Question

If 2 users connect to Access and run query that check the last number, increase this number and insert a record into the database.

Who gets the priority?

Why do I see two duplicate records with the same number?

Answer 1

Neither of them got “priority”. That is no longer a useful term when you talk about non-atomic operations. An example will illustrate the issue here.

Here’s what your script looks like:

• Read value from database.
• Increment by 1.
• Write new value to database.

This works when only one person uses the database at a time. When more than one person does, things won’t work. Here’s what happens now:

• Original value in DB is “100”.
• Client A reads “100”.
• Client A increments “100” locally to “101”, but hasn’t yet written it.
• Client B reads “100”.
• Client B increments “100” locally to “101”, but hasn’t yet written it.
• Client A writes “101”.
• Client B writes “101”.

This is called a race condition, and it happens because you didn’t use a transaction. With a transaction, here’s what might happen instead:

• Original value in DB is “100”.
• Client A begins a transaction on this value.
• Client A reads “100”.
• Client A increments “100” locally to “101”, but hasn’t yet written it.
• Client B tries to begin a transaction, but there is already a pending transaction, so it cannot start yet.
• Client A writes “101”.
• Client A closes the transaction and the value is actually written.
• Client B begins a transaction on this value.
• Client B reads “101”.
• Client B increments “101” locally to “102”, but hasn’t yet written it.
• Client B writes “102”.
• Client B closes the transaction and the value is actually written.