Many people ask me the following:
What’s the difference between JMS and JavaSpaces?
JMS is essentially a “one-way” comunication technology - fire and forget. Hope that someone on the other side accepts the message and does the right thing with it. This is okay for many things in particular certain aspects of financial systems.
JavaSpaces can certainly support this “one-way” pattern but they are designed to be more generic which means you can use them for JMS-type applications but they might not be the best choice. The key difference comes down to the fact that JavaSpaces can support a more “interactive” style of communication, typically two or multi-party, single or multi round bi-directional “conversations” which explains why they are a favourite for master/worker patterns. JavaSpaces can also be a potential solution for cases where a JMS approach is suffering from “topic explosion”.
Many people get hung up on JavaSpace.notify() because it is “not reliable”. This needs careful explanation - most JavaSpaces will endeavour to deliver all appropriate events to a client. If the client becomes permanently unavailable (NoSuchObjectException) further events will not be delivered to it.
A client that is temporarily unavailable might not receive all events with this typically occurring due to network partition. Those applications that can’t tolerate such loss tend to be deployed on reliable networks with backup infrastructure to ensure no events are lost. It has to be done this way because even JMS implementations have finite storage and if a network is down too long, there is potential for overflow leading to lost messages.
As of JavaSpace05 a client can figure out what events it missed by re-registering for notify and then using contents to locate any messages not received. Typically, such messages are leased so the client must return within the leased time period but that could be as much as 24 hours. The reason for leasing the messages is to automatically clean them up after a period of time.
Clients of JavaSpace.notify() have an additional option which is to have all their events routed to an EventMailbox located (in network hop terms) close to the JavaSpace which they can then receive events from (and as of Jini 2.1 you can have those events pulled by your client).
Update: Updated the last paragraph, as of Jini 2.1, clients can pull events - shame on me for getting that wrong.
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