Persistence unit as RESOURCE_LOCAL or JTA?
JPA implementations have the choice of managing transactions themselves (RESOURCE_LOCAL), or having them managed by the application server's JTA implementation.
In most cases, RESOURCE_LOCAL is fine. This would use basic JDBC-level transactions. The downside is that the transaction is local to the JPA persistence unit, so if you want a transaction that spans multiple persistence units (or other databases), then RESOURCE_LOCAL may not be good enough.
JTA is also used for managing transactions across systems like JMS and JCA, but that's fairly exotic usage for most of us.
To use JTA, you need support for it in your application server, and also support from the JDBC driver.
As an addition to other answers
Here is an excerpt from the extremely useful article (published on the Apache TomEE website), which can also help answer the OP's first question (the link to the article is below).
Comparing RESOURCE_LOCAL and JTA persistence contexts
With <persistence-unit transaction-type="RESOURCE_LOCAL"> YOU are responsible for EntityManager (PersistenceContext/Cache) creating and tracking...
- You must use the EntityManagerFactory to get an EntityManager
- The resulting EntityManager instance is a PersistenceContext/Cache
- An EntityManagerFactory can be injected via the @PersistenceUnit annotation only (not @PersistenceContext)
- You are not allowed to use @PersistenceContext to refer to a unit of type RESOURCE_LOCAL
- You must use the EntityTransaction API to begin/commit around every call to your EntityManger
- Calling entityManagerFactory.createEntityManager() twice results in two separate EntityManager instances and therefor two separate PersistenceContexts/Caches.
- It is almost never a good idea to have more than one instance of an EntityManager in use (don't create a second one unless you've destroyed the first)
With <persistence-unit transaction-type="JTA"> the CONTAINER will do EntityManager (PersistenceContext/Cache) creating and tracking...
- You cannot use the EntityManagerFactory to get an EntityManager
- You can only get an EntityManager supplied by the container
- An EntityManager can be injected via the @PersistenceContext annotation only (not @PersistenceUnit)
- You are not allowed to use @PersistenceUnit to refer to a unit of type JTA
- The EntityManager given by the container is a reference to the PersistenceContext/Cache associated with a JTA Transaction.
- If no JTA transaction is in progress, the EntityManager cannot be used because there is no PersistenceContext/Cache.
- Everyone with an EntityManager reference to the same unit in the same transaction will automatically have a reference to the same PersistenceContext/Cache
- The PersistenceContext/Cache is flushed and cleared at JTA commit time
Anyone interested in learning the Java Persistence API - please do yourself a favor and read the full article here: JPA Concepts: JPA 101.