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Thread: Help: Bug with removing data from a Hash Map

  1. #1
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    Default Help: Bug with removing data from a Hash Map

    Hi,

    I am working on this code..where a datacache is implemented as a HashMap. The Hashmap has a key and an string arraylist <> associated with it. There is a refresh time period and data expiry time associated with data in the hashmap. In certain scenerios, some data of certain Type in the data cache needs to be refreshed and not all.

    The way refresh is working currently, it makes a query to the database for the dataType which needs to be refershed and adds that data to the cache. Since the key for HashMap needs to be unique it overwriites any data associated with a key which already exists and adds any new data.

    However, a bug with this implemention is it does not remove any data from the cache which does not exsist in the database anymore.

    Nothing gets removed from the datacache until expiry period is reached. But it is not necessary refresh will always be called after expiry of data.

    If someone can please suggest a way I can improve the refresh method so that it removes any Key/data from the cache which is not in the database anymore?

    I do not want to clear the cache completely before refresh is called.

    Any suggestions?

    Thanks.


  2. #2
    Super Moderator Norm's Avatar
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    Default Re: Help: Bug with removing data from a Hash Map

    does not remove any data from the cache
    This sounds like a logic problem. Hard to make any suggestions without seeing the code.
    Does the code that is to remove the entries from the HashMap get called when you want it to be called?

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    Super Moderator Sean4u's Avatar
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    Default Re: Help: Bug with removing data from a Hash Map

    Caches have to be read-through, write-through or they just don't work. If you have some other access to the backing store which is modifying it independently of the cache, the only way you can fix this problem is to check the backing store to see if your cached data is stale, and bang goes your cache benefit (okay, you save some deserialization).

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    Default Re: Help: Bug with removing data from a Hash Map

    Quote Originally Posted by Norm View Post
    This sounds like a logic problem. Hard to make any suggestions without seeing the code.
    Does the code that is to remove the entries from the HashMap get called when you want it to be called?
    Yes, the code to remove the entries can be called at anytime. However, the way the remove function is implemented right now it removes only the data which has reached its expiry time. It does not check for data which may not be in the database anymore.

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    Default Re: Help: Bug with removing data from a Hash Map

    Can't you just remove the data from the Map when it is removed from the database? Without knowing more about the implementation its hard to suggest specifics, but given a simple Map object, when delete is called, just call remove on the map with the given key.

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