Add Cookbook S01-S22 (draft)

docs-src/pages/en/cookbook/s20-keep-alive.md

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+---
+title: "S20. Tune Keep-Alive"
+order: 39
+status: "draft"
+---
+
+`httplib::Server` enables HTTP/1.1 Keep-Alive automatically. From the client's perspective, connections are reused — so they don't pay the TCP handshake cost on every request. When you need to tune the behavior, there are two setters.
+
+## What you can configure
+
+| API | Default | Meaning |
+| --- | --- | --- |
+| `set_keep_alive_max_count` | 100 | Max requests served over a single connection |
+| `set_keep_alive_timeout` | 5s | How long an idle connection is kept before closing |
+
+## Basic usage
+
+```cpp
+httplib::Server svr;
+
+svr.set_keep_alive_max_count(20);
+svr.set_keep_alive_timeout(10); // 10 seconds
+
+svr.listen("0.0.0.0", 8080);
+```
+
+`set_keep_alive_timeout()` also has a `std::chrono` overload.
+
+```cpp
+using namespace std::chrono_literals;
+svr.set_keep_alive_timeout(10s);
+```
+
+## Tuning ideas
+
+**Too many idle connections eating resources**  
+Shorten the timeout so idle connections drop and release their worker threads.
+
+```cpp
+svr.set_keep_alive_timeout(2s);
+```
+
+**API is hammered and you want max reuse**  
+Raising the per-connection request cap improves benchmark numbers.
+
+```cpp
+svr.set_keep_alive_max_count(1000);
+```
+
+**Never reuse connections**  
+Set `set_keep_alive_max_count(1)` and every request gets its own connection. Mostly only useful for debugging or compatibility testing.
+
+## Relationship with the thread pool
+
+A Keep-Alive connection holds a worker thread for its entire lifetime. If `connections × concurrent requests` exceeds the thread pool size, new requests wait. For thread counts, see S21. Configure the Thread Pool.
+
+> **Note:** For the client side, see C14. Understand Connection Reuse and Keep-Alive Behavior. Even when the server closes the connection on timeout, the client reconnects automatically.