net/http server: too many open files error

Question

I'm trying to develop a simple job queue server with some worker that query it but I encountered a problem with my net/http server. I'm surely doing something bad but after ~3 minutes my server start displaying :

http: Accept error: accept tcp [::]:4200: accept4: too many open files; retrying in 1s

For information it receive 10 request per second in my test case.

Here's two files to reproduce this error :

// server.go package main  import (     "net/http" )  func main() {     http.HandleFunc("/get", func(rw http.ResponseWriter, r *http.Request) {         http.Error(rw, "Try again", http.StatusInternalServerError)     })     http.ListenAndServe(":4200", nil) }  // worker.go package main  import (     "net/http"     "time" )  func main() {     for {         res, _ := http.Get("http://localhost:4200/get")         defer res.Body.Close()          if res.StatusCode == http.StatusInternalServerError {             time.Sleep(100 * time.Millisecond)             continue         }          return     } } 

I already done some search about this error and I found some interesting response but none of these fixed my issue.

The first response I saw was to correctly close the Body in the http.Get response, as you can see I did it.

The second response was to change the file descriptor ulimit of my system but as I will not control where my app will run I prefer to not use this solution (But for information it's set at 1024 on my system)

Can someone explain me why this problem happen and how I can fix it in my code ?

Thanks a lot for your time

EDIT :

EDIT 2 : In comment Martin says that I'm not closing the Body, I tried to close it (without defer so) and it fixed the issue. Thanks Martin ! I was thinking that continue will execute my defer, I was wrong.

Answer 1

I found a post explaining the root problem in a lot more detail. Nathan Smith even explains how to control timeouts on the TCP level, if needed. Below is a summary of everything I could find on this particular problem, as well as the best practices to avoid this problem in future.

Problem

When a response is received regardless of whether response-body is required or not, the connection is kept alive until the response-body stream is closed. So, as mentioned in this thread, always close the response-body. Even if you do not need to use/read the body content:

func Ping(url string) (bool) {     // simple GET request on given URL     res, err := http.Get(url)     if err != nil {         // if unable to GET given URL, then ping must fail         return false     }      // always close the response-body, even if content is not required     defer res.Body.Close()      // is the page status okay?     return res.StatusCode == http.StatusOK } 

Best Practice

As mentioned by Nathan Smith never use the http.DefaultClient in production systems, this includes calls like http.Get as it uses http.DefaultClient at its base.

Another reason to avoid http.DefaultClient is that it is a Singleton (package level variable), meaning that the garbage collector will not try to clean it up, which will leave idling subsequent streams/sockets alive.

Instead create your own instance of http.Client and remember to always specify a sane Timeout:

func Ping(url string) (bool) {     // create a new instance of http client struct, with a timeout of 2sec     client := http.Client{ Timeout: time.Second * 2 }      // simple GET request on given URL     res, err := client.Get(url)     if err != nil {         // if unable to GET given URL, then ping must fail         return false     }      // always close the response-body, even if content is not required     defer res.Body.Close()      // is the page status okay?     return res.StatusCode == http.StatusOK } 

Safety Net

The safety net is for that newbie on the team, who does not know the shortfalls of http.DefaultClient usage. Or even that very useful, but not so active, open-source library that is still riddled with http.DefaultClient calls.

Since http.DefaultClient is a Singleton we can easily change the Timeout setting, just to ensure that legacy code does not cause idle connections to remain open.

I find it best to set this on the package main file in the init function:

package main  import (     "net/http"     "time" )  func init() {     /*     Safety net for 'too many open files' issue on legacy code.     Set a sane timeout duration for the http.DefaultClient, to ensure idle connections are terminated.     Reference: https://stackoverflow.com/questions/37454236/net-http-server-too-many-open-files-error     */     http.DefaultClient.Timeout = time.Minute * 10 }