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.

v3 upgrade

Version 3 compiles with Go modules. It also provides some basic simplifications:

• For performance, the Call property has been removed from a log15.Record.

• The CallerStackHandler, CallerFuncHandler, and CallerFileHandler have been removed (all three relied on the Call property).

• The term subpackage has been removed (there are multiple better replacements you can use now).

In most cases, however, it should be sufficient to just change your import path:

- import log "github.com/inconshreveable/log15"
+ import log "github.com/inconshreveable/log15/v3"

A lot has happened since log15 was released; you may find the log/slog package in the standard library to be a better fit for your use case.

Development of the v3 branch happens in the v3 subdirectory.

We also added a compatibility layer, which allows existing code to use the v3 library, so you can incrementally migrate.

import compat "github.com/inconshreveable/log15/compat"

• compat.CompatHandler takes a v3 Handler, and returns a legacy log15 Handler.
• compat.CompatLogger does the same with the Logger type.

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. If you want to rely on a stable API, you must vendor the library.

Importing

import log "github.com/inconshreveable/log15"

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.JsonFormat()))))

Will result in output that looks like this:

WARN[06-17|21:58:10] abnormal conn rate                       module=app/server rate=0.500 low=0.100 high=0.800
INFO[06-17|21:58:10] connection open                          module=app/server raddr=10.0.0.1

Breaking API Changes

The following commits broke API stability. This reference is intended to help you understand the consequences of updating to a newer version of log15.

• 57a084d014d4150152b19e4e531399a7145d1540 - Added a Get() method to the Logger interface to retrieve the current handler
• 93404652ee366648fa622b64d1e2b67d75a3094a - Record field Call changed to stack.Call with switch to github.com/go-stack/stack
• a5e7613673c73281f58e15a87d2cf0cf111e8152 - Restored syslog.Priority argument to the SyslogXxx handler constructors

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})

Regenerating the CONTRIBUTORS file

go get -u github.com/kevinburke/write_mailmap
write_mailmap > CONTRIBUTORS

License

Apache