Checking for Updates

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Revision as of 11:34, 18 November 2009 by SMIL T.A. (Talk | contribs)

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It is often necessary to check for an update in the playlist while the current playlist is playing. To do that, simply use the "Refresh" meta tag as shown below.

<smil>
  <head>
    <meta http-equiv="Refresh" content="60"/>
  </head>
  <body>
    ...
  </body>
</smil>

The playlist will reload itself every 60 seconds (or any specified interval).

However, each time the playlist is reloaded, playback restarts from the beginning of the SMIL document. To avoid restarting the playlist when the playlist is not changed, have your SMIL generating script support the HTTP "Last-Modified:" header (which is usually omitted in dynamic scripts). If the last modified time has not changed, SMIL player will not reload the file and therefore will not restart playback.

How do I Generate Last-Modified: Header

The easiest way is to serve your playlist as a local file that is updated by a background daemon at regular intervals. Most web servers would automatically tag on the Last-Modified: header using the timestamp on the file.

It is a good practice to support the header. Media feeds that are designed to serve a large user base typically support the Last-Modified: header, such as:

http://api.flickr.com/services/feeds/photos_public.gne

---response begin---
HTTP/1.1 200 OK
Date: Wed, 18 Nov 2009 11:16:09 GMT
P3P: policyref="http://p3p.yahoo.com/w3c/p3p.xml", CP="CAO DSP COR CUR ADM DEV TAI PSA PSD IVAi IVDi CONi TELo OTPi OUR DELi SAMi OTRi UNRi PUBi IND PHY ONL UNI PUR FIN COM NAV INT DEM CNT STA POL HEA PRE GOV"
Expires: Mon, 26 Jul 1997 05:00:00 GMT
Last-Modified: Wed, 18 Nov 2009 11:16:01 GMT
Cache-Control: no-store, no-cache, must-revalidate
Cache-Control: post-check=0, pre-check=0
Pragma: no-cache
Connection: close
Content-Type: application/atom+xml; charset=utf-8

---response end---

Last-Modified: info is used by proxies to maintain their cache. Suppose 10,000 players access your playlist once every 10 minutes, it would not be wise to serve them via a live script (which would pull on YOUR SOURCE 1,000 times per minute, wasting lots of bandwidth and processor cycles).

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