log15

README

log15

Package log15 provides an opinionated, simple toolkit for best-practice logging in Go (golang) that is both human and machine readable. It is modeled after the Go standard library's io and net/http packages and is an alternative to the standard library's log package.

Features

• A simple, easy-to-understand API
• Promotes structured logging by encouraging use of key/value pairs
• Child loggers which inherit and add their own private context
• Lazy evaluation of expensive operations
• Simple Handler interface allowing for construction of flexible, custom logging configurations with a tiny API.
• Color terminal support
• Built-in support for logging to files, streams, syslog, and the network
• Support for forking records to multiple handlers, buffering records for output, failing over from failed handler writes, + more

Versioning

The API of the master branch of log15 should always be considered unstable. Using a stable version of the log15 package is supported by gopkg.in. Include your dependency like so:

import log "gopkg.in/inconshreveable/log15.v2"

Examples

// all loggers can have key/value context
srvlog := log.New("module", "app/server")

// all log messages can have key/value context 
srvlog.Warn("abnormal conn rate", "rate", curRate, "low", lowRate, "high", highRate)

// child loggers with inherited context
connlog := srvlog.New("raddr", c.RemoteAddr())
connlog.Info("connection open")

// lazy evaluation
connlog.Debug("ping remote", "latency", log.Lazy(pingRemote))

// flexible configuration
srvlog.SetHandler(log.MultiHandler(
    log.StreamHandler(os.Stderr, log.LogfmtFormat()),
    log.LvlFilterHandler(
        log.LvlError,
        log.Must.FileHandler("errors.json", log.JsonHandler())))

FAQ

The varargs style is brittle and error prone! Can I have type safety please?

Yes. Use log.Ctx:

srvlog := log.New(log.Ctx{"module": "app/server"})
srvlog.Warn("abnormal conn rate", log.Ctx{"rate": curRate, "low": lowRate, "high": highRate})

License

Apache

Documentation

Index

• Variables
• func Crit(msg string, ctx ...interface{})
• func Debug(msg string, ctx ...interface{})
• func Error(msg string, ctx ...interface{})
• func Info(msg string, ctx ...interface{})
• func Warn(msg string, ctx ...interface{})
• type Ctx
• type Format
  • func FormatFunc(f func(*Record) []byte) Format
  • func JsonFormat() Format
  • func JsonFormatEx(pretty, lineSeparated bool) Format
  • func LogfmtFormat() Format
  • func TerminalFormat() Format
• type Handler
  • func BufferedHandler(bufSize int, h Handler) Handler
  • func CallerFileHandler(h Handler) Handler
  • func CallerStackHandler(format string, h Handler) Handler
  • func ChannelHandler(recs chan<- *Record) Handler
  • func DiscardHandler() Handler
  • func FailoverHandler(hs ...Handler) Handler
  • func FileHandler(path string, fmtr Format) (Handler, error)
  • func FilterHandler(fn func(r *Record) bool, h Handler) Handler
  • func FuncHandler(fn func(r *Record) error) Handler
  • func LazyHandler(h Handler) Handler
  • func LvlFilterHandler(maxLvl Lvl, h Handler) Handler
  • func MatchFilterHandler(key string, value interface{}, h Handler) Handler
  • func MultiHandler(hs ...Handler) Handler
  • func NetHandler(network, addr string, fmtr Format) (Handler, error)
  • func StreamHandler(wr io.Writer, fmtr Format) Handler
  • func SyncHandler(h Handler) Handler
  • func SyslogHandler(tag string, fmtr Format) (Handler, error)
  • func SyslogNetHandler(net, addr string, tag string, fmtr Format) (Handler, error)
• type Lazy
• type Logger
  • func New(ctx ...interface{}) Logger
  • func Root() Logger
• type Lvl
  • func LvlFromString(lvlString string) (Lvl, error)
  • func (l Lvl) String() string
• type Record
• type RecordKeyNames

Variables

var (
	StdoutHandler = StreamHandler(os.Stdout, LogfmtFormat())
	StderrHandler = StreamHandler(os.Stderr, LogfmtFormat())
)

var Must muster

The Must object provides the following Handler creation functions which instead of returning an error parameter only return a Handler and panic on failure: FileHandler, NetHandler, SyslogHandler, SyslogNetHandler

Functions

func Crit

func Crit(msg string, ctx ...interface{})

Crit is a convenient alias for Root().Crit

func Debug

func Debug(msg string, ctx ...interface{})

