Asynchronous processing of streaming HTTP with boost::beast

Question

I'm implementing a client which accesses a REST endpoint and then begins processing an SSE stream and monitoring events as they occur. To this end, I'm using Boost::Beast version 124 with Boost 1.63 and attempting to use async_read_some to incrementally read the body of the response.

Here's my code thus far:

namespace http = boost::beast::http;

http::response_parser<http::string_body> sse_client::m_parser;
http::response<http::string_body> sse_client::m_response;
boost::beast::flat_buffer m_buffer;

void sse_client::monitor_sse()
{
    http::request<http::empty_body> req{http::verb::get, m_target, 11};
    req.set(http::field::host, m_host);
    req.set(http::field::user_agent, BOOST_BEAST_VERSION_STRING);
    req.set(http::field::accept, "text/event-stream");
    http::async_write(m_socket, req,
            std::bind(
                &sse_client::process_sse,
                shared_from_this(),
                std::placeholders::_1,
                std::placeholders::_2));
}

void sse_client::process_sse(boost::system::error_code ec, std::size_t byte_count)
{
    http::read_header(m_socket, m_buffer, m_parser);
    http::async_read_some(m_socket, m_buffer, m_parser,
            std::bind(
                &sse_client::read_event,
                shared_from_this(),
                std::placeholders::_1));
}

void sse_client::read_event(boost::system::error_code ec)
{
    // TODO: process event
    http::async_read_some(m_socket, m_buffer, m_parser,
            std::bind(
                &sse_client::read_event,
                shared_from_this(),
                std::placeholders::_1));
}

My questions are:

1. Is this the right approach for this particular use case?
2. Is there a more appropriate type to use with response_parser and response than http::string_body?
3. When the read_event handler is invoked, how does it access the content retrieved by async_read_some? Should it be pulled from the buffer?

Answer 1

我会先回答你的问题，然后再做解释。

1. 是的，您想读取标头，然后调用 read_some（或 read，见下文），直到解析器从 is_complete() 返回 true。但是，在您的代码中，我注意到您混合了同步和异步调用（read_header 后跟 async_read_some）。最好只使用一种模型而不是混合它们。

2. 出于您的目的，您可能需要 buffer_body 而不是 string_body。文档中有一个示例，显示了如何执行此操作（http://www.boost.org/doc/libs/1_66_0/libs/beast/doc/html/beast/using_http/parser_stream_operations/incremental_read.html）

3. 您所指的“缓冲区”是传递给 HTTP 流操作的动态缓冲区参数。虽然此缓冲区将保存消息数据，但它不是由应用程序检查的。此缓冲区用于保存流算法可以读取的当前消息末尾之后的附加数据（这在http://www.boost.org/doc/libs/1_66_0/libs/beast/doc/html/ beast/using_http/message_stream_operations.html#beast.using_http.message_stream_operations.reading）。使用 buffer_body 时，您将通过检查消息正文来访问内容

http::response_parser::get() 将为您提供对正在读入的消息的访问。

对您来说最好的解决方案是在示例中使用 buffer_body，提供一个内存区域来指向它，然后在循环中调用 read 或 async_read 。每次缓冲区满时，读取都会返回错误beast::http::error::need_buffer，表明需要进一步调用。

希望这可以帮助！