What does CICS equates to in the Java world

It is always tough to explain what CICS exactly is to someone with a Java background.

One misconception I often hear is that it is similar to JBoss TS (Arjuna), the transaction manager in JBoss.

I think the issue is with the "Transaction" moniker. In the Java world, it is generally understood as a "Database transaction", something that has to do with coordinating resource updates.

In CICS, the meaning is different. If you monitor an active CICS system, what you will see are a number of active transactions. CICS transactions are primarily scheduling units. A CICS transaction is associated with an initial executable program which can in turn call other programs. CICS transactions get alloted chunks of memory for their programs to use, get CPU cycles when CICS decides, are authorized or not to a given user, etc. That's why CICS is called a Transaction Monitor: what is does is monitor CICS transactions.

A CICS transaction might very well never perform any update to any resource. It would still be a CICS transaction. A CICS transaction can send a form to a screen or receive data entered from a keyboard. CICS transactions can save conversation contexts, pretty much like servlets do.

Now, if a CICS transaction (or to be more precise some program it is associated with) does perform an update to a resource (say a VSAM file), then the CICS transaction is actually the default boundary from a "database transaction" standpoint. This means that when the transaction ends fine, all updates are committed but if the transaction fails (abends in CICS jargon), then all updates since the start of the CICS transaction are undone.

The CICS API also has COMMIT/ROLLBACK to give programs more control over the database transaction. From this standpoint, CICS is a local transaction manager. Now this is true for resources such as VSAM but more complex systems, such as DB2 or IMS-DB, have their own transaction managers. So who does the distributed transaction coordination that is needed when a CICS transaction updates both a VSAM file and a DB2 table?

Well, that would be RRS (Resource Recovery Services), a separated address space (process) in z/OS. That would be the closest thing I can think of to JBoss TS in a z/OS world.