kvlog

kvlog provides a structured logging facility. The underlying structure is based on key-value pairs. key-value pairs are rendered as JSON lines but other Formatters can be used to provide different outputs including custom ones.

Why another logging lib?

kvlog tries to find a balance between highly performance optimized libs such as zap or zerolog and those that define a "dead simple" API (such as go-kit or logrus). It does not perform as well as the first two but significantly better than the last two. kvlog tries to provide an easy-to-use API for producing structured log events while keeping up a good performance.

Usage

Installation

kvlog uses go modules and requires Go 1.16 or greater.

$ go get -u github.com/halimath/kvlog

Creating a Logger

To emit log events, you need a Logger. kvlog provides a ready-to-use Logger via the L variable. You can create a custom logger giving you more flexibility on the logger's target and format. Creating a new Logger is done via the kvlog.New function. It accepts any number of Handlers. Each Handler pairs an io.Writer as well as a Formatter.

logger := kvlog.New(kvlog.NewSyncHandler(os.Stdout, kvlog.JSONLFormatter())).
	AddHook(kvlog.TimeHook)

Handlers can be synchronous as well as asynchronous. Synchronous Handlers execute the Formatter as well as writing the output in the same goroutine that invoked the Logger. Asynchronous Handlers dispatch the log event to a different goroutine via a channel. Thus, asynchronous Handlers must be closed before shutdown in order to flush the channel and emit all log events.

Emitting Events

The easiest way to emit a simple log message is to use a Logger's Log, Log or Logf method.

kvlog.L.Logs("hello, world")
kvlog.L.Logf("hello, %s", "world)

Log will log all given key-value pairs while Logs and Logf will format a message with additional key-value pairs. With the default JSONL formatter, this produces:

{"msg":"hello"}
{"msg":"hello, world"}

If you want to add more key-value pairs - which is the primary use case for a structured logger - you can pass additional arguments to any of the log methods. key-value pairs are best created using one of the With... functions from kvlog.

kvlog.L.Logs("hello, world",
	kvlog.WithKV("tracing_id", 1234),
	kvlog.WithDur(time.Second),
	kvlog.WithErr(fmt.Errorf("some error")),
)

Deriving Loggers

Logger's can be derived from another Logger. This enables to configure a set of key-value-pairs to be added to every event emmitted via the deriverd logger. The syntax works similar to emitting log messages this time only invoking the Sub method instead of Log.

dl := l.Sub(
	kvlog.WithKV("tracing_id", "1234"),
)

Hooks

In addition to deriving loggers, any number of Hooks may be added to a logger. The hook's callback function is invoked everytime an Event is emitted via this logger or any of its derived loggers. Hooks are useful to add dynamic values, such as timestamps or anything else read from the surrounding context. Adding a timestamp to every log event is realized via the TimeHook.

l := kvlog.New(kvlog.NewSyncHandler(&buf, kvlog.JSONLFormatter())).
	AddHook(kvlog.TimeHook)

You can write your own hook by implement the kvlog.Hook interface or using the kvlog.HookFunc convenience type for a simple function.

// This is an example for some function that determines a dynamic value.
extractTracingID := func() string {
	// some real implementation here
	return "1234"
}

// Create a logger and add the hook
logger := kvlog.New(kvlog.NewSyncHandler(os.Stdout, kvlog.JSONLFormatter())).
	AddHook(kvlog.HookFunc(func(e *kvlog.Event) {
		e.AddPair(kvlog.WithKV("tracing_id", extractTracingID()))
	}))

// Emit some event
logger.Logs("request")

This example produces:

{"tracing_id":"1234","msg":"request"}

Passing a logger by Context

The go standard library provides package context to pass contextual values to operations being called downstream. kvlog provides functions that hook into a context and add a Logger which can later be retrieved. As a sub logger can be configured with context keys, you can pass that logger down via the context.

Use ContextWithLogger to create a derived context holding a logger. Calling FromContext restores the logger. If no logger is contained in the context a default NoOp logger is returned, so code can run without panic.

ctx = ContextWithLogger(ctx, subLogger)

// ...
l = FromContext(ctx)
l.Logs("my message")

Formatters

The kvlog package comes with three Formatters out of the box:

• JSONLFormatter formats events as JSON line values
• ConsoleFormatter formats events for output on a terminal which includes colorizing the event
• KVFormatter formats events in the legacy KV-Format

The JSONLFormatter features a lot of optimizations to improve time and memory behavior. The other two have a less optimized performance. While the ConsoleFormatter is intended for dev use the KVFormatter is only provided for compatibility reasons and should be considered deprecated. Use JSONLFormatter for production systems.

Custom formatters may be created by implementing the kvlog.Formatter interface or using the kvlog.FormatterFunc convenience type.

