ded/cpp-httplib @ c0248ff7fc1506c1bdd12a57cef4e2c5026032a7

title: "W02. Set a WebSocket Heartbeat" order: 52

status: "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.

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

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