How NetCache handles streaming objects

NetCache performs segmented caching of streams

NetCache caches streams by dividing them into fixed time segments. Each stream is cached as a series of objects, each containing n seconds of data. When a user plays a stream, NetCache caches each segment of streaming media as soon as it has played for enough time to be segmented.

If a client watches five minutes of a stream, NetCache caches as many objects as it can segment in that five minutes. If another client subsequently requests the same content, NetCache serves the cached content and prefetches portions of content that were not cached when the client is within n seconds of viewing it.

How streaming content is stored on disk

Because NetCache stores streams as a collection of discrete objects, entire streams do not have to be ejected from the cache when disk space is required to cache a small Web object. For example, a small portion of a large stream can be ejected to make room for new HTTP or streaming objects.

About streaming Time-To-Live

Streaming protocols do not have a formalized notion of Time-To-Live (TTL).

Standard RTSP streams

The default TTL for RTSP streams is equal to the length of the stream. For example, a two-hour film has a default TTL of two hours.

RealNetworks streams

To comply with RealNetworks licensing, the NetCache appliance reverifies RealNetworks streaming content each time the content is requested.

MMS streams

The default TTL for MMS streams is found by comparing the length of the stream to 20 percent of the last modified time. The lesser of these two values becomes the default TTL. For example, if the stream length is two hours and the stream was last modified one day earlier, the TTL for the stream will be 2 hours (because 20% of 24 hours = 4.8 hours).

Note
If the time specified in the Force Refresh Rate option (Setup tab > Streaming > Splitting and Caching Behavior) has expired, NetCache revalidates the stream, regardless of the stream's TTL. See the online Help for more information.

Modifying the default TTL

You can modify the TTL behavior for both RTSP and MMS by locking the content using the confill command or by modifying the value specified in the Force Refresh Rate option (Setup tab > Streaming > Caching and Splitting Behavior).

For more information about the confill command, see Prefilling streaming media content.

What happens when a stream expires

When a stream expires, NetCache does not immediately invalidate the object in the cache. Instead, all versions of all streams remain in the cache until NetCache needs to reclaim disk space. If a client requests a stream of one format, such as RTSP, and the same stream is cached in another format, such as MMS, the MMS stream is not ejected.

When an expired stream is referenced by a user request, NetCache sends a query to the originating streaming server to determine whether the stream in the cache is the current version, that is, whether the stream has been modified. If the stream on the streaming server has not been modified, NetCache serves the cached stream to the user and resets the stream's cache timestamp. If the stream on the streaming server has been modified, NetCache caches the new version and delivers it to the client.