DataFlex

Large Persistent Hash Table Stored on Filesystem

See more Misc Examples

Demonstrates how to implement a large, persistent hash table that is stored on the filesystem and allows for quick retrieval using a hash key. DataFlex

Use ChilkatAx-win32.pkg

Procedure Test
    Boolean iSuccess
    Handle hoCache
    String sKey
    String sETag
    String sItemValue

    Move False To iSuccess

    // Let's say we would like to implement a persistent hash table with approximately 200,000 entries.
    // We want something simple and straightforward.
    // 
    // The Chilkat Cache class is a solution that might fit.
    // 
    // Each hash table entry is a file.  Depending on the number of anticipated hash entries,
    // the files can be contained in a single directory, or a collection of 256 directories,
    // or a collection of 256x256 directories.

    // There are 3 options:
    // 
    // Level 0: All cache files are in a single directory (the cache root).
    // Level 1: Cache files are located in 256 sub-directories numbered 0 .. 255 directly under the cache root.
    // Level 2: There are two levels of sub-directories under the cache root. 
    //          The 1st level has 256 sub-directories numbered 0 .. 255 directly under the cache root. 
    //          The 2nd level allows for up to 256 sub-directories (0..255) under each level-1 directory. 
    //          Cache files are stored in the leaf directories. 

    // For this example, given that we anticipate a larger number of hash entries, we choose a "level 2" cache.
    Get Create (RefClass(cComChilkatCache)) To hoCache
    If (Not(IsComObjectCreated(hoCache))) Begin
        Send CreateComObject of hoCache
    End

    // We can also spread the cache among several root directories, but for this example we'll only use one root directory.
    // Call AddRoot once for each root directory.
    Send ComAddRoot To hoCache "c:/temp/myCache"
    Set ComLevel Of hoCache To 2

    // Add some key/values to the persisted hash table (i.e. the cache).
    // The eTag is optional metadata.
    Move "apple" To sKey
    Move "" To sETag
    Move "macos" To sItemValue
    Get ComSaveTextNoExpire Of hoCache sKey sETag sItemValue To iSuccess

    // Add more items..
    Get ComSaveTextNoExpire Of hoCache "microsoft" "" "windows" To iSuccess
    Get ComSaveTextNoExpire Of hoCache "google" "" "android" To iSuccess

    // Lookup items:
    Move "microsoft" To sKey
    Get ComFetchText Of hoCache sKey To sItemValue
    Showln sKey ": " sItemValue

    Move "apple" To sKey
    Get ComFetchText Of hoCache sKey To sItemValue
    Showln sKey ": " sItemValue


End_Procedure