too many open files when making http requests

I am trying to create a http client program in go which would make many http GET requests. I am using a buffered channel to limit the number of concurrent requests.

When I run the program I get

Get http://198.18.96.213/: dial tcp 198.18.96.213:80: too many open files

Here is my program:

package main
import (
"fmt"
"net/http"
"time"
)
func HttpGets(numRequests int, numConcurrent int, url string) map[int]int {
// I want number of requests with each status code
responseStatuses := map[int]int {
100: 0, 101 : 0, 102 : 0, 200 : 0, 201 : 0, 202 : 0, 203 : 0, 204 : 0, 205 : 0, 
206 : 0, 207 : 0, 208 : 0, 226 : 0, 300 : 0, 301 : 0, 302 : 0, 303 : 0, 304 : 0, 
305 : 0, 306 : 0, 307 : 0, 308 : 0, 400 : 0, 401 : 0, 402 : 0, 403 : 0, 404 : 0, 
405 : 0, 406 : 0, 407 : 0, 408 : 0, 409 : 0, 410 : 0, 411 : 0, 412 : 0, 413 : 0, 
414 : 0, 415 : 0, 416 : 0, 417 : 0, 421 : 0, 422 : 0, 423 : 0, 424 : 0, 425 : 0, 
426 : 0, 427 : 0, 428 : 0, 429 : 0, 430 : 0, 431 : 0, 500 : 0, 501 : 0, 502 : 0, 
503 : 0, 504 : 0, 505 : 0, 506 : 0, 507 : 0, 508 : 0, 509 : 0, 510 : 0, 511 : 0, 
}
reqsDone := 0
ch := make(chan *http.Response, numConcurrent)
for i := 0; i < numRequests; i++ {
go func(url string) {
client := &http.Client{}
req, reqErr := http.NewRequest("GET", url, nil)
if reqErr != nil {
fmt.Println(reqErr)
}
// adding connection:close header hoping to get rid 
// of too many files open error. Found this in http://craigwickesser.com/2015/01/golang-http-to-many-open-files/           
req.Header.Add("Connection", "close") 
resp, err := client.Do(req)
if (err !=nil) {
fmt.Println(err)
}
ch <- resp
}(url)
}
for {
select {
case r := <-ch:
reqsDone += 1 // probably needs a lock?
status := r.StatusCode          
if _, ok := responseStatuses[status]; ok {
responseStatuses[status] += 1           
} else {
responseStatuses[status] = 1
}
r.Body.Close() // trying to close body hoping to get rid of too many files open error 
if (reqsDone == numRequests) {
return responseStatuses
}    
}
}
return responseStatuses
}
func main() {
var numRequests, numConcurrent = 500, 10
url := "http://198.18.96.213/"
beginTime := time.Now()
results := HttpGets(numRequests, numConcurrent, url)
endTime := time.Since(beginTime)
fmt.Printf("Total time elapsed: %s\n", endTime)
for k,v := range results {
if v!=0 {
fmt.Printf("%d : %d\n", k, v)
}       
}
}

How to I ensure files/socets are closed so that I don't get this error when making multiple requests?

Basically you were spawning 100s of goroutines that will start the connection the block until they are closed.

Here's a quick (and very ugly) working code:

var (
responseStatuses = make(map[int]int, 63)
reqsDone         = 0
urlCh = make(chan string, numConcurrent)
ch    = make(chan *http.Response, numConcurrent)
)
log.Println(numConcurrent, numRequests, len(responseStatuses))
for i := 0; i < numConcurrent; i++ {
go func() {
for url := range urlCh {
client := &http.Client{}
req, reqErr := http.NewRequest("GET", url, nil)
if reqErr != nil {
fmt.Println(reqErr)
}
// adding connection:close header hoping to get rid
// of too many files open error. Found this in http://craigwickesser.com/2015/01/golang-http-to-many-open-files/
req.Header.Add("Connection", "close")
resp, err := client.Do(req)
if err != nil {
fmt.Println(err)
}
ch <- resp
}
}()
}
go func() {
for i := 0; i < numRequests; i++ {
urlCh <- url
}
close(urlCh)
}()

<kbd>playground</kbd>

You can use the following library:

Requests: A Go library for reduce the headache when making HTTP requests (20k/s req)

https://github.com/alessiosavi/Requests

The idea is to allocate a list of request, than send them with a configurable "parallel" factor that allow to run only "N" request at time.

// This array will contains the list of request
var reqs []requests.Request

// N is the number of request to run in parallel, in order to avoid "TO MANY OPEN FILES. N have to be lower than ulimit threshold"
var N int = 12

// Create the list of request
for i := 0; i < 1000; i++ {
    // In this case, we init 1000 request with same URL,METHOD,BODY,HEADERS 
    req, err := requests.InitRequest("https://127.0.0.1:5000", "GET", nil, nil, true) 
    if err != nil {
        // Request is not compliant, and will not be add to the list
        log.Println("Skipping request [", i, "]. Error: ", err)
    } else {
        // If no error occurs, we can append the request created to the list of request that we need to send
        reqs = append(reqs, *req)
    }
}

At this point, we have a list that contains the requests that have to be sent.
Let's send them in parallel!

// This array will contains the response from the givens request
var response []datastructure.Response

// send the request using N request to send in parallel
response = requests.ParallelRequest(reqs, N)

// Print the response
for i := range response {
    // Dump is a method that print every information related to the response
    log.Println("Request [", i, "] -> ", response[i].Dump())
    // Or use the data present in the response
	log.Println("Headers: ", response[i].Headers)
	log.Println("Status code: ", response[i].StatusCode)
	log.Println("Time elapsed: ", response[i].Time)
	log.Println("Error: ", response[i].Error)
	log.Println("Body: ", string(response[i].Body))
}

You can find example usage into the example folder of the repository.

SPOILER:
>! I'm the author of this little library