Lua

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. Lua

local chilkat = require("chilkat")

-- 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.
local cache = chilkat.Cache.new()

-- 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:add_root("c:/temp/myCache")
cache.level = 2

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

-- Add more items..
success = cache:save_text_no_expire("microsoft", "", "windows")
success = cache:save_text_no_expire("google", "", "android")

-- Lookup items:
key = "microsoft"
itemValue = cache:fetch_text(key)
print(key .. ": " .. itemValue)

key = "apple"
itemValue = cache:fetch_text(key)
print(key .. ": " .. itemValue)