S20. Tune Keep-Alive - Cpp Httplib

httplib::Server enables HTTP/1.1 Keep-Alive automatically. From the client's perspective, connections are reused — so they don't pay the TCP handshake cost on every request. When you need to tune the behavior, there are two setters.

What you can configure

API	Default	Meaning
`set_keep_alive_max_count`	100	Max requests served over a single connection
`set_keep_alive_timeout`	5s	How long an idle connection is kept before closing

Basic usage

cpp

httplib::Server svr;

svr.set_keep_alive_max_count(20);
svr.set_keep_alive_timeout(10); // 10 seconds

svr.listen("0.0.0.0", 8080);

set_keep_alive_timeout() also has a std::chrono overload.

cpp

using namespace std::chrono_literals;
svr.set_keep_alive_timeout(10s);

Tuning ideas

Too many idle connections eating resources
Shorten the timeout so idle connections drop and release their worker threads.

cpp

svr.set_keep_alive_timeout(2s);

API is hammered and you want max reuse
Raising the per-connection request cap improves benchmark numbers.

cpp

svr.set_keep_alive_max_count(1000);

Never reuse connections
Set set_keep_alive_max_count(1) and every request gets its own connection. Mostly only useful for debugging or compatibility testing.

Relationship with the thread pool

A Keep-Alive connection holds a worker thread for its entire lifetime. If connections × concurrent requests exceeds the thread pool size, new requests wait. For thread counts, see S21. Configure the thread pool.

Note: For the client side, see C14. Understand connection reuse and Keep-Alive behavior. Even when the server closes the connection on timeout, the client reconnects automatically.