On Thu, Jun 21, 2012 at 4:46 PM, Felix E. Klee <felix.klee_at_inka.de>
wrote:
> Helpful would be an example list of steps for a typical retrieval
> [...]

What follows is the coarse flow of a program that watches a database and
downloads data from requested URLs.

Key events for exactly one request (query), which is successful:

1. Program start

  The event loop is started.

2. Timer event (database-watcher)
  Timer event (database-watcher)
  ...

  The database is checked every half a second until finally a request
  (URL) is available.

  Once a request is available, the event handler constructs a query
  based on the URL. The associated cURL easy handle is passed to
  curl_multi_add_handle().

  curl_multi_add_handle() calls CURLMOPT_TIMERFUNCTION with a timeout of
  just 1 ms. CURLMOPT_TIMERFUNCTION adjust the timeout for the
  check-cURL-handles-watcher.

3. Timer event (check-cURL-handles-watcher)

  The event handler checks the cURL easy handles by calling
  curl_multi_socket_action() with CURL_SOCKET_TIMEOUT. Remember that in
  this example there is just one cURL easy handle.

  curl_multi_socket_action():

  a) Calls CURLMOPT_SOCKETFUNCTION, which sets up a socket-watcher for
    the socket associated with the cURL easy handle. The socket-watcher
    is instructed to watch IO-read-events (EV_READ).

  b) Calls CURLMOPT_TIMERFUNCTION with timeout 39999 ms (= 40 s = far in
    the future).

4. IO-read-event (socket-watcher)

  The event handler calls curl_multi_socket_action() for the socket.

  curl_multi_socket_action() calls CURLMOPT_SOCKETFUNCTION, which
  updates the socket-watcher to watch IO-write-events (EV_WRITE).

5. IO-write-event (socket-watcher)
  IO-write-event (socket-watcher)
  ...

  Data is available, repeatedly. Every time:

  a) The event handler calls curl_multi_socket_action() for the socket.

  b) curl_multi_socket_action() calls CURLOPT_WRITEFUNCTION, which
    appends retrieved data into the database.

6. IO-write-event (socket-watcher)

  The transfer is done.

  a) The event handler calls curl_multi_socket_action() for the socket.

  b) curl_multi_socket_action() calls CURLMOPT_SOCKETFUNCTION, which
    removes the socket-watcher.

  c) curl_multi_socket_action() calls CURLMOPT_TIMERFUNCTION with a
    timeout of -1 ms (= reach timeout immediately).

  d) CURLMOPT_TIMERFUNCTION calls curl_multi_socket_action() with
    CURL_SOCKET_TIMEOUT.

  e) CURLMOPT_TIMERFUNCTION finishes up the completed query (frees up
    resources, etc.).

  f) CURLMOPT_TIMERFUNCTION halts the check-cURL-handles-watcher.

How I named the cURL multi callback functions to make clear what is
going on:

* CURLMOPT_SOCKETFUNCTION: on_update_socket_watcher

* CURLMOPT_TIMERFUNCTION: on_timeout_changed

What is not clear to me yet: What writes into the socket?
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Received on 2012-06-23