HTTP Middleware

kvlog contains a HTTP middleware that generates an access log and supports adding a logger to the request's Context. The middleware is compatible with frameworks such as Chi that support Use but you can also use the middleware with bare net/http.

package main

import (
	"net/http"

	"github.com/halimath/kvlog"
)

func main() {
    mux := http.NewServeMux()
    // ...
	kvlog.L.Log("started")
	http.ListenAndServe(":8000", kvlog.Middleware(kvlog.L, true)(mux))
}

Default Keys

The following table lists the default keys used by kvlog. You can customize these by setting a module-level variable. These are also given in the table below.

Key	Used with	Variable to change	Description
time	TimeHook	KeyTime	The default key used to identify an event's time stamp.
err	Event.Err	KeyError	The default key used to identify an event's error.
msg	Event.Log or Event.Logf	KeyMessage	The default key used to identify an event's message.
dur	Event.Dur	KeyDuration	The default key used to identify an event's duration value.

Customizing memory behavior

kvlog includes a set of performance optimizations. Most of them work by pre-allocating memory for data structures to be reused in order to avoid allocations for each log event. These pre-allocations can be tuned in case an application has a specific load profile. This tuning can be performed by setting module-global variables.

There are two primary areas, where pre-allocation is used.

1. New Events are not created every time a logger's With method is called. Instead, most of the time a pre-existing Event is pulled from a sync.Pool and put back after the event has been formatted. Such a pool exists for each root logger - that is every logger created with kvlog.New. The pool is pre-filled with a number of events. All the events pre-filled into the pool also have a pre-allocated number of key-value-pair slots which are re-used by overwriting them whenever a KV method is called. Both numbers - initial pool size and pre-allocated number of pairs - can be changed.
2. When using an asynchronous handler, the handler's formatter is invoked synchronously. The output is written to a bytes.Buffer. This buffer comes from a sync.Pool and has a pre-allocated bytes slice. After the event has been formatted, the buffer is sent over a bufferd channel. The channel is consumed by another goroutine, which copies the buffer's bytes on the output writer. After that, the buffer is put back into the pool. Pool size, buffer size and channel buffer size can be customized.

Changes to these variables only take effect for loggers/handlers created after the variable have been assigned. Use at your own risk.

Variable	Default Value	Description
DefaultEventSize	16	The default size Events created from an Event pool.
InitialEventPoolSize	128	Number of events to allocate for a new Event pool.
AsyncHandlerBufferSize	2048	Defines the size of an async handler's buffer that is preallocated.
AsyncHandlerPoolSize	64	Defines the number of preallocated buffers in a pool of buffers.
AsyncHandlerChannelSize	1024	Number of log events to buffer in an async handler's channel.

Benchmarks

The benchmarks directory contains some benchmarking tests which compare different logging libs with a structured log event consisting of several key-value-pairs of different types. These are the results run on a developers laptop.

Library	ns/op	B/op	allocs/op
kvlog (sync handler)	1527	152	8
kvlog (async handler)	1392	153	8
zerolog	352.9	0	0
logrus	4430	2192	34
go-kit/log	2201	970	18

Changelog

0.12.0

• HTTP middleware's responeWrapper satifies additional go http interfaces:
  • http.Flusher
  • http.Hijacker
  • http.Pusher

0.11.0

⚠️ breaking change: This version changes the API of the HTTP middleware function

• New HTTP middleware function compatible with other frameworks (such as Chi)
• Middleware can add logger to Context

0.10.0

• Context API
• NoOp Logger

0.9.0

⚠️ breaking change: This version provides a new API which is not compatible to the API exposed before. This involves the way loggers and other components are configured (which normally only affects a small portion of the using code) as well as the way log events are emitted. The interface emitting log messages remains similar.

• New API
• Performance improvements

0.8.1 - retracted

• Fix: add sync.Mutex to lock Handler

0.8.0 - retracted

• added NoOpHandler to easily silence logging output

0.7.0 - retracted

⚠️ breaking change: This version provides a new API which is not compatible to the API exposed before. This involves the way loggers and other components are configured as well as how log events are emitted.

• New chaining API to create messages
• Performance optimization

0.6.0

⚠️ breaking change: This version provides a new API which is in parts not compatible to the API exposed before. This basically involves the way loggers and other components are configured (which normally only affects a small portion of the using code). The interface to creating and emitting log messages stays the same.

• Nested loggers allow adding default key value pairs to add to all logger (i.e. for use with a category)
• Reorganization into several packages; root package kvlog acts as a facade
• Renamed some types (mostly interfaces) to better match the new package name (i.e. handler.Interface instead of handler.Handler)
• Added jsonl formatter which creates JSON lines output

0.5.0

• KVFormatter sorts pairs based on key
• New TerminalFormatter providing colored output on terminals
• Moved to github

0.4.0

• Export package level logger instance L

0.3.0

⚠️ breaking changes: This version provides a new API which is not compatible to the API exposed before.

• Introduction of new component structure (see description above)

0.2.0

• Improve log message rendering

0.1.0

• Initial release

License

Copyright 2019 - 2025 Alexander Metzner.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.