cpp-httplib v0.42.0
DRAFT

C14. Understand Connection Reuse and Keep-Alive

When you send multiple requests through the same httplib::Client instance, the TCP connection is reused automatically. HTTP/1.1 Keep-Alive does the work for you — you don't pay the TCP and TLS handshake cost on every call.

Connections are reused automatically

httplib::Client cli("https://api.example.com");

auto res1 = cli.Get("/users/1");
auto res2 = cli.Get("/users/2"); // reuses the same connection
auto res3 = cli.Get("/users/3"); // reuses the same connection

httplib::Client cli("https://api.example.com");

auto res1 = cli.Get("/users/1");
auto res2 = cli.Get("/users/2"); // reuses the same connection
auto res3 = cli.Get("/users/3"); // reuses the same connection

No special config required. Just hold on to cli — internally, the socket stays open across calls. The effect is especially noticeable over HTTPS, where the TLS handshake is expensive.

Disable Keep-Alive explicitly

To force a fresh connection every time, call set_keep_alive(false). Mostly useful for testing.

cli.set_keep_alive(false);

cli.set_keep_alive(false);

For normal use, leave it on (the default).

Don't create a Client per request

If you create a Client inside a loop and let it fall out of scope each iteration, you lose the reuse benefit. Create the instance outside the loop.

// Bad: a new connection every iteration
for (auto id : ids) {
  httplib::Client cli("https://api.example.com");
  cli.Get("/users/" + id);
}

// Good: the connection is reused
httplib::Client cli("https://api.example.com");
for (auto id : ids) {
  cli.Get("/users/" + id);
}

// Bad: a new connection every iteration
for (auto id : ids) {
  httplib::Client cli("https://api.example.com");
  cli.Get("/users/" + id);
}

// Good: the connection is reused
httplib::Client cli("https://api.example.com");
for (auto id : ids) {
  cli.Get("/users/" + id);
}

Concurrent requests

If you want to send requests in parallel from multiple threads, give each thread its own Client instance. A single Client uses a single TCP connection, so firing concurrent requests at the same instance ends up serializing them anyway.

Note: If the server closes the connection after its Keep-Alive timeout, cpp-httplib reconnects and retries transparently. You don't need to handle this in application code.

ESC