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Package log

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

Overview ▾

Package log provides a structured logger.

Deprecated: Use github.com/go-kit/log instead.

Structured logging produces logs easily consumed later by humans or machines. Humans might be interested in debugging errors, or tracing specific requests. Machines might be interested in counting interesting events, or aggregating information for off-line processing. In both cases, it is important that the log messages are structured and actionable. Package log is designed to encourage both of these best practices.

Basic Usage

The fundamental interface is Logger. Loggers create log events from key/value data. The Logger interface has a single method, Log, which accepts a sequence of alternating key/value pairs, which this package names keyvals. 

type Logger interface {
    Log(keyvals ...interface{}) error
}

Here is an example of a function using a Logger to create log events. 

func RunTask(task Task, logger log.Logger) string {
    logger.Log("taskID", task.ID, "event", "starting task")
    ...
    logger.Log("taskID", task.ID, "event", "task complete")
}

The keys in the above example are "taskID" and "event". The values are task.ID, "starting task", and "task complete". Every key is followed immediately by its value.

Keys are usually plain strings. Values may be any type that has a sensible encoding in the chosen log format. With structured logging it is a good idea to log simple values without formatting them. This practice allows the chosen logger to encode values in the most appropriate way.

Contextual Loggers

A contextual logger stores keyvals that it includes in all log events. Building appropriate contextual loggers reduces repetition and aids consistency in the resulting log output. With, WithPrefix, and WithSuffix add context to a logger. We can use With to improve the RunTask example. 

func RunTask(task Task, logger log.Logger) string {
    logger = log.With(logger, "taskID", task.ID)
    logger.Log("event", "starting task")
    ...
    taskHelper(task.Cmd, logger)
    ...
    logger.Log("event", "task complete")
}

The improved version emits the same log events as the original for the first and last calls to Log. Passing the contextual logger to taskHelper enables each log event created by taskHelper to include the task.ID even though taskHelper does not have access to that value. Using contextual loggers this way simplifies producing log output that enables tracing the life cycle of individual tasks. (See the Contextual example for the full code of the above snippet.)

Dynamic Contextual Values

A Valuer function stored in a contextual logger generates a new value each time an event is logged. The Valuer example demonstrates how this feature works.

Valuers provide the basis for consistently logging timestamps and source code location. The log package defines several valuers for that purpose. See Timestamp, DefaultTimestamp, DefaultTimestampUTC, Caller, and DefaultCaller. A common logger initialization sequence that ensures all log entries contain a timestamp and source location looks like this: 

logger := log.NewLogfmtLogger(log.NewSyncWriter(os.Stdout))
logger = log.With(logger, "ts", log.DefaultTimestampUTC, "caller", log.DefaultCaller)

Concurrent Safety

Applications with multiple goroutines want each log event written to the same logger to remain separate from other log events. Package log provides two simple solutions for concurrent safe logging.

NewSyncWriter wraps an io.Writer and serializes each call to its Write method. Using a SyncWriter has the benefit that the smallest practical portion of the logging logic is performed within a mutex, but it requires the formatting Logger to make only one call to Write per log event.

NewSyncLogger wraps any Logger and serializes each call to its Log method. Using a SyncLogger has the benefit that it guarantees each log event is handled atomically within the wrapped logger, but it typically serializes both the formatting and output logic. Use a SyncLogger if the formatting logger may perform multiple writes per log event.

Error Handling

This package relies on the practice of wrapping or decorating loggers with other loggers to provide composable pieces of functionality. It also means that Logger.Log must return an error because some implementations—especially those that output log data to an io.Writer—may encounter errors that cannot be handled locally. This in turn means that Loggers that wrap other loggers should return errors from the wrapped logger up the stack.

Fortunately, the decorator pattern also provides a way to avoid the necessity to check for errors every time an application calls Logger.Log. An application required to panic whenever its Logger encounters an error could initialize its logger as follows. 

fmtlogger := log.NewLogfmtLogger(log.NewSyncWriter(os.Stdout))
logger := log.LoggerFunc(func(keyvals ...interface{}) error {
    if err := fmtlogger.Log(keyvals...); err != nil {
        panic(err)
    }
    return nil
})

▾ Example (Basic)

Code:

logger := log.NewLogfmtLogger(os.Stdout)

type Task struct {
    ID int
}

RunTask := func(task Task, logger log.Logger) {
    logger.Log("taskID", task.ID, "event", "starting task")

    logger.Log("taskID", task.ID, "event", "task complete")
}

RunTask(Task{ID: 1}, logger)

Output:

taskID=1 event="starting task"
taskID=1 event="task complete"

▾ Example (Contextual)

Code:

logger := log.NewLogfmtLogger(os.Stdout)

type Task struct {
    ID  int
    Cmd string
}

taskHelper := func(cmd string, logger log.Logger) {
    // execute(cmd)
    logger.Log("cmd", cmd, "dur", 42*time.Millisecond)
}

