How do I design a “conveyor” of operations with OmniThreadLibrary?

Question

I have a Windows Delphi application with "Start" and "Stop" menu items accessible via a notification icon. After click on "Start", I need to do the following (as I see implementation):

1. ThreadMonitor: The first thread is waiting for the appearance of the specified file in the specified folder.

2. ThreadParse: Once the file appears, it should be transferred to another thread (for parsing content) and continue monitoring for the next file.

3. ThreadDB: Once all data are parsed, save them into MySQL DB. (Another background thread with active DB connection?)

4. ThreadLog: If there are any errors in the steps 1–3, write them to a log file (another background thread?) without interrupting the steps 1–3.

That is, it turns out that something like a continuous conveyor, whose work is stopped only by pressing Stop. What should I use from a whole variety of methods of OmniThreadLibrary?

Answer 1

It would probably be best to use Parallel.BackgroundWorker for logging and Parallel.Pipeline for data processing. Here's a sketch of a solution (compiles, but is not fully implemented):

unit PipelineDemo1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls,
  OtlCommon, OtlCollections, OtlParallel;

type
  TfrmPipelineDemo = class(TForm)
    btnStart: TButton;
    btnStop: TButton;
    procedure btnStartClick(Sender: TObject);
    procedure btnStopClick(Sender: TObject);
  private
    FLogger  : IOmniBackgroundWorker;
    FPipeline: IOmniPipeline;
  strict protected //asynchronous workers
    procedure Asy_LogMessage(const workItem: IOmniWorkItem);
    procedure Asy_Monitor(const input, output: IOmniBlockingCollection);
    procedure Asy_Parser(const input: TOmniValue; var output: TOmniValue);
    procedure Asy_SQL(const input, output: IOmniBlockingCollection);
  public
  end;

var
  frmPipelineDemo: TfrmPipelineDemo;

implementation

uses
  OtlTask;

{$R *.dfm}

procedure TfrmPipelineDemo.Asy_LogMessage(const workItem: IOmniWorkItem);
begin
  //log workItem.Data
end;

procedure TfrmPipelineDemo.Asy_Monitor(const input, output: IOmniBlockingCollection);
begin
  while not input.IsCompleted do begin
    if FileExists('0.0') then
      output.TryAdd('0.0');
    Sleep(1000);
  end;
end;

procedure TfrmPipelineDemo.Asy_Parser(const input: TOmniValue; var output: TOmniValue);
begin
  // output := ParseFile(input)
  FLogger.Schedule(FLogger.CreateWorkItem('File processed: ' + input.AsString));
end;

procedure TfrmPipelineDemo.Asy_SQL(const input, output: IOmniBlockingCollection);
var
  value: TOmniValue;
begin
  //initialize DB connection
  for value in input do begin
    //store value into database
  end;
  //close DB connection
end;

procedure TfrmPipelineDemo.btnStartClick(Sender: TObject);
begin
  FLogger := Parallel.BackgroundWorker.NumTasks(1).Execute(Asy_LogMessage);

  FPipeline := Parallel.Pipeline
    .Stage(Asy_Monitor)
    .Stage(Asy_Parser)
    .Stage(Asy_SQL)
    .Run;
end;

procedure TfrmPipelineDemo.btnStopClick(Sender: TObject);
begin
  FPipeline.Input.CompleteAdding;
  FPipeline := nil;
  FLogger.Terminate(INFINITE);
  FLogger := nil;
end;

end.