JBO-30003: The application pool failed ...
Lately I've been troubleshooting "JBO-30003: The application pool failed to checkout an application module" error with Oracle Application Server 10g. The person which reported the error, said that this is definitely server side issue. So I started to debug this problematic java application.
Oracle Metalink and Oracle forums claims that this is DB connection related error. So after dealing with some OC4J different configurations I started to debug this application on OS level.
First thing I've done, I've checked with tcpdump where this application is trying to connect, and found out that it's trying to connect to wrong DB server. Somebody hardcoded wrong connection in the application. Is that server side error? I don't think so.
To be 100% sure that it's gonna work with proper connection, I've redirected traffic addressed to wrong DB server to proper one and application started to work flawlessly.
So when you see this error you should really check DB connections.