This is one error which can be attributed to a lot of things. The first time I encountered this, there was not much help I could find on Google that would specifically pinpoint to my problem. Some sites tells of limitations to virtual hosts, some to DB failures, and there are still some forums wherein the problem was not even answered accordingly. So I just tried to connect the dots of every bit of information I can find from the search engines in order to fix this error in one of my Apache web servers.
As a background, HTTP 503 errors or a “Service Temporarily Unavailable” error page is commonly caused by a problem with web services components or modules. This simply means, that the web server is up and running but it can’t service the request due to a failure somewhere in the web components, DB server, or application server integrated into it. There are even times wherein only specific virtual hosts are affected by this. If found it quite weird, but then it’s happening.
In my situation, I found the problem to be in my Resin application server which I use for JSP/Servlets support for my web sites. I can’t really remember what caused the web server restart, but when it started all of the components of my web services started except for Resin. It didn’t take me long to fix the problem, with the knowledge of its probable causes. Basically, since I’m pretty sure that the problem is not within Apache’s core itself, I tried looking at the different modules loaded with Apache. The first thing I checked is if my MySQL server is running fine and it was ok. Then I tried checking PHP support (mod_php), then moved onto checking Perl (mod_perl), and then some other modules such as mod_security but have seen no problems with it.
And then I noticed the following lines:
LoadModule caucho_module /etc/httpd/modules/mod_caucho.so
ResinConfigServer localhost 6802
So I checked if the Resin application server was running, and found out that it didn’t start. So.. BINGO!!!.. there’s the culprit! I then started Resin and everything worked fine. That was pretty simple, but then the information from the Internet or search engines would never make sense at first glance, but if you just try to understand the nature of the problem, and the possible solutions, then you’ll be good to go in not time. 
[tags]Java, Linux, Apache, Resin Application Server, HTTP 503, Service Temporarily Unavailable[/tags]
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