RunTask := func(task Task, logger log.Logger) {
    logger = log.With(logger, "taskID", task.ID)
    logger.Log("event", "starting task")

    taskHelper(task.Cmd, logger)

    logger.Log("event", "task complete")
}

RunTask(Task{ID: 1, Cmd: "echo Hello, world!"}, logger)

Output:

taskID=1 event="starting task"
taskID=1 cmd="echo Hello, world!" dur=42ms
taskID=1 event="task complete"

▾ Example (DebugInfo)

Code:

logger := log.NewLogfmtLogger(os.Stdout)

// make time predictable for this test
baseTime := time.Date(2015, time.February, 3, 10, 0, 0, 0, time.UTC)
mockTime := func() time.Time {
    baseTime = baseTime.Add(time.Second)
    return baseTime
}

logger = log.With(logger, "time", log.Timestamp(mockTime), "caller", log.DefaultCaller)

logger.Log("call", "first")
logger.Log("call", "second")

// ...

logger.Log("call", "third")

Output:

time=2015-02-03T10:00:01Z caller=example_test.go:93 call=first
time=2015-02-03T10:00:02Z caller=example_test.go:94 call=second
time=2015-02-03T10:00:03Z caller=example_test.go:98 call=third

▾ Example (SyncWriter)

Code:

w := log.NewSyncWriter(os.Stdout)
logger := log.NewLogfmtLogger(w)

type Task struct {
    ID int
}

var wg sync.WaitGroup

RunTask := func(task Task, logger log.Logger) {
    logger.Log("taskID", task.ID, "event", "starting task")

    time.Sleep(time.Duration(rand.Intn(200)) * time.Millisecond)

    logger.Log("taskID", task.ID, "event", "task complete")
    wg.Done()
}

wg.Add(2)

go RunTask(Task{ID: 1}, logger)
go RunTask(Task{ID: 2}, logger)

wg.Wait()

Output:

taskID=1 event="starting task"
taskID=2 event="starting task"
taskID=1 event="task complete"
taskID=2 event="task complete"

▾ Example (Valuer)

Code:

logger := log.NewLogfmtLogger(os.Stdout)

count := 0
counter := func() interface{} {
    count++
    return count
}

logger = log.With(logger, "count", log.Valuer(counter))

logger.Log("call", "first")
logger.Log("call", "second")

Output:

count=1 call=first
count=2 call=second

Variables

var (
    // DefaultTimestamp is a Valuer that returns the current wallclock time,
    // respecting time zones, when bound.
    DefaultTimestamp = log.DefaultTimestamp

    // DefaultTimestampUTC is a Valuer that returns the current time in UTC
    // when bound.
    DefaultTimestampUTC = log.DefaultTimestampUTC

    // DefaultCaller is a Valuer that returns the file and line where the Log
    // method was invoked. It can only be used with log.With.
    DefaultCaller = log.DefaultCaller
)

ErrMissingValue is appended to keyvals slices with odd length to substitute the missing value. 

var ErrMissingValue = log.ErrMissingValue

func NewStdlibAdapter 

func NewStdlibAdapter(logger Logger, options ...StdlibAdapterOption) io.Writer

NewStdlibAdapter returns a new StdlibAdapter wrapper around the passed logger. It's designed to be passed to log.SetOutput.

func NewSyncWriter 

func NewSyncWriter(w io.Writer) io.Writer

NewSyncWriter returns a new writer that is safe for concurrent use by multiple goroutines. Writes to the returned writer are passed on to w. If another write is already in progress, the calling goroutine blocks until the writer is available.

If w implements the following interface, so does the returned writer. 

interface {
    Fd() uintptr
}

type Logger

Logger is the fundamental interface for all log operations. Log creates a log event from keyvals, a variadic sequence of alternating keys and values. Implementations must be safe for concurrent use by multiple goroutines. In particular, any implementation of Logger that appends to keyvals or modifies or retains any of its elements must make a copy first. 

type Logger = log.Logger

func NewJSONLogger 

func NewJSONLogger(w io.Writer) Logger

NewJSONLogger returns a Logger that encodes keyvals to the Writer as a single JSON object. Each log event produces no more than one call to w.Write. The passed Writer must be safe for concurrent use by multiple goroutines if the returned Logger will be used concurrently.

func NewLogfmtLogger 

func NewLogfmtLogger(w io.Writer) Logger

NewLogfmtLogger returns a logger that encodes keyvals to the Writer in logfmt format. Each log event produces no more than one call to w.Write. The passed Writer must be safe for concurrent use by multiple goroutines if the returned Logger will be used concurrently.

func NewNopLogger 

func NewNopLogger() Logger

NewNopLogger returns a logger that doesn't do anything.

func NewSyncLogger 

func NewSyncLogger(logger Logger) Logger

NewSyncLogger returns a logger that synchronizes concurrent use of the wrapped logger. When multiple goroutines use the SyncLogger concurrently only one goroutine will be allowed to log to the wrapped logger at a time. The other goroutines will block until the logger is available.

func With 

func With(logger Logger, keyvals ...interface{}) Logger

With returns a new contextual logger with keyvals prepended to those passed to calls to Log. If logger is also a contextual logger created by With, WithPrefix, or WithSuffix, keyvals is appended to the existing context.

