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    <p>You basically have two issues here:</p> <ol> <li><p>You want a browser to be able to receive asynchronous events from the web application server without polling in a tight loop.</p></li> <li><p>You want the web application to be able to receive asynchronous events from the database without polling in a tight loop.</p></li> </ol> <p><strong>For Problem #1</strong></p> <p>See these wikipedia links for the type of techniques I think you are looking for:</p> <ul> <li><a href="http://en.wikipedia.org/wiki/Comet_%28programming%29" rel="nofollow noreferrer">Comet</a></li> <li><a href="http://en.wikipedia.org/wiki/Reverse_Ajax" rel="nofollow noreferrer">Reverse AJAX</a></li> <li><a href="http://en.wikipedia.org/wiki/Push_technology#HTTP_server_push" rel="nofollow noreferrer">HTTP Server Push</a></li> </ul> <p>EDIT: 19 Mar 2009 - Just came across <a href="http://www.reversehttp.net/" rel="nofollow noreferrer">ReverseHTTP</a> which might be of interest for Problem #1.</p> <p><strong>For Problem #2</strong></p> <p>The solution is going to be specific to which database you are using and probably the database driver your server uses too. For instance, with <a href="http://www.postgresql.org/" rel="nofollow noreferrer">PostgreSQL</a> you would use <a href="http://www.postgresql.org/docs/8.3/interactive/sql-listen.html" rel="nofollow noreferrer">LISTEN</a> and <a href="http://www.postgresql.org/docs/8.3/interactive/sql-notify.html" rel="nofollow noreferrer">NOTIFY</a>. (And at the risk of being down-voted, you'd probably use database triggers to call the NOTIFY command upon changes to the table's data.)</p> <p>Another possible way to do this is if the database has <a href="http://www.postgresql.org/docs/8.3/interactive/spi.html" rel="nofollow noreferrer">an interface</a> to create stored procedures or triggers that link to a dynamic library (i.e., a DLL or .so file). Then you could write the server signalling code in <a href="http://www.postgresql.org/docs/8.3/interactive/triggers.html" rel="nofollow noreferrer">C or whatever</a>.</p> <p>On the same theme, some databases allow you to write stored procedures in languages such as <a href="http://www.postgresql.org/docs/8.3/interactive/external-pl.html" rel="nofollow noreferrer">Java, Ruby</a>, <a href="http://www.postgresql.org/docs/8.3/interactive/plpython.html" rel="nofollow noreferrer">Python</a> and <a href="http://www.postgresql.org/docs/8.3/interactive/external-pl.html" rel="nofollow noreferrer">others</a>. You might be able to use one of these (instead of something that compiles to a machine code DLL like C does) for the signalling mechanism.</p> <p>Hope that gives you enough ideas to get started.</p>
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    1. POWhat's the best way for the client app to immediately react to an update in the database?
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    1. COThis is precisely what I was looking for. Thanks!
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    2. COI don't know why but server push is rarely utilized well.
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