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Re: [MirageOS-devel] Cohttp Design -- LWT, Async, JS, Mirage Compatibility

On 28 Mar 2015, at 14:24, Trevor Smith <trevorsummerssmith@xxxxxxxxx> wrote:

Hi all,

I was wondering if there is a document somewhere describing why the different backends to cohttp don't have a unified client/server interface? It seems like it would be such a boon for the user to be able to write their code but be able to choose the backends. I realize that this must have already received much discussion but am not sure where it is located.

Hi Trevor,

There's no design document describing this, mainly because CoHTTP started as an informal bet between me and Yaron Minsky that the current design would be impossible/a bad idea.  The jury's still out on the verdict, but I don't think I've lost yet :-)

I wanted to build an HTTP implementation as an "onion", with the portable parsing core progressively introducing I/O, and then higher-level abstractions for various HTTP operations.  Here's my description of each layer (that eventually ought to go into a design doc in the CoHTTP repo to make it more accessible to newcomers to the codebase):

- The very first layer (in `lib/`) is a pure OCaml, non-blocking layer that handles simple parts of the HTTP protocol such as parsing requests and responses, various header parsers (e.g. cookies) and codes.

- Some layers of HTTP need some notion of I/O, and so there is a set of signatures in `lib/s.mli` that defines some common module types that can be used to build parameterised modules (also known as functors).  The first one used in the `lib/` layer is the IO module type, which defines the minimal collection of functions used by cooperative threading libraries.  The pure HTTP core uses this IO module to capture IO-based operations, such as Transfer_IO (for transfer encoding).

- There are three implementations that satisfy the IO module in the tree: Lwt, Async and String.  The first two are full cooperative threading libraries, and the latter is used by the js_of_ocaml backend to read/write between Strings.

- Now that IO has been handled, we can send HTTP requests and responses from Lwt or Async.  However, at this point some differences appear in the implementations of Async and Lwt, notably in how they handle cancellation of threads and also higher-level iterators (e.g. Async has Pipes, and Lwt has Lwt_stream -- both quite different).  Therefore, we build backend-specific Client and Server modules that use their respective threading libraries in as native a style as possible, but still reusing the core HTTP library from `lib/`.  These can be found in `Cohttp_lwt` and `Cohttp_async` respectively.  Dave Scott also wrote an (as yet not merged) POSIX blocking version that they use in the XenAPI daemon.

- Lwt comes with an additional twist -- it is portable to both Unix *and* the MirageOS, which has no Unix at all!  Lwt makes it possible to define a "Lwt core" that uses the portable Lwt thread abstractions, but doesn't use any OS-specific functionality.  Thus we can define an HTTP Client and Server in Cohttp_lwt, but still not tie ourself to one particular OS.  This Cohttp_lwt is then used by the Cohttp_lwt_unix and Cohttp_mirage backends to hook it into the operating system.

- There's no commonality at present between Cohttp_async and Cohttp_lwt, but that's the topic of a design discussion at the moment.  It should be possible to build a common signature between the two, and Rudi Grinberg took a shot at this a while back.  I'm not sure that it's worth the trouble right now.

- Andy Ray did something interesting with the Lwt backend: he ported it to _javascript_ by implementing an IO backend that marshals the requests to and from strings.  This allows REST API users built over Cohttp (such as ocaml-github) to compile to pure _javascript_ as well.


- The heavy use of functors does make it hard to navigate the 'end user' API, even though those interfaces never expose any functors (for instance, you just use Cohttp_lwt_unix directly in most cases). This is a drawback of current OCaml tooling, and Merlin (for IDEs) and Codoc (for cross-referenced documentation) will fix this soon.

- A bigger problem that needs to be addressed in Cohttp2 is body handling, which we basically got wrong in this iteration.  The Body module is not idempotent, so to_string does not always return the same value if called multiple times.  The caller can currently be careful, but this is just an awful part of the API.  There are enough users of Cohttp that we'll leave it for 1.0, but hopefully fix it quite rapidly for 2.0.

- Cohttp is not a complete HTTP client, and doesn't implement the full logic for redirections, loop detection and so on.  That's the job of a library built over it, and there is some nascent code in opam-mirror that can do this [1].  Before building this, David Sheets and I want to look at some of the more larger API clients built using it (such as Vincent Bernardoff's BitStamp API [2]) and take a shot at a portable client API that will work with both Lwt and Async.

So was functorising this heavily such a good idea?  I think so -- the litmus test is whether or not there is more than one different implementation for each parameterised module, and this has worked out particularly well for the Cohttp_lwt backend, where there are now 4 (!) very different implementations.

Hope this helps,

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