The pkg/klog package of Kitex defines several logger interfaces: Logger, CtxLogger and FormatLogger. It also provides a default implementation of FormatLogger, the instance of which can be retrieved by klog.DefaultLogger().

The pkg/klog package also provides some global functions that invoke the corresponding methods of the default logger to help simplifying the usage, such as klog.Info, klog.Errorf and others.

Note that the default logger uses the log.Logger from the standard library, the log entry in the log message that indicates where the logging function is called depends on the setting of call depth of the logger. So wrapping the implementation provided by klog might cause the file name and line number in the log message become inaccurate.

Customized Logger Implementation

The client module and server module of Kitex both have a WithLogger option to serve the purpose of injecting customized logger implementations. Middlewares and the other parts of the framework will use it to output logs.

By default, the framework uses klog.DefaultLogger().

Logging in a Middleware

When you create a middleware and wish to write some logs in it using the logger provided by the framework while getting along with the logger injected by the WithLogger option, just inject the middleware with WithMiddlewareBuilder instead of the WithMiddleware option.

The endpoint.MiddlewareBuilder injected by the WithMiddlewareBuilder will be invoked with a context.Context argument that carries the logger being injected. The latter can be retrieved by ctx.Value(endpoint.CtxLoggerKey).

func MyMiddlewareBuilder(mwCtx context.Context) endpoint.Middleware { // middleware builder

    logger := mwCtx.Value(endpoint.CtxLoggerKey).(klog.FormatLogger)  // get the logger

    return func(next endpoint.Endpoint) endpoint.Endpoint {           // middleware
        return func(ctx context.Context, request, response interface{}) error {

            err := next(ctx, request, response)

            logger.Debugf("This is a log message from MyMiddleware! err: %v", err)

            return err

Redirecting the Output of the Default Logger

The klog.SetOutput can be used to redirect the output of the default logger provided by the pkg/klog package.

Note that this function does not affect those customized implementations injected using WithLogger options.

For example, to redirect the output of the default logger to a file name ./output.log under the launch directory, a possible implementation might be:

package main

import (


func main() {
    f, err := os.OpenFile("./output.txt", os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
    if err != nil {
    defer f.Close()

    ... // continue to set up your server