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#

bool 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.
Chilkat.Cache cache = new Chilkat.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.Level = 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";
success = cache.SaveTextNoExpire(key,eTag,itemValue);

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

// Lookup items:
key = "microsoft";
itemValue = cache.FetchText(key);
Debug.WriteLine(key + ": " + itemValue);

key = "apple";
itemValue = cache.FetchText(key);
Debug.WriteLine(key + ": " + itemValue);