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Class Nethttpd_kernel.http_protocol


class http_protocol : #http_protocol_config -> Unix.file_descr -> object .. end
The core event loop of the HTTP daemon

Exchange of HTTP messages

In fd one must pass the already connected socket. It must be in non- blocking mode.

How to use this class: Basically, one invokes cycle until the whole message exchange on fd is processed. cycle receives data from the socket and sends data to the socket. There are two internal queues:

The receive queue stores parts of received requests as req_token. One can take values from the front of this queue by calling receive.

The response queue stores http_response objects. Each of the objects corresponds to a request that was received before. This queue is handled fully automatically, but one can watch its length to see whether all responses are actually transmitted over the wire.

The basic algorithm to process messages is:

 let rec next_token () =
   if proto # recv_queue_len = 0 then (
     proto # cycle ();
     next_token()
   )
   else
     proto # receive()

 let cur_token = ref (next_token()) in
 while !cur_token <> `Eof do
   (* Process first token of next request: *)
   match !cur_token with
    | `Req_header(req_line, header, resp) ->
         (* Depending on [req_line], read further tokens until [`Req_end] *)
         ...
         (* Switch to the first token of the next message: *)
         cur_token := next_token()
    | `Timeout -> ...
    | `Bad_request_error(e,resp) -> 
          (* Generate 400 error, send it to [resp] *)
          ...
          (* Switch to the first token of the next message: *)
          cur_token := next_token()
    | `Fatal_error e -> failwith "Crash"
    | _ -> assert false
 done;
 while proto # resp_queue_len > 0 do
   proto # cycle ();
 done;
 proto # shutdown()
 

See the file tests/easy_daemon.ml for a complete implementation of this.

As one can see, it is essential to watch the lengths of the queues in order to figure out what has happened during cycle.

When the body of the request is empty, `Req_body tokens are omitted. Note that for requests like GET that always have an empty body, it is still possible that an errorneous client sends a body, and that `Req_body tokens arrive. One must accept and ignore these tokens.

Error handling: For serious errors, the connection is immediately aborted. In this case, receive returns a `Fatal_error token. Note that the queued responses cannot be sent! An example of this is `Broken_pipe.

There is a large class of non-serious errors, esp. format errors in the header and body. It is typical of these errors that one cannot determine the end of the request properly. For this reason, the daemon stops reading further data from the request, but the response queue is still delivered. For these errors, receive returns a `Bad_request_error token. This token contains a http_response object that must be filled with a 400 error response.

method cycle : ?block:float -> unit -> unit
Looks at the file descriptor. If there is data to read from the descriptor, and there is free space in the input buffer, additional data is read into the buffer. It is also tried to interpret the new data as req_tokens, and if possible, new req_tokens are appended to the receive queue.

If the response queue has objects, and there is really data one can send, and if the socket allows one to send data, it is tried to send as much data as possible.

The option block (default: 0) can be set to wait until data can be exchanged with the socket. This avoids busy waiting. The number is the duration in seconds to wait until the connection times out (0 means not to wait at all, -1 means to wait infinitely). When a timeout happens, and there is nothing to send, and the last request was fully processed, receive will simply return `Timeout (i.e. when waiting_for_next_message is true). Otherwise, the fatal error `Timeout is generated.

method receive : unit -> req_token
Returns the first req_token from the receive queue. Raises Recv_queue_empty when the queue is empty (= has no new data)
method peek_recv : unit -> req_token
Peeks the first token, but leaves it in the queue. Raises Recv_queue_empty when the queue is empty.
method recv_queue_len : int
Returns the length of the receive queue (number of tokens)
method resp_queue_len : int
Returns the length of the internal response queue (number of http_response objects that have not yet fully processed)
method pipeline_len : int
Returns the number of unanswered requests = Number of received `Req_end tokens minus number of responses in state `Processed. Note that pipeline_len can become -1 when bad requests are responded.
method recv_queue_byte_size : int
Returns the (estimated) size of the input queue in bytes
method waiting_for_next_message : bool
Whether the kernel is currently waiting for the beginning of a new arriving HTTP request. This is false while the request is being received.
method input_timeout_class : [ `Next_message | `None | `Normal ]
Suggests the calculation of a timeout value for input:
  • `Normal: The normal timeout value applies
  • `Next_message: The timeout value applies while waiting for the next message
  • `None: The connection is output-driven, no input timeout value

method shutdown : unit -> unit
Shuts the socket down. Note: the descriptor is not closed.
method timeout : unit -> unit
Process a timeout condition as cycle does
method abort : fatal_error -> unit
Stops the transmission of data. The receive queue is cleared and filled with the two tokens `Fatal_error and `Eof. The response queue is cleared. The cycle method will return immediately without doing anything.
method fd : Unix.file_descr
Just returns the socket
method do_input : bool
Returns true iff the protocol engine is interested in new data from the socket. Returns false after EOF and after errors.
method do_output : bool
Returns true iff the protocol engine has data to output to the socket
method need_linger : bool
Returns true when a lingering close operation is needed to reliably shut down the socket. In many cases, this expensive operation is not necessary. See the class lingering_close below.
method config : http_protocol_config
Just returns the configuration
method test_coverage : string list
For testing: returns a list of tokens indicating into which cases the program ran.
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