Difference between revisions of "Checking for Updates"

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(Added section to recommend using static SMIL as best practice)
(Added use of HEAD and GET response headers)
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</source>
 
</source>
  
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The playlist will reload itself every 60 seconds (or any specified interval).  
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The playlist will check back every 60 seconds (or any specified interval), and reload itself if it is changed (determined by issuing an HTTP HEAD request and comparing the Last-Modifed response header against the previously version in-use).  
  
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== Best Practice: Using Static SMIL File ==
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== Best Practice: Using a Static SMIL File ==
  
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You may be tempted to use a server-side script to dynamically feed your playlist to the players. However, the simplest and most scalable way to host the SMIL playlist is to store it as a static file on the server. Updates are applied to the file when user chooses to update the schedule. When you perform the update, make sure the change is "atomic" so that players will not download a partial SMIL while it is being updated. Create the new SMIL file separately, and renaming it to the name of the old SMIL version is one way to do this.
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You may be tempted to use a server-side script to dynamically feed your playlist to the players. However, the simplest and most scalable way to host the SMIL playlist is to store it as a static file on the server. Updates are applied to the file when user chooses to update the schedule.  
  
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If you are using a server-side script as the "master entry point" for all your players, it is recommended that you use an HTTP "Location:" header to redirect your player to its corresponding SMIL file once you have determined the player's identity (see [[Player ID]]).  
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When you perform the update, make sure the change is "atomic" so that players will not download a partial SMIL while it is being updated. Create the new SMIL file separately, and renaming it to the name of the old SMIL version is one way to do this.
 +
 
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If you are using a server-side script as the "master entry point" for all your players, it is recommended that you use an HTTP "Location:" response header to redirect your player to its corresponding SMIL file once you have determined the player's identity (see [[Player ID]]).  
  
 
== Using a Dynamic Server-side Script ==
 
== Using a Dynamic Server-side Script ==
  
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If you must use a dynamic server-side script to generate your SMIL file, make sure you supply an HTTP "Last-Modified:" header so the player does not abruptly restart its playback each time it checks for an update. If the last modified time has not changed, SMIL player will not reload the file and therefore will not restart playback.
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If you must use a dynamic server-side script to generate your SMIL file, make sure you supply an HTTP "Last-Modified:" header so the player does not abruptly restart its playback each time it checks for an update.  
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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:
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http://api.flickr.com/services/feeds/photos_public.gne
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<PRE>
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During each Refresh cycle, the server receives two requests from the player: a HEAD followed by a GET request. The HEAD request only needs the HTTP header with an empty body (and therefore doesn't need to have the SMIL actually generated).
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---response begin---
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HTTP/1.1 200 OK
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...
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Last-Modified: Wed, 18 Nov 2009 11:16:01 GMT
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...
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---response end---
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The player looks at the HEAD response (mainly for Last-Modified: and Content-Length:), and decides whether it needs to do a GET for the updated SMIL file and restart playback.
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</PRE>
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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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Note that with the HEAD response you need to have all response headers IDENTICAL to the GET response, especially "Content-Length:". In order for you to return that number back to the player, you may need to cache that information (length of the SMIL file) after you have generated the SMIL file the first time. You must make sure the Content-Length returned in HEAD response is EXACTLY the same as GET, otherwise the player will think the GET does not match HEAD and keep asking until they are identical.
  
 
== Related ==
 
== Related ==

Revision as of 08:05, 9 July 2010

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 check back every 60 seconds (or any specified interval), and reload itself if it is changed (determined by issuing an HTTP HEAD request and comparing the Last-Modifed response header against the previously version in-use).

Best Practice: Using a Static SMIL File

You may be tempted to use a server-side script to dynamically feed your playlist to the players. However, the simplest and most scalable way to host the SMIL playlist is to store it as a static file on the server. Updates are applied to the file when user chooses to update the schedule.

When you perform the update, make sure the change is "atomic" so that players will not download a partial SMIL while it is being updated. Create the new SMIL file separately, and renaming it to the name of the old SMIL version is one way to do this.

If you are using a server-side script as the "master entry point" for all your players, it is recommended that you use an HTTP "Location:" response header to redirect your player to its corresponding SMIL file once you have determined the player's identity (see Player ID).

Using a Dynamic Server-side Script

If you must use a dynamic server-side script to generate your SMIL file, make sure you supply an HTTP "Last-Modified:" header so the player does not abruptly restart its playback each time it checks for an update.

During each Refresh cycle, the server receives two requests from the player: a HEAD followed by a GET request. The HEAD request only needs the HTTP header with an empty body (and therefore doesn't need to have the SMIL actually generated).

The player looks at the HEAD response (mainly for Last-Modified: and Content-Length:), and decides whether it needs to do a GET for the updated SMIL file and restart playback.

Note that with the HEAD response you need to have all response headers IDENTICAL to the GET response, especially "Content-Length:". In order for you to return that number back to the player, you may need to cache that information (length of the SMIL file) after you have generated the SMIL file the first time. You must make sure the Content-Length returned in HEAD response is EXACTLY the same as GET, otherwise the player will think the GET does not match HEAD and keep asking until they are identical.

Related

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