Add http.Handler to enable runtime changes (#58)

[pravj]

• signed Uber's Contributor License Agreement
• added tests to cover your changes.
• ran the test suite locally (make test).
• ran the linters locally (make lint).

I have already signed the CLA for my previous pull request #53.

---

The pull request implements a HTTPHandler struct that takes a Logger instance as a field.
It provides a ChangeLogLevel method that accepts a Level argument and returns a http.Handler.

Right now the handler will set the new log level and returns the response after checking the level via Enabled method. The response string is "true" if the level is in effect and "false" otherwise.

• What are your comments on this response workflow?
• I feel that a good enhancement would be passing the log level through the request argument, what do you think?

[pravj]

This is a sample code (gist) I have written that demonstrates its working.

It can be viewed as an application which is trying to load its configurations initially.
So the log level is Debug in starting. The application is emitting both Info and Debug messages.

Once the config files are distributed perfectly (simulated by time.Sleep). It makes a request using the provided Handler and the log level is changed to Info. After that, the Debug messages are ignored.

package main

import (
    "os"
    "fmt"
    "time"
    "net/http"
    "github.com/uber-go/zap"
)

func main() {
    writeSyncer := zap.AddSync(os.Stderr)
    logger := zap.NewJSON(
        zap.Debug,
        zap.Output(writeSyncer),
        zap.ErrorOutput(writeSyncer),
    )

    go func() {
        var i int
        for i <= 4 {
            logger.Debug(fmt.Sprintf("Debug - loading config - time left: %v", 5 - i))
            logger.Info(fmt.Sprintf("Info - loading config - time left: %v", 5 - i))

            time.Sleep(time.Millisecond * 1000)
            i = i + 1
        }

        _, err := http.Get("http://localhost:3000/change")
        if err != nil {
            logger.Fatal("Error in GET request")
        }

        // intentionally leaving the check for response data

        i = 1
        for i <= 4 {
            logger.Debug(fmt.Sprintf("Debug - loaded config - since: %v", i))
            logger.Info(fmt.Sprintf("Info - loaded config - since: %v", i))

            time.Sleep(time.Millisecond * 1000)
            i = i + 1
        }

        os.Exit(0)
    }()

    hh := zap.NewHTTPHandler(logger)

    mux := http.NewServeMux()
    mux.Handle("/change", hh.ChangeLogLevel(zap.Info))

    http.ListenAndServe(":3000", mux)
}

[akshayjshah]

Thanks for catching this 👍

[akshayjshah]

Great - thanks for being so flexible, @pravj! There are a few small style nits remaining, but I can fix those up in a follow-up change - I'll tag you in the PR so you can see the changes.

[pravj]

@akshayjshah @prashantv
Thanks for your guidance, I believe I have been disturbing you with my questions.

I have learned so much just by reading the source code of zap, it's awesome.

• Should we also log events happening in the handler like 'receiving a request', 'bad request' etc.?

[akshayjshah]

Nope, this is great as-is. At least for now, let's not log.