add graceful shutdown system

guide/src/qs_3_5.md

@@ -164,3 +164,73 @@ fn index(req: HttpRequest) -> HttpResponse {
 }
 # fn main() {}
 ```
+
+## Graceful shutdown
+
+Actix http server support graceful shutdown. After receiving a stop signal, workers
+have specific amount of time to finish serving requests. Workers still alive after the
+timeout are force dropped. By default shutdown timeout sets to 30 seconds.
+You can change this parameter with `HttpServer::shutdown_timeout()` method.
+
+You can send stop message to server with server address and specify if you what 
+graceful shutdown or not. `start()` or `spawn()` methods return address of the server.
+
+```rust
+# extern crate futures;
+# extern crate actix;
+# extern crate actix_web;
+# use futures::Future;
+use actix_web::*;
+
+fn main() {
+    let addr = HttpServer::new(
+        || Application::new()
+             .resource("/", |r| r.h(httpcodes::HTTPOk)))
+        .bind("127.0.0.1:0").expect("Can not bind to 127.0.0.1:0")
+        .shutdown_timeout(60)    // <- Set shutdown timeout to 60 seconds
+        .spawn();
+
+    let _ = addr.call_fut(
+         dev::StopServer{graceful:true}).wait(); // <- Send `StopServer` message to server.
+}
+```
+
+It is possible to use unix signals on compatible OSs. "signal" feature needs to be enabled
+in *Cargo.toml* for *actix-web* dependency.
+
+```toml
+[dependencies]
+actix-web = { git = "https://github.com/actix/actix-web", features=["signal"] }
+```
+
+Then you can subscribe your server to unix signals. Http server handles three signals:
+
+* *SIGINT* - Force shutdown workers
+* *SIGTERM* - Graceful shutdown workers
+* *SIGQUIT* - Force shutdown workers
+
+```rust,ignore
+# extern crate futures;
+# extern crate actix;
+# extern crate actix_web;
+use actix_web::*;
+use actix::actors::signal::{ProcessSignals, Subscribe};
+
+fn main() {
+    let sys = actix::System::new("signals");
+
+    let addr = HttpServer::new(|| {
+        Application::new()
+            .resource("/", |r| r.h(httpcodes::HTTPOk))})
+        .bind("127.0.0.1:8080").unwrap()
+        .start();
+
+    // Subscribe to unix signals
+    let signals = Arbiter::system_registry().get::<ProcessSignals>();
+    signals.send(Subscribe(addr.subscriber()));
+
+    println!("Started http server: 127.0.0.1:8080");
+    #     actix::Arbiter::system().send(actix::msgs::SystemExit(0));
+    let _ = sys.run();
+}
+```