Messaging systems Compare to RPC-based systems:

• In RPC, the client invokes a method on a distributed object on a server. Until the method invocation returns, the client is blocked; it must wait for the method invocation to end before it can execute the next instruction.
• In JMS, a client delivers a message to a virtual channel (topic or queue) to which other JMS clients subscribe or listen. When a JMS client sends a message, it doesn't wait for a reply. It executes the send operation and moves on to the next instruction. The message might eventually be forwarded to one client or many, none of which need reply.