Sporadically http clients show the error that the server connection has been reset

[RainerSchielke]

Sporadically our java client logs the message "HttpURLConnection.getInputStream java.io.IOException: Error writing to server" which ocurred because the receiving server closed the connection.

Our Windows-http client shows the windows error code 12030, which means "The connection with the server has been reset or terminated"

I increased the receive buffer size to 256k and the number of threads to 64. This reduced the occurence to about once in 1000 requests.

Then I added a retry in my clients, which helped. But I am not happy with that.

Btw. we do not use "keep alive", at server initialization we call set_keep_alive_timeout (0). Without that, every requests delays for 5 seconds.

Why resets the httplib server the connection sporadically ?

[lukasberbuer]

I face the same problem getting sporadic ERR_CONNECTION_RESET from the JavaScript client with following setup:

// single threaded server
#define CPPHTTPLIB_THREAD_POOL_COUNT 1
#include <httplib.h>

httplib::Server server;
server.set_keep_alive_timeout(0);

// setup endpoints...

server.listen("0.0.0.0", 8080);

Any ideas?

Edit: I'm using version 0.12.2

[RainerSchielke]

Looks like a keep-alive problem. I came up with it when I read the issue #1825 which was fixed with disabling keep-alive on the client side. Probably the function process_server_socket_core causes this:

I assume that if the number of keep alive connections is exhausted, a connection is closed and that causes the client error.
The solution is quite simple: call set_keep_alive_max_count () to increase the keep_alive_max_count to more than 5 e.g. 100 and the problem disappears. Other servers like tomcat or jetty allow 100 to 256 keep alive connections.
My very well working setup ist now:

set_keep_alive_timeout (5); // default
set_keep_alive_max_count (300);

@yhirose I suggest to increase the default value for keep_alive_max_count_ or at least mention that in the documentation.

[lukasberbuer]

Thank you for the deep dive @RainerSchielke.

Following settings seem to work for me:

#define CPPHTTPLIB_THREAD_POOL_COUNT 2
#define CPPHTTPLIB_KEEPALIVE_TIMEOUT_SECOND 5
#define CPPHTTPLIB_KEEPALIVE_MAX_COUNT 300

Whenever I switch to a single-threaded server (#define CPPHTTPLIB_THREAD_POOL_COUNT 1), it seems to work but I face another issues of hanging requests (#454).

[yhirose]

@RainerSchielke thanks for the report. The documentation already mentions it as below, and I feel it's enough for those who know the HTTP/1.1 Keep-Alive connection spec.

https://github.com/yhirose/cpp-httplib?tab=readme-ov-file#keep-alive-connection

But of course, any pull request which updates README is welcome!