EJBs are built on top of RMI. So RMI and EJB both imply Java clients and beans. If your clients
need to be written in something else (e.g., .NET, PHP, etc.) go with Web services or something
else that speaks a platform-agnostic wire protocol, like HTTP or XML over HTTP or SOAP.
 

If you choose RMI, you don't need a Java EE EJB app server. You have to keep client and server
JVMs in synch; you can't upgrade the client without upgrading the server.You have to write
all the services that the EJB Container provides for you (e.g., naming and directory services,
pooling, request queuing,transactions, etc.).

Web services have some gotchas that you need to watch out for,  Marshalling from XML to
objects and back consumes CPU cycles that aren't providing any business value besides
allowing your clients to speak a platform-agnostic protocol.

SOAP is a standard that is becoming more bloated and complex every day, but it has lots of
tool support. Vendors like it because it helps drive sales of ESBs. REST is simple but not
as well understood. It's not supported by tools.

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