What does a Terracotta server do when it is used as a backend for EHCache with Hibernate?

Question

My DAL is implemented with Hibernate and I want to use EHCache as its second level cache with its distributed capabilities (for scalability and HA).
Seeing as EHCache provides distributed caching only with Terracotta my question is what is the role of the Terracotta server instance? Does it also hold data? Does it only coordinate the distribution between the partitioned cache parts?
My confusion derives mainly from this explanation regarding TSA which says the server holds the data but I think that maybe in my scenario the cache and the Terracotta server are sort of merged. Am I correct?
If the server does hold data then why shouldn't the bottleneck just move from the db to the Terracotta server?

Update: Affe's answer answered the second part of my question which was the important part but just in case someone comes by looking for the first part I'll say that the TC server has to hold all the data that the EHCache in memory holds and so if you want a distributed cache (not replicated) then the L2 (TC server) must hold all the objects itself as well.

Thanks in advance,
Ittai

Answer 1

The idea is it's still significantly faster to contact the terracotta cluster via the terracotta driver and do what's basically a Map lookup, than to acquire a database connection and execute an SQL statement. Even if that does become the application's choke point, overall throughput would be expected to still be significantly higher than a JDBC Connection + SQL choke point. Open connections and open cursors are big resource hogs in the database, an open socket to the terracotta cluster is not!

Answer 2

You can get ehcache clustered without using terracotta. They have documentation for doing it via RMI, JGroups and JMS. We are using JMS since we have a significant JMS infrastructure to handle the communication already. I don't know how well it will scale in the long term, but our current concern is just HA.