Add serializer design notes

Author: ashtum

doc/modules/ROOT/pages/serializer.adoc

@@ -0,0 +1,112 @@
+//
+// Copyright (c) 2024 Vinnie Falco ([email protected])
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at https://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/cppalliance/http_proto
+//
+
+= Serializer Design Requirements
+
+
+== Use Cases and Interfaces
+
+
+=== Empty Body
+
+Useful for sending bodiless requests or responses, it should be possible to reuse the existing `serializer` and `request`/`response` objects without any need for extra memory allocation.
+
+
+[source,cpp]
+----
+system::result<void>
+handle_request(
+  serializer& sr,
+  response& res,
+  request_view req)
+{
+    res.set_start_line(status::not_found, req.version());
+    res.set_keep_alive(req.keep_alive());
+    sr.start(res);
+    return {};
+}
+----
+
+
+=== Source Body
+
+A `source`-like body allows for algorithms that can generate body contents directly in the `serializer`'s internal buffer in one or multiple steps, for example for sending contents of a file. The `serializer` should take ownership of the `source` object and is responsible for driving the algorithm and offering a `MutableBufferSequence` (by calling the related virtual interfaces on the `source` object).
+
+
+[source,cpp]
+----
+system::result<void>
+handle_request(
+  serializer& sr,
+  response& res,
+  request_view req)
+{
+    res.set_start_line(status::ok, req.version());
+    res.set_keep_alive(req.keep_alive());
+    res.set_chunked(true);
+
+    http_proto::file file;
+    system::error_code ec;
+    file.open("./index.html", file_mode::scan, ec);
+    if (ec.failed())
+        return ec;
+
+    sr.start<file_body>(res, std::move(file));
+    return {};
+}
+----
+
+=== ConstBufferSequence Body
+
+The `serializer` should be able to use body contents passed as a `ConstBufferSequence` without copying it into its internal buffer.
+
+[source,cpp]
+----
+system::result<void>
+handle_request(
+  serializer& sr,
+  response& res,
+  request_view req)
+{
+    res.set_start_line(status::not_found, req.version());
+    res.set_keep_alive(req.keep_alive());
+
+    // assume caller has an stable reference to static_pages
+    sr.start(res, buffers::make_buffer(static_pages.not_found));
+    return {};
+}
+----
+
+
+=== ConstBufferSequence Body with Ownership Transfer
+
+This is useful when the caller wants to create a `ConstBufferSequence` from an object which lives on the caller's stack or for any reason it is inconvenient for the caller to keep the object alive until the send operation is complete.
+
+[source,cpp]
+----
+system::result<void>
+handle_request(
+  serializer& sr,
+  response& res,
+  request_view req)
+{
+    res.set_start_line(status::ok, req.version());
+    res.set_keep_alive(req.keep_alive());
+
+    std::string body;
+    body += "<section>";
+    body += "<b>";
+    body += "HOWDY!";
+    body += "</b>";
+    body += "</section>";
+
+    sr.start(res, [body = std::move(body)]{ return buffers::make_buffer(body);});
+    return {};
+}
+----