C

Large Persistent Hash Table Stored on Filesystem

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

#include <C_CkCache.h>

void ChilkatSample(void)
    {
    BOOL success;
    HCkCache cache;
    const char *key;
    const char *eTag;
    const char *itemValue;

    success = FALSE;

    // 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.
    cache = CkCache_Create();

    // 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.
    CkCache_AddRoot(cache,"c:/temp/myCache");
    CkCache_putLevel(cache,2);

    // Add some key/values to the persisted hash table (i.e. the cache).
    // The eTag is optional metadata.
    key = "apple";
    eTag = "";
    itemValue = "macos";
    success = CkCache_SaveTextNoExpire(cache,key,eTag,itemValue);

    // Add more items..
    success = CkCache_SaveTextNoExpire(cache,"microsoft","","windows");
    success = CkCache_SaveTextNoExpire(cache,"google","","android");

    // Lookup items:
    key = "microsoft";
    itemValue = CkCache_fetchText(cache,key);
    printf("%s: %s\n",key,itemValue);

    key = "apple";
    itemValue = CkCache_fetchText(cache,key);
    printf("%s: %s\n",key,itemValue);


    CkCache_Dispose(cache);

    }