Question

I have a tasks thread running in two separate instances of tomcat.
The Task threads concurrently reads (using select) TASKS table on certain where condition and then does some processing.

Issue is ,sometimes both the threads pick the same task , because of which the task is executed twice.
My question is how do i make both thread not to read the same set of data from the TASKS table

Answer 1

I think you need have some variable (column) where you keep last modified date of rows. Your threads can read same set of data with same modified date limitation.

Edit:
I did not see “not to read”

In this case you need have another table TaskExecutor (taskId , executorId) , and when some thread runs task you put data to TaskExecutor; and when you start another thread it just checks that task is already executing or not (Select … from RanTask where taskId = …).
Нou also need to take care of isolation level for transaсtions.