WebSocket

The WebSocket Protocol enables two-way communication between a client running untrusted code in a controlled environment to a remote host that has opted-in to communications from that code. The protocol consists of an opening handshake followed by basic message framing, layered over TCP. The goal of this technology is to provide a mechanism for browser-based applications needing two-way communication with servers without relying on opening multiple HTTP connections.

Beast provides developers with a robust WebSocket implementation built on Boost.Asio with a consistent asynchronous model using a modern C++ approach.

This documentation assumes familiarity with __Asio__ and

Construction

A WebSocket connection requires a stateful object, represented in Beast by a single class template [link beast.ref.boost__beast__websocket__stream `websocket::stream`]. The interface uses the layered stream model. A websocket stream object contains another stream object, called the "next layer", which it uses to perform I/O. Descriptions of each template parameter follow:

WebSocket Stream Template Parameters

Name

Description

`NextLayer`

The type of the next layer. An object of this type will be constructed and maintained for the lifetime of the stream. All reads and writes will go through the next layer. This type must meet the requirements of either __SyncStream__, __AsyncStream__, or both, depending on the style of I/O that is to be performed.

`deflateSupported`

When this value is `true`, the stream will support (but not require) the [@https://tools.ietf.org/html/rfc7692 permessage-deflate extension]. Whether or not the stream actually requests or accepts the extension during a handshake depends on a separate configurable option.

When the value is `false` the extension is disabled. Streams will never request the extension in the client role or accept a request for the extension in the server role. An additional benefit of disabling the extension is that compilation will be faster, and the resulting program executable will contain less code.

When a stream is constructed, any arguments provided to the constructor are forwarded to the next layer object's constructor. This declares a stream over a plain TCP/IP socket using an I/O context:

Websocket streams use their own protocol-specific timeout feature. When

As with most I/O objects, a websocket stream is [*not thread-safe]. Undefined behavior results if two different threads access the object concurrently. For multi-threaded programs, the `tcp_stream` can be constructed from an executor, in this case a strand. The stream declared below will use a strand to invoke all completion handlers:

If the next layer supports move-construction, then the websocket stream can be constructed from a moved-from object.

The next layer may be accessed by calling [link beast.ref.boost__beast__websocket__stream.next_layer.overload1 `stream::next_layer`].

Using SSL

To use WebSockets over SSL, use an instance of the __ssl_stream__ class template as the template type for the stream. The required __io_context__ and __ssl_context__ arguments are forwarded to the wrapped stream's constructor: