Xbase++

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. Xbase++

LOCAL nSuccess
LOCAL oCache
LOCAL cKey
LOCAL cETag
LOCAL cItemValue

nSuccess := 0

//  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.
oCache := CreateObject("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.
oCache:AddRoot("c:/temp/myCache")
oCache:Level := 2

//  Add some key/values to the persisted hash table (i.e. the cache).
//  The eTag is optional metadata.
cKey := "apple"
cETag := ""
cItemValue := "macos"
nSuccess := oCache:SaveTextNoExpire(cKey, cETag, cItemValue)

//  Add more items..
nSuccess := oCache:SaveTextNoExpire("microsoft", "", "windows")
nSuccess := oCache:SaveTextNoExpire("google", "", "android")

//  Lookup items:
cKey := "microsoft"
cItemValue := oCache:FetchText(cKey)
? cKey + ": " + cItemValue

cKey := "apple"
cItemValue := oCache:FetchText(cKey)
? cKey + ": " + cItemValue

oCache:destroy()