Java

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. Chilkat2 Java

import com.chilkatsoft.chilkat2.Cache;

public class ChilkatExample {

    public static void main(String[] args) {
        // 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 cache = new Cache();

        // 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.
        cache.addRoot("c:/temp/myCache");
        cache.setLevel(2);

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

        // Add more items..
        cache.saveTextNoExpire("microsoft", "", "windows");
        cache.saveTextNoExpire("google", "", "android");

        // Lookup items:
        key = "microsoft";
        itemValue = cache.fetchText(key);
        System.out.println(key + ": " + itemValue);

        key = "apple";
        itemValue = cache.fetchText(key);
        System.out.println(key + ": " + itemValue);
    }
}