en	HTTP Comparison to Other Libraries

en	There are a few C++ published libraries which implement some of the HTTP protocol. We analyze the message model chosen by those libraries and discuss the advantages and disadvantages relative to Beast.

en	The general strategy used by the author to evaluate external libraries is as follows:

en	* Review the message model. Can it represent a complete request or

en	* Review the stream abstraction. This is the type of object, such as

en	* Check treatment of buffers. Does the library manage the buffers

en	* How does the library handle corner cases such as trailers,

en	Declarations examples from external libraries have been edited:

en	cpp-netlib

en	is a network programming library previously intended for Boost but not having gone through formal review. As of this writing it still uses the Boost name, namespace, and directory structure although the project states that Boost acceptance is no longer a goal. The library is based on Boost.Asio and bills itself as ['"a collection of network related routines/implementations geared towards providing a robust cross-platform networking library"]. It cites ['"Common Message Type"] as a feature. As of the branch previous linked, it uses these declarations: ``` template <class Tag> struct basic_message {

en	}; ```

en	This container is the base class template used to represent HTTP messages. It uses a "tag" type style specializations for a variety of trait classes, allowing for customization of the various parts of the message. For example, a user specializes `headers_container<T>` to determine what container type holds the header fields. We note some problems with the container declaration:

en	* The header and body containers may only be default-constructed.

en	* No stateful allocator support.

en	* There is no way to defer the commitment of the type for `body_` to

en	* The message model includes a "source" and "destination." This is

en	* The use of `string_type` (a customization point) for source,

en	* The library uses specializations of `string<Tag>` to change the type

en	* The specialized trait classes generate a proliferation of small

en	* The `string<Tag> customization point constrains user defined body types

en	The design of the message container in this library is cumbersome with its system of customization using trait specializations. The use of these customizations is extremely limited due to the way they are used in the container declaration, making the design overly complex without corresponding benefit.

en	Boost.HTTP

en	is a library resulting from the 2014 Google Summer of Code. It was submitted for a Boost formal review and rejected in 2015. It is based on Boost.Asio, and development on the library has continued to the present. As of the branch previously linked, it uses these message declarations: ``` template<class Headers, class Body> struct basic_message {

en	private:

en	};

en	typedef basic_message<boost::http::headers, std::vector<std::uint8_t>> message;

en	template<class Headers, class Body> struct is_message<basic_message<Headers, Body>>: public std::true_type {}; ```

en	* This container cannot model a complete message. The ['start-line] items

en	* `headers_`, `body_`, and `trailers_` may only be default-constructed,

en	* There is no way to defer the commitment of the [*Body] type to after

en	* No stateful allocator support. This follows from the previous limitation

en	* The trailers are stored in a separate object. Aside from the combinatorial

en	* The declarations imply that `std::vector` is a model of [*Body].

en	* The [*Body] customization point constrains user defined types to

en	This representation addresses a narrow range of use cases. It has limited potential for customization and performance. It is more difficult to use because it excludes the start line fields from the model.

en	C++ REST SDK (cpprestsdk)

en	is a Microsoft project which ['"...aims to help C++ developers connect to and interact with services"]. It offers the most functionality of the libraries reviewed here, including support for Websocket services using its websocket++ dependency. It can use native APIs such as HTTP.SYS when building Windows based applications, and it can use Boost.Asio. The WebSocket module uses Boost.Asio exclusively.

en	As cpprestsdk is developed by a large corporation, it contains quite a bit of functionality and necessarily has more interfaces. We will break down the interfaces used to model messages into more manageable pieces. This is the container used to store the HTTP header fields: ``` class http_headers { public:

en	private:

en	}; ```

en	This declaration is quite bare-bones. We note the typical problems of most field containers:

en	* The container may only be default-constructed.

en	* No support for allocators, stateful or otherwise.

en	* There are no customization points at all.

en	Now we analyze the structure of the larger message container. The library uses a handle/body idiom. There are two public message container interfaces, one for requests (`http_request`) and one for responses (`http_response`). Each interface maintains a private shared pointer to an implementation class. Public member function calls are routed to the internal implementation. This is the first implementation class, which forms the base class for both the request and response implementations: ``` namespace details {

en	class http_msg_base { public:

en	protected:

en	}; ```

en	To understand these declarations we need to first understand that cpprestsdk uses the asynchronous model defined by Microsoft's [@https://msdn.microsoft.com/en-us/library/dd504870.aspx [*Concurrency Runtime]]. Identifiers from the [@https://msdn.microsoft.com/en-us/library/jj987780.aspx [*`pplx` namespace]] define common asynchronous patterns such as tasks and events. The `concurrency::streams::istream` parameter and `m_data_available` data member indicates a lack of separation of concerns. The representation of HTTP messages should not be conflated with the asynchronous model used to serialize or parse those messages in the message declarations.

en	The next declaration forms the complete implementation class referenced by the handle in the public interface (which follows after): ``` /// Internal representation of an HTTP request message. class _http_request final : public http::details::http_msg_base, public std::enable_shared_from_this<_http_request> { public:

en	private:

en	};

en	} // namespace details ```

en	As before, we note that the implementation class for HTTP requests concerns itself more with the mechanics of sending the message asynchronously than it does with actually modeling the HTTP message as described in __rfc7230__:

en	* The constructor accepting `std::unique_ptr<http::details::_http_server_context`

en	* The "cancellation token" is stored inside the message. This breaks the

en	* The `_reply_impl` function implies that the message implementation also

en	Finally, here is the public class which represents an HTTP request: ``` class http_request { public:

en	private:

en	}; ```

en	It is clear from this declaration that the goal of the message model in this library is driven by its use-case (interacting with REST servers) and not to model HTTP messages generally. We note problems similar to the other declarations:

en	* There are no compile-time customization points at all. The only

en	* The extraction of the body is conflated with the asynchronous model.

en	* No way to define an allocator for the container used when extracting

en	* A body can only be extracted once, limiting the use of this container

en	* Setting the body requires either a vector or a `concurrency::streams::istream`.

en	* The HTTP request container conflates HTTP response behavior (see the

en	The general theme of the HTTP message model in cpprestsdk is "no user definable customizations". There is no allocator support, and no separation of concerns. It is designed to perform a specific set of behaviors. In other words, it does not follow the open/closed principle.

en	Tasks in the Concurrency Runtime operate in a fashion similar to `std::future`, but with some improvements such as continuations which are not yet in the C++ standard. The costs of using a task based asynchronous interface instead of completion handlers is well documented: synchronization points along the call chain of composed task operations which cannot be optimized away. See: [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3747.pdf [*A Universal Model for Asynchronous Operations]] (Kohlhoff).