[Lazarus] RE : RE : How to use critical sections to lookup database info inmain thread?

Ludo Brands ludo.brands at free.fr
Mon Oct 3 13:33:54 CEST 2011

The minimum requirement for multithreaded db access is to use a separate db
connection for every thread and transaction isolation.

I am not sure if the database library code is faulty or looking up the
values in a thread is the problem

Some database client libraries have problems with multithreading or have
threadsafe versions (fe. libmysql.dll is not threadsafe in the standard

The database lookup code is in a data module running in the main thread. Do
I have to use a critical section in data module containing the threads to
the database lookup?

No. That is trying to solve the problem in the wrong place.


If I put a TCP component such as an Indy http component on that data module,
and get all the lookups to be done over the http socket, is that likely to
fix the problem, rather than calling the database lookup code in the data
module directly from the threads?

You mean you have a 2nd path to your database over http? As long as you
don't use the same database connection (session) from the different threads
and are sure there are no transaction isolation issues you can use whatever
communication protocol you like. Seems complicating things though.
Why don't you write a data lookup class in your data module that uses it's
own Txxxconnection,TSQLTransaction and TSqlQuery and have each thread use
its own instance? Destroy after use or, when frequent data updates are
needed, give it a DisConnect method so that the number of sessions remains
reasonable. If you have a lot of threads and the number of simultanuous
database sessions is an issue, then write one lookup procedure using a
criticial section but, again, using its own database connection: enter
criticial section, connect to database, run sql, retrieve data, disconnect,
exit critical section. 
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