Debug is a convenient alias for Root().Debug

func Error

func Error(msg string, ctx ...interface{})

Error is a convenient alias for Root().Error

func Info

func Info(msg string, ctx ...interface{})

Info is a convenient alias for Root().Info

func Warn

func Warn(msg string, ctx ...interface{})

Warn is a convenient alias for Root().Warn

Types

type Ctx

type Ctx map[string]interface{}

Ctx is a map of key/value pairs to pass as context to a log function Use this only if you really need greater safety around the arguments you pass to the logging functions.

type Format

type Format interface {
	Format(r *Record) []byte
}

func FormatFunc

func FormatFunc(f func(*Record) []byte) Format

FormatFunc returns a new Format object which uses the given function to perform record formatting.

func JsonFormat

func JsonFormat() Format

JsonFormat formats log records as JSON objects separated by newlines. It is the equivalent of JsonFormatEx(false, true).

func JsonFormatEx

func JsonFormatEx(pretty, lineSeparated bool) Format

JsonFormatEx formats log records as JSON objects. If pretty is true, records will be pretty-printed. If lineSeparated is true, records will be logged with a new line between each record.

func LogfmtFormat

func LogfmtFormat() Format

LogfmtFormat prints records in logfmt format, an easy machine-parseable but human-readable format for key/value pairs.

For more details see: http://godoc.org/github.com/kr/logfmt

func TerminalFormat

func TerminalFormat() Format

TerminalFormat formats log records optimized for human readability on a terminal with color-coded level output and terser human friendly timestamp. This format should only be used for interactive programs or while developing.

[TIME] [LEVEL] MESAGE key=value key=value ...

Example:

[May 16 20:58:45] [DBUG] remove route ns=haproxy addr=127.0.0.1:50002

type Handler

type Handler interface {
	Log(r *Record) error
}

A Logger prints its log records by writing to a Handler. The Handler interface defines where and how log records are written. Handlers are composable, providing you great flexibility in combining them to achieve the logging structure that suits your applications.

func BufferedHandler

func BufferedHandler(bufSize int, h Handler) Handler

BufferedHandler writes all records to a buffered channel of the given size which flushes into the wrapped handler whenever it is available for writing. Since these writes happen asynchronously, all writes to a BufferedHandler never return an error and any errors from the wrapped handler are ignored.

func CallerFileHandler

func CallerFileHandler(h Handler) Handler

CallerFileHandler returns a Handler that adds the line number and file of the calling function to the context with key "caller".

func CallerStackHandler

func CallerStackHandler(format string, h Handler) Handler

CallerStackHandler returns a Handler that adds a stack trace to the context with key "stack". The stack trace is formated as a space separated list of call sites inside matching []'s. The most recent call site is listed first. Each call site is formatted according to format. See the documentation of log15/stack.Call.Format for the list of supported formats.

func ChannelHandler

func ChannelHandler(recs chan<- *Record) Handler

ChannelHandler writes all records to the given channel. It blocks if the channel is full. Useful for async processing of log messages, it's used by BufferedHandler.

func DiscardHandler

func DiscardHandler() Handler

DiscardHandler reports success for all writes but does nothing. It is useful for dynamically disabling logging at runtime via a Logger's SetHandler method.

func FailoverHandler

func FailoverHandler(hs ...Handler) Handler

A FailoverHandler writes all log records to the first handler specified, but will failover and write to the second handler if the first handler has failed, and so on for all handlers specified. For example you might want to log to a network socket, but failover to writing to a file if the network fails, and then to standard out if the file write fails:

log.FailoverHandler(
    log.Must.NetHandler("tcp", ":9090", log.JsonFormat()),
    log.Must.FileHandler("/var/log/app.log", log.LogfmtFormat()),
    log.StdoutHandler)

All writes that do not go to the first handler will add context with keys of the form "failover_err_{idx}" which explain the error encountered while trying to write to the handlers before them in the list.

func FileHandler

func FileHandler(path string, fmtr Format) (Handler, error)

FileHandler returns a handler which writes log records to the give file using the given format. If the path already exists, FileHandler will append to the given file. If it does not, FileHandler will create the file with mode 0644.

func FilterHandler

func FilterHandler(fn func(r *Record) bool, h Handler) Handler

FilterHandler returns a Handler that only writes records to the wrapped Handler if the given function evaluates true. For example, to only log records where the 'err' key is not nil:

logger.SetHandler(FilterHandler(func(r *Record) bool {
    for i := 0; i < len(r.Ctx); i += 2 {
        if r.Ctx[i] == "err" {
            return r.Ctx[i+1] != nil
        }
    }
    return false
}, h))

func FuncHandler

func FuncHandler(fn func(r *Record) error) Handler

FuncHandler returns a Handler that logs records with the given function.

