struct {
    Name      string
    Namespace string
    Secret    string
}

func (ref ComplexObjectRef) MarshalLog() any {
    return struct {
        Name, Namespace string
    }{
        Name:      ref.Name,
        Namespace: ref.Namespace,
    }
}

var _ logr.Marshaler = ComplexObjectRef{}

func ExampleMarshaler_secret() {
    l := NewStdoutLogger()
    secret := ComplexObjectRef{Namespace: "kube-system", Name: "some-secret", Secret: "do-not-log-me"}
    l.Info("simplified", "secret", secret)
    // Output:
    // "level"=0 "msg"="simplified" "secret"={"Name"="some-secret" "Namespace"="kube-system"}
}

type RuntimeInfo

RuntimeInfo holds information that the logr "core" library knows which LogSinks might want to know. 

type RuntimeInfo struct {
    // CallDepth is the number of call frames the logr library adds between the
    // end-user and the LogSink.  LogSink implementations which choose to print
    // the original logging site (e.g. file & line) should climb this many
    // additional frames to find it.
    CallDepth int
}

type SlogSink

SlogSink is an optional interface that a LogSink can implement to support logging through the slog.Logger or slog.Handler APIs better. It then should also support special slog values like slog.Group. When used as a slog.Handler, the advantages are:

Both APIs (Logger and slog.Logger/Handler) then are supported equally well. Developers can pick whatever API suits them better and/or mix packages which use either API in the same binary with a common logging implementation.

This interface is necessary because the type implementing the LogSink interface cannot also implement the slog.Handler interface due to the different prototype of the common Enabled method.

An implementation could support both interfaces in two different types, but then additional interfaces would be needed to convert between those types in FromSlogHandler and ToSlogHandler. 

type SlogSink interface {
    LogSink

    Handle(ctx context.Context, record slog.Record) error
    WithAttrs(attrs []slog.Attr) SlogSink
    WithGroup(name string) SlogSink
}

type Underlier

Underlier is implemented by the LogSink returned by NewFromLogHandler. 

type Underlier interface {
    // GetUnderlying returns the Handler used by the LogSink.
    GetUnderlying() slog.Handler
}

Subdirectories

Name Synopsis
..
examples
slog Package main is an example of using slogr.
funcr Package funcr implements formatting of structured log messages and optionally captures the call site and timestamp.
example Package main is an example of using funcr.
slogr Package slogr enables usage of a slog.Handler with logr.Logger as front-end API and of a logr.LogSink through the slog.Handler and thus slog.Logger APIs.
testing Package testing provides support for using logr in tests.
testr Package testr provides support for using logr in tests.