Golang simple http file server concurrent connections limitation on Windows

Question

I am trying to create a fairly simple http file server in Go. Ideally, it should be able to support multiple concurrent file downloads from 2 to 20 clients simultaneously.

I have tried several different approaches including the using the "net/http" and "gorilla" libraries with the same unsuccessful results: the first two clients connect and begin downloading the file normally. Any additional connections fail until one of the first two completes.

Is there a maximum number of concurrent connections / downloads of the same file and can this be increased?

Operating system: Windows 10 Enterprise - max 2 concurrent
Operating system: Ubuntu 22.04 - unknown max, but at least 15
Operating system: Raspbian v11 - unknown max, but at least 15

Very simplified code example below:

package main

import (
    "log"
    "net/http"
)

func main() {
    log.Printf("\nListening... [:80]")

    mux := http.NewServeMux()
    mux.Handle("/", http.FileServer(http.Dir(".")))

    if err := http.ListenAndServe(":80", mux); err != nil {
        log.Fatalf("Could not start server: \n %v", err)
    }

}

The maximum is system dependent, as is how you would change that. 2-20 is trivial, so you would ned to show a minimal reproducible example of the problem for anyone to offer a specific answer. — Mr_Pink
– Mr_Pink, Commented Jul 18, 2022 at 13:17
Be sure any concurrent clients are properly closing the body response of any web calls. Without this the server will not be able to do connection reuse & resources will eventually became exhausted as new unclosed connections pile up. — colm.anseo
– colm.anseo, Commented Jul 18, 2022 at 15:03
@JimB added detail that original test machine is Win10 Enterprise. Confirmed Unbuntu 22.04 supports at least 6 concurrent connections. On Win10 can this limitation be increased? — Peter
– Peter, Commented Jul 18, 2022 at 18:39
@Peter: you need to provide more info or a working example. When I say that you can increase the maximum, I mean starting in the thousands. If you cannot handle 2-20 concurrent connections then there is something else going on. — Mr_Pink
– Mr_Pink, Commented Jul 18, 2022 at 18:47
@JimB Thank you for assisting - the code I included is a working example I think? The same code on Win10 does not allow more than 2 concurrent connections while on Ubuntu 22.04 I was able to have 15 concurrent downloads. As you suggested, it appears to be OS specific. Can this be modified for Win10? — Peter
– Peter, Commented Jul 18, 2022 at 18:53

Shibi J M · Accepted Answer · 2025-05-21 19:55:38Z

2

I ran into the same issue and investigated it.

Go uses a Windows API function called TransmitFile to transmit file data over connected sockets. Workstation and client versions of Windows limit the number of concurrent TransmitFile operations allowed on the system to a maximum of two. This is what causes the issue.

I reported this and submitted a change that makes Go avoid TransmitFile in such cases. The change has been merged and should be included in the next release of Go.

See:

https://github.com/golang/go/issues/73746

https://go-review.googlesource.com/c/go/+/673855

edited May 21 at 19:55

answered May 21 at 19:53

Shibi J M

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