Another common service often found in some applications are Java Message Service (JMS) services. JMS services are message services that allow applications that run on the JVM to communicate with each other through message passing. There are two main channels of communication: Topics (pub/sub) or Queues (p2p). With Topics, a producer sends a message to a destination and all subscribers to that channel get the message. You can think of this as a mailing list. When you are subscribed or opt in for mails from a vendor, then you are added to their e-mail or mail distribution, and you receive all communication sent by the vendor afterwards. What subscribers choose to do with the received message is entirely up to them. With Queues, the producer sends a message to a destination and only one subscriber gets to pick up the message at a time. A classic use of a Queue for example could be sending out a registration mail to a new registrant of a website or sending a payment invoice...