Parsoid

Parsoid is a service that parses converts between wikitext and HTML. The HTML contains additional metadata that allows it to be converted back ("round-tripped") to wikitext. VisualEditor fetches the HTML for a given page from Parsoid, edits it, then delivers the modified HTML to Parsoid, which converts it back to wikitext.

Data flow

Parsoid runs entirely on an internal subnet, so requests to it are proxied through the ve-parsoid API module. This module is implemented in extensions/VisualEditor/ApiVisualEditor.php and is invoked with a POST request to /w/api.php?action=ve-parsoid. The API module then sends a request to Parsoid, either GET /$prefix/$pagename to get the HTML for a page, or POST /$prefix/$pagename to submit HTML and get wikitext back. Parsoid itself also issues requests to /w/api.php to get the wikitext of the requested page and to do template expansion.

Once the ve-parsoid API module receives a response from Parsoid, it either relays it back to the client (when requesting HTML), or saves the returned wikitext to the page (when submitting HTML).

                (POST /w/api.php?action=ve-parsoid)              (GET /en/Barack_Obama)                (requests for page content and template expansions)
Client browser ------------------------------------------> API ---------------------------->  Parsoid -----------------------------------------------------> API
    ^                                                      | ^                                 |   ^                                                          |
    |                  (response)                          | |      (HTML)                     |   |                   (responses)                            |
    +------------------------------------------------------+ +---------------------------------+   +----------------------------------------------------------+


                (POST /w/api.php?action=ve-parsoid)              (POST /en/Barack_Obama)
Client browser ------------------------------------------> API ---------------------------->  Parsoid
                                                           | ^                                 |
                                               (save page) | |      (wikitext)                 |
                                                           | +---------------------------------+
                                                           |
                                                        Database