Introduction

Package xylog is designed for leveled and structured logging, dynamic fields, high performance, zone management, simple configuration, and readable syntax.

The library is combined by python logging design and zap encoding approach.

Quick start

You can easily configure a logger with SimpleConfig.

There are some fields you can modify with this way (note that all fields are optional):

• Name is the name of Logger. It can be used later with GetLogger function. Default to an empty name (the root logger).

• Encoding to format the output. Default to TextEncoding.

• Filename specifies that Logger will write the output to a file. Do NOT use together with Writer.

• Filemode specifies the mode to open file. Default to APPEND | CREATE | WRONLY.

• Fileperm specifies the permission when creating the file. Default to 0666.

• Level specifies the logging level. Default to WARNING.

• TimeLayout when format the time string. Default to RFC3339Nano.

• Writer specifies that Logger will write the output to a file. Do NOT use together with Filename.

var config = &xylog.SimpleConfig{
    Name:   "simple-logger",
    Level:  xylog.DEBUG,
    Writer: os.Stdout,
}

var logger, err = config.AddMacro("level", "levelname").Apply()
if err != nil {
    fmt.Println("An error occurred:", err)
    os.Exit(1)
}
defer xylog.Flush()

logger.Debug("logging message")
logger.Event("create-user").Field("username", "foo").
    Field("email", "[email protected]").Field("Age", 25).Info()

// Output:
// level=DEBUG messsage="logging message"
// level=INFO event=create-user username=foo [email protected] Age=25

Full configuration

Logger is directly used by application code. Logger names are dot-separated hierarchical names, such as "a", "a.b", "a.b.c" or similar. For "a.b.c", its parents are "a" and "a.b".

A Logger is obtained using GetLogger method. If the Logger with that name didn't exist before, the method will create a new one. The Logger with empty name is the root one.

var logger = xylog.GetLogger("example")
defer xylog.Flush()

Handler is responsible for generating logging messages. Like Logger, Handler is also identified by its name, however, the name is not hierarchical. Every GetHandler call with the same name gives the same Handler.

Exception: Handlers with the empty names are always different.

var handler = xylog.GetHandler("handler")

Emitter writes logging messages to the specified output. Currently, only StreamEmitter is supported. You can use any Writer in this Emitter type.

var emitter = xylog.NewStreamEmitter(os.Stdout)

When a logging method is called, the Logger creates a LogRecord and sends it to underlying Handlers. Handlers convert LogRecord to text and send it to Emitters.

A Logger can have multiple Handlers, and a Handler can have multiple Emitters.

handler.AddEmitter(emitter)
logger.AddHandler(handler)

After preparing Logger, Handler, and Emitter, you can log the first messages.

logger.Debug("foo") // This message is blocked by Logger's preferred level.
logger.Warning("bar")

// Output:
// message=bar

Logging level

Both Logger and Handler has its own preferred level. If a logging level is lower than the preferred one, the message will not be logged.

By default:

• Handler's preferred level is NOTSET (it logs all logging levels).
• Logger's preferred level depends on its parents. When a Logger is newly created, its preferred level is the nearest parent's one. The root logger's preferred level is WARNING.

You can set a new preferred level for both Logger and Handler.

logger.SetLevel(xylog.DEBUG)

logger.Debug("foo")
logger.Warning("bar")

// Output:
// message=foo
// message=bar

In the following example, however, the first message with DEBUG can bypass the Logger, but will be prevented by Handler.

logger.SetLevel(xylog.DEBUG)
handler.SetLevel(xylog.INFO)

logger.Debug("foo")
logger.Warning("bar")

// Output:
// message=bar

The numeric values of logging levels are given in the following table. If you define a level with the same numeric value, it overwrites the predefined value.

Level	Numeric value
CRITICAL	50
ERROR/FATAL	40
WARN/WARNING	30
INFO	20
DEBUG	10
NOTSET	0

Structured logging

If the logging message has more than one field, EventLogger can help.

logger.Event("add-user").Field("name", "david").Field("email", "[email protected]").Info()

// Output:
// event=add-user name=david [email protected]

You also add a field to Logger or Handler permanently. All logging messages will always include permanent fields.

logger.AddField("host", "localhost")
handler.AddField("port", 3333)

logger.Info("start server")

// Output:
// host=localhost port=3333 message="start server"

NOTE: Fixed fields added to Handler will log faster than the one added to Logger

Handler can support different encoding types. By default, it is TextEncoding.

You can log the message with JSON format too.

import "github.com/xybor-x/xylog/encoding"

handler.SetEncoding(encoding.NewJSONEncoding())

logger.Warning("this is a message")
logger.Event("failed").Field("id", 1).Error()

// Output:
// {"message":"this is a message"}
// {"event":"failed","id": 1}

Macros

You can log special fields whose values change every time you log. These fields called macros.

