`HttpServer` accepts an application factory as a parameter, and the application factory must have `Send` + `Sync` boundaries. More about that in the _multi-threading_ section.
To bind to a specific socket address, [`bind()`][bindmethod] must be used, and it may be called multiple times. To bind ssl socket, [`bind_openssl()`][bindopensslmethod] or [`bind_rustls()`][bindrusttls] should be used. To run the HTTP server, use the `HttpServer::run()` method.
`HttpServer` automatically starts a number of HTTP _workers_, by default this number is equal to the number of logical CPUs in the system. This number can be overridden with the [`HttpServer::workers()`][workers] method.
Once the workers are created, they each receive a separate _application_ instance to handle requests. Application state is not shared between the threads, and handlers are free to manipulate their copy of the state with no concurrency concerns.
To share state between worker threads, use an `Arc`. Special care should be taken once sharing and synchronization are introduced. In many cases, performance costs are inadvertently introduced as a result of locking the shared state for modifications.
In some cases these costs can be alleviated using more efficient locking strategies, for example using [read/write locks](https://doc.rust-lang.org/std/sync/struct.RwLock.html) instead of [mutexes](https://doc.rust-lang.org/std/sync/struct.Mutex.html) to achieve non-exclusive locking, but the most performant implementations often tend to be ones in which no locking occurs at all.
Since each worker thread processes its requests sequentially, handlers which block the current thread will cause the current worker to stop processing new requests:
For this reason, any long, non-cpu-bound operation (e.g. I/O, database operations, etc.) should be expressed as futures or asynchronous functions. Async handlers get executed concurrently by worker threads and thus don't block execution:
The same limitation applies to extractors as well. When a handler function receives an argument which implements `FromRequest`, and that implementation blocks the current thread, the worker thread will block when running the handler. Special attention must be given when implementing extractors for this very reason, and they should also be implemented asynchronously where needed.
> **Note**: the _HTTP/2.0_ protocol requires [tls alpn][tlsalpn]. At the moment, only `openssl` has `alpn` support. For a full example, check out [examples/openssl][exampleopenssl].
If the first option above is selected, then _keep alive_ state is calculated based on the response's _connection-type_. By default `HttpResponse::connection_type` is not defined. In that case _keep alive_ is defined by the request's HTTP version.
`HttpServer` supports graceful shutdown. After receiving a stop signal, workers have a specific amount of time to finish serving requests. Any workers still alive after the timeout are force-dropped. By default the shutdown timeout is set to 30 seconds. You can change this parameter with the [`HttpServer::shutdown_timeout()`][shutdowntimeout] method.
