The reason we need clustered mongrels is because of shortcomings in the way Rails handles threads. No other framework/system I have used has required clustering except in a physical sense (clustering across physical nodes). However, because of these limitations we need to deploy with non-physical clustering just to get throughput with all the complexity and extra management overhead this entails (apache/nginx requirements, mod_proxy, multiple servers). Not to mention the extra server capacity required (50mb RAM for each instance).
PHP apps don't require this extra setup and configuration, neither do Java or .Net ... or ColdFusion or even good old-fashioned original recipe ASP.
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