Delta supports ACID transactions and guarantees between reads and writes.
Delta uses Optimistic concurrency control mechanism to handle concurrent transactions, there are two types of locking mechanism.
Pessimistic concurrency control -> it assumes that two or more users will want to update the same record at the same time, and then prevents that possibility by locking the record, no matter how unlikely conflicts actually are.The locks are placed as soon as any piece of the row is accessed, making it impossible for two or more users to update the row at the same time. Depending on the lock mode (shared, exclusive, or update), other users might be able to read the data even though a lock has been placed. For more details on the lock modes, see Lock modes: shared, exclusive, and update.
Optimistic concurrency control -> Instead of locking every record every time that it is used, the system merely looks for indications that two users actually did try to update the same record at the same time. If that evidence is found, then one userโs updates are discarded and the user is informed.
To use an analogy with banks, pessimistic locking is like having a guard at the bank door who checks your account number when you try to enter; if someone else (a spouse, or a merchant to whom you wrote a check) is already in the bank accessing your account, then you cannot enter until that other person finishes her transaction and leaves. Optimistic locking, on the other hand, allows you to walk into the bank at any time and try to do your business, but at the risk that as you are walking out the door the bank guard will tell you that your transaction conflicted with someone elseโs and you will have to go back and do the transaction again.
more details are available here w.r.t Delta
https://docs.databricks.com/delta/concurrency-control.html#conflict-exceptions
https://docs.databricks.com/delta/optimizations/isolation-level.html
