Handling concurrent access
lazarus / 16-Databases

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Leledumbo · 2010-12-30 15:17

I'm currently writing a web service with data upload and retrieval. As
a web service, it will surely be used concurrently. All provided
services do database access. The problem is when the data size is
quite large (when uploading big files for instance), another incoming
request causes internal server error. If not big, two request incoming
in almost the same time could cause that too (tried from extjs). How
do I solve this?

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typo · 2010-12-30 17:02

Probably you need a Mutex,
http://en.wikipedia.org/wiki/Mutual_exclusion

For instance, this code avoids this program to be executed twice:

Code: [Select]program project1;

{$mode objfpc}{$H+}

uses
  {$IFDEF UNIX}{$IFDEF UseCThreads}
  cthreads,
  {$ENDIF}{$ENDIF}
  Interfaces, // this includes the LCL widgetset
  Forms, Unit1
  { you can add units after this }
  ,windows;

{$R *.res}

var
  hMutex: THandle;
begin
  hMutex := Crea]"]>Blockedtex(nil, True, PChar('MyUniqueName'));
  if (hMutex <> 0) and (GetLastError = 0) then
  begin
    Application.Initialize;
    Application.CreateForm(TForm1, Form1);
    Application.Run;
  end;
end.

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Leledumbo · 2010-12-31 04:39

Well, what should I say to the currently waiting request then? Reject
with "database is busy" or what? I wonder how PHP handles this.

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typo · 2010-12-31 10:07

You could wait on a loop until a timeout and use a message like
"Timeout reached ".

Someting like:

Code: [Select]  TimeOut := GetTickCount + 10000;
  repeat
    hMutex := Crea]"]>Blockedtex(nil, True, PChar('MyUniqueName2'));
    Application.ProcessMessages;
  until ((hMutex <> 0) and (GetLastError = 0)) or (GetTickCount >
TimeOut);
  if (hMutex <> 0) and (GetLastError = 0) then
  begin
    try 
      //
      // your code
      //
    finally
      ReleaseMutex(hMutex);
    end;
  end
  else
    ShowMessage('Timeout reached');

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Leledumbo · 2010-12-31 10:48

Thanks, but without asking, I know that it's Windows only. The
deployment platform is a Linux box. Perhaps I need a critical section?

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typo · 2010-12-31 11:15

I don't know if there is any implementation of Mutexes for Linux in
Lazarus.

AFAIK, Linux Mutexes can't be used to different processes, maybe you
need a Semaphore, more specifically a Binary Semaphore.

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McCorder · 2011-01-03 17:53

Probably a silly comment, but for web services shouldn't you pass off
each request onto a separate thread allowing more than one to run
concurrently ? You could limit the numbers of threads by having a pool
of available ones for use.

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typo · 2011-01-03 18:05

Mutexes allow different threads or processes to share resources that
should be accessed by one at a time.

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Leledumbo · 2011-01-04 14:36

QuoteProbably a silly comment, but for web services shouldn't you pass
off each request onto a separate thread allowing more than one to run
concurrently ? You could limit the numbers of threads by having a pool
of available ones for use.With one database server, that wouldn't make
any difference.

Either mutex or semaphore, I don't really care. It's just something
that can block for a while when another operation is using the
database.

Update: I've got an idea to use empty temp file to indicate whether a
process is using the database or not and wait until that file gets
deleted.

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typo · 2011-01-04 14:48

Except for the fact that file creation and deletion can have their own
errors, mutexes or binary semaphores have the same effect.

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McCorder · 2011-01-08 23:47

Quote from: Leledumbo on January 04, 2011, 03:36:45 pmQuoteProbably a
silly comment, but for web services shouldn't you pass off each
request onto a separate thread allowing more than one to run
concurrently ? You could limit the numbers of threads by having a pool
of available ones for use.With one database server, that wouldn't make
any difference.

Either mutex or semaphore, I don't really care. It's just something
that can block for a while when another operation is using the
database.

Update: I've got an idea to use empty temp file to indicate whether a
process is using the database or not and wait until that file gets
deleted.

The database server (if it is a server) will be able to handle
multiple connections. So you have a pool of them (say 10) which you
pass off to a new thread in response to a new request. If you need to
block then your server is not a server really, it can only handle one
request.

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Leledumbo · 2011-01-09 02:58

QuoteThe database server (if it is a server) will be able to handle
multiple connections. So you have a pool of them (say 10) which you
pass off to a new thread in response to a new request. If you need to
block then your server is not a server really, it can only handle one
request.Yeah, probably the client library isn't compiled with thread
safe option enabled.