Pascal (Lazarus/Delphi)

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. Pascal (Lazarus/Delphi)

program ChilkatDemo;

// Demonstrates using the Chilkat Pascal wrapper via the C bridge DLL.
// Builds as a console application under Lazarus (FPC) or Delphi.

{$IFDEF FPC}
  {$MODE DELPHI}
{$ENDIF}
{$APPTYPE CONSOLE}

uses
  {$IFDEF UNIX}
  cthreads,
  {$ENDIF}
  SysUtils,
  CkDllLoader,
  Chilkat.Cache;

// ---------------------------------------------------------------------------

procedure RunDemo;
var
  success: Boolean;
  cache: TCache;
  key: string;
  eTag: string;
  itemValue: string;

begin
  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.
  cache := TCache.Create;

  //  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.
  key := 'apple';
  eTag := '';
  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);
  WriteLn(key + ': ' + itemValue);

  key := 'apple';
  itemValue := cache.FetchText(key);
  WriteLn(key + ': ' + itemValue);


  cache.Free;

end;

// ---------------------------------------------------------------------------

begin

  try
    RunDemo;
  except
    on E: Exception do
      WriteLn('Unhandled exception: ', E.ClassName, ': ', E.Message);
  end;

  WriteLn;
  {$IFDEF MSWINDOWS}
  WriteLn('Press Enter to exit...');
  ReadLn;
  {$ENDIF}
end.