Why does my Go app deployment on Heroku succeed, yet the application displays an 'Application Error,' despite the build succeeding?

Deploying a golang app on Heroku: Build succeeds but application shows error

Problem

When deploying a Go application on Heroku using godep support, the build succeeds, but accessing the endpoint results in an "Application Error."

Code and Logs

The following code is used in the application:

import (
    "log"
    "fmt"
    "net/http"
    "os"

    "github.com/gorilla/mux"
    "github.com/gorilla/context"

    "gopkg.in/paytm/grace.v1"
)

func main() {
    log.Println("$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ CHIT STARTED $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$")
    log.Println("$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$")

    muxRouter := mux.NewRouter()

    muxRouter.HandleFunc("/", Articles)
    http.Handle("/", muxRouter)

    port := os.Getenv("PORT")
    if port == "" {
        port = "9000" // Default port if not specified
    }

    err := grace.Serve(":" + port, context.ClearHandler(http.DefaultServeMux))
    if err != nil {
        log.Println("[ERROR GRACEFUL]", err)
        os.Exit(1)
    }

    os.Exit(0)
}

func Articles(w http.ResponseWriter, r *http.Request) {
    // vars := mux.Vars(r)
    w.WriteHeader(http.StatusOK)
    fmt.Fprintf(w, "Hello")
    // r.Close = true
    // w.Header().Set("Content-Type", "application/json")
    // w.Header().Set("Access-Control-Allow-Origin", "*")

    /*if r.Method == "OPTIONS" {
        w.WriteHeader(http.StatusOK)
        return
    }

    if err := fn(w, r); err != nil {
        log.Println(err)
        apiObject := ConstructAPIError(http.StatusInternalServerError, ErrGeneral, SysMsgErrGeneral, MsgErrGeneral)
        SendAPIObject(w, apiObject)
        return
    }*/
}

The Heroku logs during deployment are as follows:

-----> Go app detected
-----> Fetching jq... done
-----> Fetching stdlib.sh.v8... done
-----> Checking Godeps/Godeps.json file.
-----> New Go Version, clearing old cache
-----> Installing go1.12.6
-----> Fetching go1.12.6.linux-amd64.tar.gz... done
-----> Running: go install -v -tags heroku ./... 
bitbucket.org/michaelchandrag/chit/pkg
bitbucket.org/michaelchandrag/chit/vendor/github.com/gorilla/context
bitbucket.org/michaelchandrag/chit/vendor/github.com/gorilla/mux
bitbucket.org/michaelchandrag/chit/vendor/gopkg.in/tylerb/graceful.v1
bitbucket.org/michaelchandrag/chit/vendor/gopkg.in/paytm/grace.v1
bitbucket.org/michaelchandrag/chit/pkg/util
bitbucket.org/michaelchandrag/chit

       Installed the following binaries:
            ./bin/chit
-----> Discovering process types
       Procfile declares types -> web
-----> Compressing...
       Done: 7.5M
-----> Launching...
       Released v3
       https://michaelchandrag-project.herokuapp.com/ deployed to Heroku

The application logs before and after accessing the endpoint are:

2:47.954106+00:00 heroku[web.1]: Starting process with command `chit`
2019-07-08T05:02:49.413453+00:00 app[web.1]: 2019/07/08 05:02:49 $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ CHIT STARTED $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
2019-07-08T05:02:49.413476+00:00 app[web.1]: 2019/07/08 05:02:49 $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
2019-07-08T05:02:49.413647+00:00 app[web.1]: 2019/07/08 05:02:49 starting serve on  :9000
2019-07-08T05:03:48.131507+00:00 heroku[web.1]: Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch
2019-07-08T05:03:48.131595+00:00 heroku[web.1]: Stopping process with SIGKILL
2019-07-08T05:03:48.214979+00:00 heroku[web.1]: State changed from starting to crashed
2019-07-08T05:03:48.193205+00:00 heroku[web.1]: Process exited with status 137
2019-07-08T10:38:59.721224+00:00 heroku[web.1]: State changed from crashed to starting
2019-07-08T10:39:00.359017+00:00 heroku[web.1]: Starting process with command `chit`
2019-07-08T10:39:02.232435+00:00 app[web.1]: 2019/07/08 10:39:02 $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$ CHIT STARTED $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
2019-07-08T10:39:02.232458+00:00 app[web.1]: 2019/07/08 10:39:02 $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
2019-07-08T10:39:02.232583+00:00 app[web.1]: 2019/07/08 10:39:02 starting serve on  :9000
2019-07-08T10:40:00.462841+00:00 heroku[web.1]: Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch
2019-07-08T10:40:00.462974+00:00 heroku[web.1]: Stopping process with SIGKILL
2019-07-08T10:40:00.555959+00:00 heroku[web.1]: Process exited with status 137
2019-07-08T10:40:00.573427+00:00 heroku[web.1]: State changed from starting to crashed

Solution

The application starts but is getting killed because it does not bind to the specified port. This is evident from the Heroku log message:

2019-07-08T05:03:48.131507+00:00 heroku[web.1]: Error R10 (Boot timeout) -> Web process failed to bind to $PORT within 60 seconds of launch

To resolve this issue, the webserver must be bound to the port specified by the PORT environment variable. On Heroku, the HTTP server is publicly available on default HTTP and HTTPS ports through Heroku gateways.

Therefore, the application should bind to the specified port instead of :9000.

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