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Ejb cache with singelton implementation  RSS feed

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I would appreciate your advice about an issue I have.

I'm working with jboss, EJB 3.1 environment.

each time the user entering a specific screen, it should be a trigger for creating a tree data type which based on a data that saved on the database. this tree calculation can take some time, and it's can be heavy on performance.

the following requests are sent from UI :

  • 1. refreshTree - a trigger for building the tree
    2. isTreeReady - indicating whether the tree is ready to use, and called every x amount of time
    3. getTree - return the tree.

  • when building this I should take into consideration that multiple users can try to perform each one of those actions simultaneously. it's also true that 2 users share the same tree if any refresh hasn't been done.

    I've thought about implementing it as a cache as follow :

    the issue I have with that is that there can be a scenario in which :

    first user refreshing the tree - and the tree is built.
    then second user start to build the tree, and initialize the isReady flag to false, before the first user has noticed about the tree calculation - in this case the first user will need to wait to the calculation to be completed (even though he could have use the tree).

    I'm trying to think a bout using a read lock in some way (instead of the isTreeReady flag), but I can't think about any that will fit my needs.

    do you have an idea what can I do?

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