Tcl

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. Tcl

load ./chilkat.dll

set success 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.
set cache [new_CkCache]

# 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.
CkCache_AddRoot $cache "c:/temp/myCache"
CkCache_put_Level $cache 2

# Add some key/values to the persisted hash table (i.e. the cache).
# The eTag is optional metadata.
set key "apple"
set eTag ""
set itemValue "macos"
set success [CkCache_SaveTextNoExpire $cache $key $eTag $itemValue]

# Add more items..
set success [CkCache_SaveTextNoExpire $cache "microsoft" "" "windows"]
set success [CkCache_SaveTextNoExpire $cache "google" "" "android"]

# Lookup items:
set key "microsoft"
set itemValue [CkCache_fetchText $cache $key]
puts "$key: $itemValue"

set key "apple"
set itemValue [CkCache_fetchText $cache $key]
puts "$key: $itemValue"

delete_CkCache $cache