Science Fair Project Encyclopedia
Publish and Subscribe
Publish and Subscribe was a document linking model introduced by Apple Computer in System 7. It extended the existing cut and paste editing model with a notification system; "subscribers" could include parts of "published" documents within themselves, and changes to the original published document would be noticed and updated by the subscribers. To the user the system was very similar to cut and paste in concept; material would be selected from the source document and published, creating a clipping file, then placed into the subscriber by selecting that clipping file and positioning it inside the document.
In general terms the concept was very similar to Microsoft's Object Linking and Embedding system. Unlike OLE, P&S was horribly complex from a programming standpoint. Suffering from second system effect, it included features intended to make it better than OLE, including support for non-rectangular areas, network notifications and an extensive user interface. A result of this complexity was poor uptake among developers, and P&S-enabled applications were few and far between. One of the few products to use it effectively was Claris's suite of software, including ClarisWorks which had already built a system somewhat similar in concept.
At the time many in the industry felt this was the "next big thing". Apple and Microsoft were not the only two companies trying to introduce such a system; most major software vendors attempted to introduce similar systems, and unsurprisingly NeXTSTEP included a powerful and very easy-to-implement version. However it did not take long to notice that the feature was essentially useless, users could find few real-world uses for the system. Further it was sometimes confusing to use, if the document included live links it was no longer possible to simply copy it to a floppy disk (for instance), the clipping file had to be copied as well. Eventually most vendors simply adbandoned the concept, while Microsoft re-positioned theirs several times, eventually as a plugin system known as ActiveX.
Publish and Subscribe is also a message distrubution paradigm (e.g. email or Teletype messages). If I want to send a message to a list of interested parties but I don't know exactly who to send to or whether they will want to be a recipient for very long, I could use a publish and subscripe method. I send the message to a P&S message switching box. The box has a configurable user interface that autorised users can use to subscribe themselves to. Therefore, I publish the message and it is recieved by all who subscribed. It's a flexible and maintenable solution which sheilds the sender from the responsibility of knowing all the correct destinations.
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