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Mainly build for the outdated android application but also used in rare occasions in other modules is the xhrapp.php front controller. This should always return pure json data (well, sometimes that might as well contain HTML/CSS/JS components). It may also, controlled by the client, use a jsonp remote procedure call.
Second API
When Matthias Larisch wanted to implement token authenticated API access, a new front controller was added: api.php. This currently only supports outputting various calendar events in ICAL format.
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