Bean scopes are quite important for the cases when state is kept in the application for longer than the duration of a single request.

If the whole business process can be implemented in a stateless way, by just executing some logic and discarding all state afterwards, scope definitions are pretty straightforward. Stateless session beans with dependent scoped CDI beans already fulfill a lot of these cases.

The EJB singleton scope and the CDI application scope, respectively, are used quite frequently as well. Single instances of a bean type are a straightforward way to store or cache information that have a long lifespan. Besides all the sophisticated caching technology, a singleton containing simple collections or maps with managed concurrency is still the most simple way to design application-specific, volatile stores. Singletons also provide a single point of responsibility for functionality that for some reason needs to be accessed in a restricted way.

The last scope of both EJBs and CDI beans is the session scope, which is bound to the client's HTTP session. Beans of this scope will be active and reused with all their states as long as the user's session is active. However, storing session data in stateful beans introduces the challenge that clients need to reconnect to the same application server again. This is certainly possible but prevents designing stateless applications which are easier to manage. If the application becomes unavailable, all temporary session data is lost as well. In modern enterprise applications, state is typically kept in the database or in caches for optimization purposes. Therefore, session scoped beans aren't used too much anymore.

CDI managed beans come with more built-in scopes, namely the conversation scope or the default dependent scope. There are also possibilities for adding custom scopes for special requirements. However, experience shows that the built-in scopes are usually sufficient for the majority of enterprise applications. The CDI specification provides further information how to extend the platform and develop custom scopes.

As you have seen, we can already achieve a lot with these Java EE core components. Before looking into integration technologies, such as HTTP communication or database access, let's have a closer look into design patterns used in our core domain.