How to write and execute a thread

Question

I have never worked with streams before, and now I'm trying to create a stream with a query to check the status of the database. The request is as follows:

select (*) as DBCount from v$datafile where status in 'OFFLINE'; .

This query returns the total number of all offline databases. Now I want to create a thread in Delphi that will execute this request against the background of my application when I run it and show the results on a shortcut.

Answer 1

Multithreading is tough. You might be better off using a simple stream infrastructure like AsyncCalls.

After the StackOverlow threads, you can get more information on how to solve your problem:

Delphi - Threading Framework

How to choose different ways to do threading in Delphi?

A delphi thread that waits for data, processes it, and then resumes waiting

Answer 2

This is pretty easy to do with AsyncCalls. Suppose your non-stream code looks like this (ignoring all errors):

 Query := 'select (*) as DBCount from...'; ExecuteSQL(SqlConnection,Query); SqlResult := GetSqlResult(SqlConnection); SqlRow := GetSqlRow(SqlResult); MyLabel.Text := SqlRow[0]; ...Go on to do other things... 

If the second line is blocked (waiting for a server response). Your new code will look like this:

 uses AsyncCalls; //added to your existing uses statement ... procedure DoesSomething(); var Thread: TAsyncCall; //your interface to AsyncCalls procedure AsyncSqlCall(); //this is a LOCAL procedure Query := 'select (*) as DBCount from...'; ExecuteSQL(SqlConnection,Query); SqlResult := GetSqlResult(SqlConnection); SqlRow := GetSqlRow(SqlResult); EnterMainThread; try Assert(GetCurrentThreadId = MainThreadId); MyLabel.Text := SqlRow[0]; finally LeaveMainThread; end; begin //this begins proc DoSomething() ... Thread := LocalAsyncCall(@AsyncSqlCall); ...Go on to do other things... end; 

All we did is put the blocking SQL calls in the local proc and tell AsyncCalls to execute it on another thread while the main thread continues to execute. The only tricky part is to use a VCL that is not thread safe. So I used this line in the main thread.

If at some point you need to make sure that the Async thread is completed, you must run this line to block the main thread before AsyncSqlCall completes:

 Thread.sync; 

It is very nice here that AsyncCalls handles everything related to creating a thread pool, creating threads, etc. Although not shown in this example, you can pass variables to your stream and return a value. You do not need to use local proc, but this gives it access to all local vars. You can make it all global and then run the Async thread in one routine and test its completion in another.

Limitations:

Your Async thread should not touch (read or write) anything but its own variables, and your main thread should not touch them while the Async thread is running. You must encode it that way. Nothing will stop you from creating complete chaos. In the above example, the main thread should not touch queries, SqlConnection, SqlResult, and SqlRow. If any part of your code used one of these vars before calling Thread.sync, your code would work - but make exceptions in strange places that you never expected. So keep it simple.

Your Async thread should not use VCL. The above example shows one of several ways to safely overcome this limitation.

Finally:

AsyncCalls is not a complete multithreaded structure. This is just a way to call processes and functions asynchronously (i.e., without waiting). Do not try to push it too far - I mean, do not try to make it the basis for a fully multitasking program.

Answer 3

Delphi has a TThread class. You can create such a stream using a simple wizard from the Delphi IDE (in Turbo Delphi: File-> New-> Delphi Projects-> Delphi Files-> Thread Obcjet). In its constructor, create a connection to the database and in the Execute method you can create a loop that queries the database, update some global variable (NOT declared as threadvar ) and sleep for some time.

Answer 4

Here you can find a discussion of using streams in a DataBase.

Delphi Simple Example

There is a code that may be useful for you.

Sincerely.