cpp-httplib/docs-src/pages/en/cookbook/w02-websocket-ping.md

title, order, status

title	order	status
W02. Set a WebSocket Heartbeat	52	draft

WebSocket connections stay open for a long time, and proxies or load balancers will sometimes drop them for being "idle." To prevent that, you periodically send Ping frames to keep the connection alive. cpp-httplib can do this for you automatically.

Server side

svr.set_websocket_ping_interval(30); // ping every 30 seconds

svr.WebSocket("/chat", [](const auto &req, auto &ws) {
  // ...
});

Just pass the interval in seconds. Every WebSocket connection this server accepts will be pinged on that interval.

There's a std::chrono overload too.

using namespace std::chrono_literals;
svr.set_websocket_ping_interval(30s);

Client side

The client has the same API.

httplib::ws::WebSocketClient cli("ws://localhost:8080/chat");
cli.set_websocket_ping_interval(30);
cli.connect();

Call it before connect().

The default

The default interval is set by the build-time macro CPPHTTPLIB_WEBSOCKET_PING_INTERVAL_SECOND. Usually you won't need to change it, but adjust downward if you're dealing with an aggressive proxy.

What about Pong?

The WebSocket protocol requires that Ping frames are answered with Pong frames. cpp-httplib responds to Pings automatically — you don't need to think about it in application code.

Picking an interval

Environment	Suggested
Normal internet	30–60s
Strict proxies (e.g. AWS ALB)	15–30s
Mobile networks	60s+ (too short drains battery)

Too short wastes bandwidth; too long and connections get dropped. As a rule of thumb, target about half the idle timeout of whatever's between you and the client.

Warning: A very short ping interval spawns background work per connection and increases CPU usage. For servers with many connections, keep the interval modest.

Detecting an unresponsive peer

Sending pings alone doesn't tell you anything if the peer just silently dies — the TCP socket might still look open while the process on the other end is long gone. To catch that, enable the max-missed-pongs check: if N consecutive pings go unanswered, the connection is closed.

cli.set_websocket_max_missed_pongs(2); // close after 2 consecutive unacked pings

The server side has the same set_websocket_max_missed_pongs().

With a 30-second ping interval and max_missed_pongs = 2, a dead peer is detected within roughly 60 seconds and the connection is closed with CloseStatus::GoingAway and the reason "pong timeout".

The counter is reset whenever read() consumes an incoming Pong frame, so this only works if your code is actively calling read() in a loop — which is what a normal WebSocket client does anyway.

Why the default is 0

max_missed_pongs defaults to 0, which means "never close the connection because of missing pongs." Pings are still sent on the heartbeat interval, but their responses aren't checked. If you want unresponsive-peer detection, set it explicitly to 1 or higher.

Even with 0, a dead connection won't linger forever: while your code is inside read(), CPPHTTPLIB_WEBSOCKET_READ_TIMEOUT_SECOND (default 300 seconds = 5 minutes) acts as a backstop and read() fails if no frame arrives in time. Think of max_missed_pongs as the knob for detecting an unresponsive peer faster than that.

For handling a closed connection, see W03. Handle connection close.