I am seeing this issue frequently after I have implemented the ORDS in R12.2.9 upgrade. Our ORDS is hosted on a weblogic server this issue occurs when there are 10 connections updating a single table? Is there any setup for a maximum connection control?
Complete Error: ORDS was unable to make a connection to the database. This can occur if the database is unavailable, the maximum number of sessions has been reached or the pool is not correctly configured. The connection pool named: |apex|pu| had the following error(s): Exception occurred while getting connection: oracle.ucp.UniversalConnectionPoolException: All connections in the Universal Connection Pool are in use
That error means the pool has been exhausted. 10 is the DEFAULT pool size, and is almost NEVER correct for a production deployment.
It's very likely a decently active application will use all 10 connection from a pool, resulting in the exact error you are seeing.
So the answer: increase the max connections property for your pool, and restart ORDS. The hard part is: based on your application performance and activity profile, how large should the pool be?
Some good advice can be found here from our Real World Performance Team.