B4X

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. B4X

Dim success As Boolean = 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.
Dim cache As ChilkatCache
cache.Initialize

'  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.
Dim key As String = "apple"
Dim eTag As String = ""
Dim itemValue As String = "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)
Log(key & ": " & itemValue)

key = "apple"
itemValue = cache.FetchText(key)
Log(key & ": " & itemValue)