Contextual Logging In Go
By: Clavin June
Tags: Golang, Go, Logging, Context, Development

Photo by @borisview on Unsplash

I’ve just woke up and somehow I remember that when I created taboo to trace the error log. And then I think, would it be better if I pass the logger instead of the error log itself?

As the context passed, it will contain a sub-logger that has x-request-id inside it. It'll trace all the requested event from HTTP Handler to the most corner of the function.

I don’t know if this gonna be good because I wrote this immediately right after I woke up. This might be not optimized or perhaps an anti-pattern. But at least this could be a reference for you to create a contextual logger.

Logger

func configureBaseLogger() {

output := zerolog.ConsoleWriter{

Out: os.Stdout,

FormatTimestamp: func(i interface{}) string {

parse, _ := time.Parse(time.RFC3339, i.(string))

return parse.Format("2006-01-02 15:04:05")

},

FormatLevel: func(i interface{}) string {

return strings.ToUpper(fmt.Sprintf(" %-6s ", i))

},

} log.Logger = zerolog.New(output).With().

Timestamp().CallerWithSkipFrameCount(2).Logger()

}

First thing first, you need a logger. Might be anything, but I will use zerolog in this post. I won’t explain about zerolog since you can use anything you want.

Format the base logger to match your logging style.

Let’s test our base logger

Output:

2020-12-19 01:01:37 INFO logger/main.go:57 > testing

LGTM! Let’s continue creating our handler!

Middleware

type HandlerFuncWithStatus func(writer http.ResponseWriter, request *http.Request) int const (

LoggerCtxKey = iota

) func middleware(next HandlerFuncWithStatus) http.HandlerFunc {

return func(writer http.ResponseWriter, request *http.Request) {

requestId := request.Header.Get("x-request-id") if requestId == "" {

requestId = uuid.New().String()

} subLogger := log.With().Str("requestId", requestId).Logger() subLogger.Info().

Str("path", request.URL.Path).

Str("method", request.Method).Msg("in") ctx := context.WithValue(context.Background(), LoggerCtxKey, subLogger)

statusCode := next(writer, request.WithContext(ctx))

writer.WriteHeader(statusCode) subLogger.Info().Int("status", statusCode).Msg("out")

}

}

Here I’ll log the request using the middleware. So I’m gonna make a new type of handler to matched my middleware.

The main idea is:

Get x-request-id if exists, otherwise create one with UUIDv4 format Create a sub-logger from our base-logger and then initiate it with the request id Log the method and path, you can do anything in this step tho Put the logger into the context and pass it to the http handler Finally set and log the http status code

HTTP Handler

func anotherFunc(ctx context.Context) {

logger := ctx.Value(LoggerCtxKey).(zerolog.Logger)

logger.Info().Msg("inside anotherFunc")

} func handler (writer http.ResponseWriter, request *http.Request) int {

logger := request.Context().Value(LoggerCtxKey).(zerolog.Logger)

logger.Info().Msg("inside handler")



anotherFunc(request.Context()) return 204

}

Now we’re gonna create the http handler that will use our middleware. It doesn’t need to be fancy.

So now all the function that has the context passed from the middleware can access the logger by the context key.

Let’s assemble all the codes.

Final

package main import (

"context"

"fmt"

"github.com/google/uuid"

"github.com/rs/zerolog"

"github.com/rs/zerolog/log"

"net/http"

"os"

"strings"

"time"

) type HandlerFuncWithStatus func(writer http.ResponseWriter, request *http.Request) int const (

LoggerCtxKey = iota

) func middleware(next HandlerFuncWithStatus) http.HandlerFunc {

return func(writer http.ResponseWriter, request *http.Request) {

requestId := request.Header.Get("x-request-id") if requestId == "" {

requestId = uuid.New().String()

} subLogger := log.With().Str("requestId", requestId).Logger() subLogger.Info().

Str("path", request.URL.Path).

Str("method", request.Method).Msg("in") ctx := context.WithValue(context.Background(), LoggerCtxKey, subLogger)

statusCode := next(writer, request.WithContext(ctx))

writer.WriteHeader(statusCode) subLogger.Info().Int("status", statusCode).Msg("out")

}

} func anotherFunc(ctx context.Context) {

logger := ctx.Value(LoggerCtxKey).(zerolog.Logger)

logger.Info().Msg("inside anotherFunc")

} func handler (writer http.ResponseWriter, request *http.Request) int {

logger := request.Context().Value(LoggerCtxKey).(zerolog.Logger)

logger.Info().Msg("inside handler") anotherFunc(request.Context()) return 204

} func configureBaseLogger() {

output := zerolog.ConsoleWriter{

Out: os.Stdout,

FormatTimestamp: func(i interface{}) string {

parse, _ := time.Parse(time.RFC3339, i.(string))

return parse.Format("2006-01-02 15:04:05")

},

FormatLevel: func(i interface{}) string {

return strings.ToUpper(fmt.Sprintf(" %-6s ", i))

},

} log.Logger = zerolog.New(output).With().

Timestamp().CallerWithSkipFrameCount(2).Logger()

} func main() {

configureBaseLogger()

log.Info().Msg("testing") http.Handle("/", middleware(handler)) if err := http.ListenAndServe(":8000", nil) ; err != nil {

log.Error().Msg(err.Error())

}

}

This will be the final look of our code.

Try it by accessing http://localhost:8000. The output looks like this if I accessed it twice.

2020-12-19 01:25:16 INFO logger/main.go:75 > testing 2020-12-19 01:25:20 INFO logger/main.go:33 > in method=GET path=/ requestId=6eb1f209-dac9-42b2-8ba4-883efffcbd9e 2020-12-19 01:25:20 INFO logger/main.go:50 > inside handler requestId=6eb1f209-dac9-42b2-8ba4-883efffcbd9e 2020-12-19 01:25:20 INFO logger/main.go:45 > inside anotherFunc requestId=6eb1f209-dac9-42b2-8ba4-883efffcbd9e 2020-12-19 01:25:20 INFO logger/main.go:39 > out requestId=6eb1f209-dac9-42b2-8ba4-883efffcbd9e status=204 2020-12-19 01:25:31 INFO logger/main.go:33 > in method=GET path=/ requestId=88a140b8-2aed-4d82-bf56-0ab9a1c7cce0 2020-12-19 01:25:31 INFO logger/main.go:50 > inside handler requestId=88a140b8-2aed-4d82-bf56-0ab9a1c7cce0 2020-12-19 01:25:31 INFO logger/main.go:45 > inside anotherFunc requestId=88a140b8-2aed-4d82-bf56-0ab9a1c7cce0 2020-12-19 01:25:31 INFO logger/main.go:39 > out requestId=88a140b8-2aed-4d82-bf56-0ab9a1c7cce0 status=204

Now if you want to trace all the event you can log it to the file and grep the request id.

That should be enough for the reference. Please modify the code according to your usage. I didn’t test for the performance nor benchmarking it so take it with a grain of salt.

Thank you for reading!