The returned Logger replaces all value elements (odd indexes) containing a Valuer with their generated value for each call to its Log method.

func WithPrefix 

func WithPrefix(logger Logger, keyvals ...interface{}) Logger

WithPrefix returns a new contextual logger with keyvals prepended to those passed to calls to Log. If logger is also a contextual logger created by With, WithPrefix, or WithSuffix, keyvals is prepended to the existing context.

The returned Logger replaces all value elements (odd indexes) containing a Valuer with their generated value for each call to its Log method.

func WithSuffix 

func WithSuffix(logger Logger, keyvals ...interface{}) Logger

WithSuffix returns a new contextual logger with keyvals appended to those passed to calls to Log. If logger is also a contextual logger created by With, WithPrefix, or WithSuffix, keyvals is appended to the existing context.

The returned Logger replaces all value elements (odd indexes) containing a Valuer with their generated value for each call to its Log method.

type LoggerFunc

LoggerFunc is an adapter to allow use of ordinary functions as Loggers. If f is a function with the appropriate signature, LoggerFunc(f) is a Logger object that calls f. 

type LoggerFunc = log.LoggerFunc

type StdlibAdapter

StdlibAdapter wraps a Logger and allows it to be passed to the stdlib logger's SetOutput. It will extract date/timestamps, filenames, and messages, and place them under relevant keys. 

type StdlibAdapter = log.StdlibAdapter

type StdlibAdapterOption

StdlibAdapterOption sets a parameter for the StdlibAdapter. 

type StdlibAdapterOption = log.StdlibAdapterOption

func FileKey 

func FileKey(key string) StdlibAdapterOption

FileKey sets the key for the file and line field. By default, it's "caller".

func MessageKey 

func MessageKey(key string) StdlibAdapterOption

MessageKey sets the key for the actual log message. By default, it's "msg".

func Prefix 

func Prefix(prefix string, joinPrefixToMsg bool) StdlibAdapterOption

Prefix configures the adapter to parse a prefix from stdlib log events. If you provide a non-empty prefix to the stdlib logger, then your should provide that same prefix to the adapter via this option.

By default, the prefix isn't included in the msg key. Set joinPrefixToMsg to true if you want to include the parsed prefix in the msg.

func TimestampKey 

func TimestampKey(key string) StdlibAdapterOption

TimestampKey sets the key for the timestamp field. By default, it's "ts".

type StdlibWriter

StdlibWriter implements io.Writer by invoking the stdlib log.Print. It's designed to be passed to a Go kit logger as the writer, for cases where it's necessary to redirect all Go kit log output to the stdlib logger.

If you have any choice in the matter, you shouldn't use this. Prefer to redirect the stdlib log to the Go kit logger via NewStdlibAdapter. 

type StdlibWriter = log.StdlibWriter

type SwapLogger

SwapLogger wraps another logger that may be safely replaced while other goroutines use the SwapLogger concurrently. The zero value for a SwapLogger will discard all log events without error.

SwapLogger serves well as a package global logger that can be changed by importers. 

type SwapLogger = log.SwapLogger

type Valuer

A Valuer generates a log value. When passed to With, WithPrefix, or WithSuffix in a value element (odd indexes), it represents a dynamic value which is re-evaluated with each log event. 

type Valuer = log.Valuer

func Caller 

func Caller(depth int) Valuer

Caller returns a Valuer that returns a file and line from a specified depth in the callstack. Users will probably want to use DefaultCaller.

func Timestamp 

func Timestamp(t func() time.Time) Valuer

Timestamp returns a timestamp Valuer. It invokes the t function to get the time; unless you are doing something tricky, pass time.Now.

Most users will want to use DefaultTimestamp or DefaultTimestampUTC, which are TimestampFormats that use the RFC3339Nano format.

func TimestampFormat 

func TimestampFormat(t func() time.Time, layout string) Valuer

TimestampFormat returns a timestamp Valuer with a custom time format. It invokes the t function to get the time to format; unless you are doing something tricky, pass time.Now. The layout string is passed to Time.Format.

Most users will want to use DefaultTimestamp or DefaultTimestampUTC, which are TimestampFormats that use the RFC3339Nano format.

Subdirectories

Name Synopsis
..
deprecated_levels Package levels implements leveled logging on top of Go kit's log package.
level Package level implements leveled logging on top of Go kit's log package.
logrus Package logrus provides an adapter to the go-kit log.Logger interface.
syslog Deprecated: Use github.com/go-kit/log/syslog instead.
term Package term provides tools for logging to a terminal.
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