yhirose
9bfd2ddf1c
Bring the READMEs and the docs pages (en and ja) in line with the current header: - Fix the client sample that does not compile, the Transfer-Encoding statement for default headers, the verifier callback comment, the thread pool spawn rule, the sanitize_filename description, and the inverted Accept-Encoding sentence. - Describe recent behaviour: TLS errors behind Read/Write failures, Host validation, Connection: close for a body without length, content_reader rules, refused CONNECT, WebSocketClient proxies, CPPHTTPLIB_INTERIM_RESPONSE_MAX_COUNT, the SSE event size limit, and the payload limit of the Stream API. - Complete the API references of WebSocket, WebSocketClient and the stream:: helpers, and the SSE reconnect rules. - Fix cookbook pages: keychain loading on every backend, received Close status, a multipart sample that trusted the client's file name, an echo sample that sent binary as text, SNI, pre-request handler scope, stop() with WebSocket handlers, and the thread pool notes. - Restore the mutex in the LLM app's Chapter 5 code.
3.6 KiB
title, order, status
| title | order | status |
|---|---|---|
| W02. Set a WebSocket Heartbeat | 53 | 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 answers Pings automatically inside read() — as long as your code keeps calling read(), you don't need to think about it.
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 60 to 90 seconds after it stops answering, and the connection is closed with CloseStatus::GoingAway and the reason "pong timeout". A read() waiting on the peer at that moment returns Fail.
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.
On the server side, even with 0, a dead connection won't linger forever: while a handler is inside read(), CPPHTTPLIB_WEBSOCKET_SERVER_READ_TIMEOUT_SECOND (default 300 seconds = 5 minutes) acts as a backstop. A client has no backstop of its own — it waits forever unless you set a read timeout — so there max_missed_pongs is what notices an unresponsive peer at all. On either side, it is also how you notice one faster than that 5-minute fallback.
For handling a closed connection, see W03. Handle connection close.