func LazyHandler

func LazyHandler(h Handler) Handler

LazyHandler writes all values to the wrapped handler after evaluating any lazy functions in the record's context. It is already wrapped around StreamHandler and SyslogHandler in this library, you'll only need it if you write your own Handler.

func LvlFilterHandler

func LvlFilterHandler(maxLvl Lvl, h Handler) Handler

LvlFilterHandler returns a Handler that only writes records which are less than the given verbosity level to the wrapped Handler. For example, to only log Error/Crit records:

log.LvlFilterHandler(log.Error, log.StdoutHandler)

func MatchFilterHandler

func MatchFilterHandler(key string, value interface{}, h Handler) Handler

MatchFilterHandler returns a Handler that only writes records to the wrapped Handler if the given key in the logged context matches the value. For example, to only log records from your ui package:

log.MatchFilterHandler("pkg", "app/ui", log.StdoutHandler)

func MultiHandler

func MultiHandler(hs ...Handler) Handler

A MultiHandler dispatches any write to each of its handlers. This is useful for writing different types of log information to different locations. For example, to log to a file and standard error:

log.MultiHandler(
    log.Must.FileHandler("/var/log/app.log", log.LogfmtFormat()),
    log.StderrHandler)

func NetHandler

func NetHandler(network, addr string, fmtr Format) (Handler, error)

NetHandler opens a socket to the given address and writes records over the connection.

func StreamHandler

func StreamHandler(wr io.Writer, fmtr Format) Handler

StreamHandler writes log records to an io.Writer with the given format. StreamHandler can be used to easily begin writing log records to other outputs.

StreamHandler wraps itself with LazyHandler and SyncHandler to evaluate Lazy objects and perform safe concurrent writes.

func SyncHandler

func SyncHandler(h Handler) Handler

SyncHandler can be wrapped around a handler to guarantee that only a single Log operation can proceed at a time. It's necessary for thread-safe concurrent writes.

func SyslogHandler

func SyslogHandler(tag string, fmtr Format) (Handler, error)

SyslogHandler opens a connection to the system syslog daemon by calling syslog.New and writes all records to it.

func SyslogNetHandler

func SyslogNetHandler(net, addr string, tag string, fmtr Format) (Handler, error)

SyslogHandler opens a connection to a log daemon over the network and writes all log records to it.

type Lazy

type Lazy struct {
	Fn interface{}
}

Lazy allows you to defer calculation of a logged value that is expensive to compute until it is certain that it must be evaluated with the given filters.

Lazy may also be used in conjunction with a Logger's New() function to generate a child logger which always reports the current value of changing state.

You may wrap any function which takes no arguments to Lazy. It may return any number of values of any type.

type Logger

type Logger interface {
	// New returns a new Logger that has this logger's context plus the given context
	New(ctx ...interface{}) Logger

	// SetHandler updates the logger to write records to the specified handler.
	SetHandler(h Handler)

	// Log a message at the given level with context key/value pairs
	Debug(msg string, ctx ...interface{})
	Info(msg string, ctx ...interface{})
	Warn(msg string, ctx ...interface{})
	Error(msg string, ctx ...interface{})
	Crit(msg string, ctx ...interface{})
}

A Logger writes key/value pairs to a Handler

func New

func New(ctx ...interface{}) Logger

New returns a new logger with the given context. New is a convenient alias for Root().New

func Root

func Root() Logger

Root returns the root logger

type Lvl

type Lvl int

const (
	LvlCrit Lvl = iota
	LvlError
	LvlWarn
	LvlInfo
	LvlDebug
)

func LvlFromString

func LvlFromString(lvlString string) (Lvl, error)

Returns the appropriate Lvl from a string name. Useful for parsing command line args and configuration files.

func (Lvl) String

func (l Lvl) String() string

Returns the name of a Lvl

type Record

type Record struct {
	Time     time.Time
	Lvl      Lvl
	Msg      string
	Ctx      []interface{}
	CallPC   [1]uintptr
	KeyNames RecordKeyNames
}

A Record is what a Logger asks its handler to write

type RecordKeyNames

type RecordKeyNames struct {
	Time string
	Msg  string
	Lvl  string
}