Only the Handler can add macros.

handler.AddMacro("level", "levelname")

logger.Warning("this is a warning message")

// Output:
// level=WARNING message="this is a warning message"

The following table shows supported macros.

MACRO	DESCRIPTION
asctime	Textual time when the LogRecord was created.
created	Time when the LogRecord was created (time.Now().Unix() return value).
filename*	Filename portion of pathname.
funcname*	Function name logged the record.
levelname	Text logging level for the message ("DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL").
levelno	Numeric logging level for the message (DEBUG, INFO, WARNING, ERROR, CRITICAL).
lineno*	Source line number where the logging call was issued.
module*	The module called log method.
msecs	Millisecond portion of the creation time.
name	Name of the logger.
pathname	Full pathname of the source file where the logging call was issued.
process	Process ID.
relativeCreated	Time in milliseconds between the time LogRecord was created and the time the logging module was loaded.

* These are macros that are only available if xylog.SetFindCaller is called with true.

Filter

Filter can be used by Handlers and Loggers for more sophisticated filtering than is provided by levels.

A Filter instance needs to define Filter(LogRecord) method, which returns true if it allows logging the LogRecord, and vice versa.

type NameFilter struct {
    name string
}

func (f *NameFilter) Filter(record xylog.LogRecord) bool {
    return f.name == record.Name
}

handler.AddFilter(&NameFilter{"example.user"})

var userLogger = xylog.GetLogger("example.user")
var serviceLogger = xylog.GetLogger("example.service")

userLogger.Warning("this is the user logger")
serviceLogger.Warning("this is the service logger")

// Output:
// message="this is the user logger"

Hierarchical logger

As the first section mentioned, the Logger's name is hierarchical. With this feature, you can setup a common Logger with a specified configuration and uses in different application zones.

// common/setup.go
func init() {
    var emitter = xylog.NewStreamEmitter(os.Stderr)
    var handler = xylog.GetHandler("")
    handler.AddEmitter(emitter)
    handler.SetEncoding(encoding.NewJSONEncoding())
    handler.AddMacro("time", "asctime")
    handler.AddMacro("level", "levelname")

    var logger = xylog.GetLogger("parent")
    logger.AddHandler(handler)
    logger.SetLevel(xylog.WARNING)
}

// user/foo.go
import _ "common"

var logger = xylog.GetLogger("parent.user")
defer xylog.Flush()
logger.SetLevel(xylog.INFO)
logger.AddField("module", "user")

logger.Info("this is user module")
logger.Debug("this is a not logged message")

// Output:
// time=[time] level=INFO module=user message="this is user module"

// service/bar.go
import _ "common"

var logger = xylog.GetLogger("parent.service")
defer xylog.Flush()
logger.AddField("module", "service")
logger.AddField("service", "bar")

logger.Warning("this is service module")

// Output:
// time=[time] level=INFO module=service service=bar message="this is service module"

Benchmark

CPU: AMD Ryzen 7 5800H (3.2Ghz)

These benchmark of xylog are measured with SetFindCaller is called with false.

NOTE: The benchmarks are run on a different CPU from the origin, so the benchmark values may be different too.

Log a message and 10 fields:

Package	Time	Time % to zap	Objects Allocated
:zap: zap	1707 ns/op	+0%	5 allocs/op
:zap: zap (sugared)	2043 ns/op	+20%	10 allocs/op
zerolog	884 ns/op	-48%	1 allocs/op
go-kit	6255 ns/op	+266%	58 allocs/op
logrus	8384 ns/op	+391%	80 allocs/op
apex/log	22707 ns/op	+1230%	65 allocs/op
log15	25461 ns/op	+1391%	75 allocs/op
:rocket: xylog	3518 ns/op	+106%	77 allocs/op

Log a message with a logger that already has 10 fields of context:

Package	Time	Time % to zap	Objects Allocated
:zap: zap	140 ns/op	+0%	0 allocs/op
:zap: zap (sugared)	181 ns/op	+29%	1 allocs/op
zerolog	89 ns/op	-36%	0 allocs/op
go-kit	5963 ns/op	+4159%	57 allocs/op
logrus	6590 ns/op	+4607%	69 allocs/op
apex/log	21777 ns/op	+15455%	54 allocs/op
log15	15124 ns/op	+10702%	71 allocs/op
:rocket: xylog	416 ns/op	+197%	6 allocs/op

Log a static string, without any context or printf-style templating:

Package	Time	Time % to zap	Objects Allocated
:zap: zap	154 ns/op	+0%	0 allocs/op
:zap: zap (sugared)	195 ns/op	+27%	1 allocs/op
zerolog	87 ns/op	-44%	0 allocs/op
go-kit	382 ns/op	+148%	10 allocs/op
logrus	1008 ns/op	+554%	24 allocs/op
apex/log	1744 ns/op	+1032%	6 allocs/op
log15	4246 ns/op	+2657%	21 allocs/op
:rocket: xylog	447 ns/op	+190%	6 allocs/op