Photo Caching

AirPlay supports preloading picture data to improve transition latency. This works by preloading a few pictures (most likely the ones before and after the current picture) just after displaying one.

Preloading is achieved using the cacheOnly asset action. Upon receiving this request, a server stores the picture in its cache. Later, a client can request the display of this picture using the displayCached asset action and the same asset key. This is much faster than a full picture upload because no additional data is transmitted.

When asked for a picture which is no longer in the cache, a server replies with an HTTP 412 error code (Precondition Failed).

Example 1: cache a picture for future display

PUT /photo HTTP/1.1
X-Apple-AssetAction: cacheOnly
X-Apple-AssetKey: B0DDE2C0-6FDD-48F8-9E5B-29CE0618DF5B
Content-Length: 462848
User-Agent: MediaControl/1.0
X-Apple-Session-ID: 1bd6ceeb-fffd-456c-a09c-996053a7a08c

<JPEG DATA>

HTTP/1.1 200 OK
Date: Thu, 23 Feb 2012 17:33:45 GMT
Content-Length: 0

Example 2: show a cached picture

PUT /photo HTTP/1.1
X-Apple-AssetAction: displayCached
X-Apple-AssetKey: B0DDE2C0-6FDD-48F8-9E5B-29CE0618DF5B
X-Apple-Transition: Dissolve
Content-Length: 0
User-Agent: MediaControl/1.0
X-Apple-Session-ID: 1bd6ceeb-fffd-456c-a09c-996053a7a08c

HTTP/1.1 200 OK
Date: Thu, 23 Feb 2012 17:33:45 GMT
Content-Length: 0