cpp-httplib v0.42.0
DRAFT

E03. Handle SSE Reconnection

SSE connections drop for all sorts of network reasons. Clients automatically try to reconnect, so it's a good idea to make your server resume from where it left off.

Read Last-Event-ID

When the client reconnects, it sends the ID of the last event it received in the Last-Event-ID header. The server reads that and picks up from the next one.

svr.Get("/events", [](const httplib::Request &req, httplib::Response &res) {
  auto last_id = req.get_header_value("Last-Event-ID");
  int start = last_id.empty() ? 0 : std::stoi(last_id) + 1;

  res.set_chunked_content_provider(
    "text/event-stream",
    [start](size_t offset, httplib::DataSink &sink) mutable {
      static int next_id = 0;
      if (next_id < start) { next_id = start; }

      std::string msg = "id: " + std::to_string(next_id) + "\n"
                      + "data: event " + std::to_string(next_id) + "\n\n";
      sink.write(msg.data(), msg.size());
      ++next_id;

      std::this_thread::sleep_for(std::chrono::seconds(1));
      return true;
    });
});

svr.Get("/events", [](const httplib::Request &req, httplib::Response &res) {
  auto last_id = req.get_header_value("Last-Event-ID");
  int start = last_id.empty() ? 0 : std::stoi(last_id) + 1;

  res.set_chunked_content_provider(
    "text/event-stream",
    [start](size_t offset, httplib::DataSink &sink) mutable {
      static int next_id = 0;
      if (next_id < start) { next_id = start; }

      std::string msg = "id: " + std::to_string(next_id) + "\n"
                      + "data: event " + std::to_string(next_id) + "\n\n";
      sink.write(msg.data(), msg.size());
      ++next_id;

      std::this_thread::sleep_for(std::chrono::seconds(1));
      return true;
    });
});

On the first connect, Last-Event-ID is empty, so start from 0. On reconnect, resume from the next ID. Event history is the server's responsibility — you need to keep recent events around somewhere.

Set the reconnect interval

Sending a retry: field tells the client how long to wait before reconnecting, in milliseconds.

std::string msg = "retry: 5000\n\n";  // reconnect after 5 seconds
sink.write(msg.data(), msg.size());

std::string msg = "retry: 5000\n\n";  // reconnect after 5 seconds
sink.write(msg.data(), msg.size());

Usually you send this once at the start. During peak load or maintenance windows, a longer retry interval helps reduce reconnect storms.

Buffer recent events

To support reconnection, keep a rolling buffer of recent events on the server.

struct EventBuffer {
  std::mutex mu;
  std::deque<std::pair<int, std::string>> events; // {id, data}
  int next_id = 0;

  void push(const std::string &data) {
    std::lock_guard<std::mutex> lock(mu);
    events.push_back({next_id++, data});
    if (events.size() > 1000) { events.pop_front(); }
  }

  std::vector<std::pair<int, std::string>> since(int id) {
    std::lock_guard<std::mutex> lock(mu);
    std::vector<std::pair<int, std::string>> out;
    for (const auto &e : events) {
      if (e.first >= id) { out.push_back(e); }
    }
    return out;
  }
};

struct EventBuffer {
  std::mutex mu;
  std::deque<std::pair<int, std::string>> events; // {id, data}
  int next_id = 0;

  void push(const std::string &data) {
    std::lock_guard<std::mutex> lock(mu);
    events.push_back({next_id++, data});
    if (events.size() > 1000) { events.pop_front(); }
  }

  std::vector<std::pair<int, std::string>> since(int id) {
    std::lock_guard<std::mutex> lock(mu);
    std::vector<std::pair<int, std::string>> out;
    for (const auto &e : events) {
      if (e.first >= id) { out.push_back(e); }
    }
    return out;
  }
};

When a client reconnects, call since(last_id) to send any events it missed.

How much to keep

The buffer size is a tradeoff between memory and how far back a client can resume. It depends on the use case:

  • Real-time chat: a few minutes to half an hour
  • Notifications: the last N items
  • Trading data: persist to a database and pull from there

Warning: Last-Event-ID is a client-provided value — don't trust it blindly. If you read it as a number, validate the range. If it's a string, sanitize it.

ESC