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https://github.com/yhirose/cpp-httplib.git
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6bdd657713
- Added `set_websocket_max_missed_pongs` method to configure unresponsive-peer detection. - Updated README and documentation to clarify WebSocket limitations and features. - Introduced tests for detecting non-responsive peers and ensuring responsive peers do not trigger timeouts.
81 lines
3.4 KiB
Markdown
81 lines
3.4 KiB
Markdown
---
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title: "W02. Set a WebSocket Heartbeat"
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order: 52
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status: "draft"
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---
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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.
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## Server side
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```cpp
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svr.set_websocket_ping_interval(30); // ping every 30 seconds
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svr.WebSocket("/chat", [](const auto &req, auto &ws) {
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// ...
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});
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```
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Just pass the interval in seconds. Every WebSocket connection this server accepts will be pinged on that interval.
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There's a `std::chrono` overload too.
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```cpp
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using namespace std::chrono_literals;
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svr.set_websocket_ping_interval(30s);
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```
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## Client side
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The client has the same API.
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```cpp
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httplib::ws::WebSocketClient cli("ws://localhost:8080/chat");
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cli.set_websocket_ping_interval(30);
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cli.connect();
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```
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Call it before `connect()`.
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## The default
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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.
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## What about Pong?
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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.
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## Picking an interval
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| Environment | Suggested |
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| --- | --- |
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| Normal internet | 30–60s |
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| Strict proxies (e.g. AWS ALB) | 15–30s |
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| Mobile networks | 60s+ (too short drains battery) |
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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.
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> **Warning:** A very short ping interval spawns background work per connection and increases CPU usage. For servers with many connections, keep the interval modest.
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## Detecting an unresponsive peer
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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.
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```cpp
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cli.set_websocket_max_missed_pongs(2); // close after 2 consecutive unacked pings
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```
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The server side has the same `set_websocket_max_missed_pongs()`.
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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"`.
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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.
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### Why the default is 0
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`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.
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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.
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> For handling a closed connection, see [W03. Handle connection close](w03-websocket-close).
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