How to use a JTA transaction with two databases?

App1 interacts with App2(EJB application) using some client api exposed by App2.Uses CMT managed JTA transaction in Jboss.We are getting the UserTransaction from App2(Jboss) using JNDI look up.

1. App1 makes a call to App2 to insert data into DS2 using UserTransaction's begin() and commit().
2. App1 makes a call to DS1 using Hibernate JPA to insert data into DS1 using JPATransaction Manager.
3. Is it possible to wrap above both DB operations in a single transaction(Distributed transaction)

PFB the image which describes requirement

To do this it´s necessary to implement your own transactional resource, capable of joining an ongoing JTA transaction. See this answer as well for some guidelines, one way to see how this is done is to look at XA driver code for a database or JMS resource, and base yourself on that.

This is not trivial to do and a very rare use case, usually solved in practice by adopting an alternative design. One way would be to extract the necessary code from App2 into a jar library, and use it in Tomcat with a JTA transaction manager like Atomikos connected to two XA JTA datasources.

Another way is to flush the SQL statements into the database into tomcat and see if that works, before sending a synchronous call to JBoss, returning the result if the transaction in JBoss went through.

Depending on that commit/rollback in tomcat. This does not guarantee that will work 100% of the times (network failure etc) but might be acceptable depending on what the system does and the business consequences of a failed transaction.

Yet another way is to make the operation revertable in JBoss side and expose a compensate service used by tomcat in case errors are detected. For that and making the two servers JBoss you could take advantage of the JBoss Narayana engine, see also this answer.

Which way is better it depends on the use case, but implementing your own XA transactional services is a big undertaking, I would be simpler to change the design. The reason that very few projects are doing it is doing it is that it´s complex and there are simpler alternatives.