How are locking mechanisms (Pessimistic/Optimistic) related to database transaction isolation levels?

Question

I am writing a web application where two different users can update a list of things, to do list, for example. I have come to realize that, optimistic locking mechanism works best since I don't expect high contention.

I was looking at transaction isolation levels and now I am a little confused. Looks like different transaction isolation levels also solve similar problems.

How are these two different concepts related to each other? If possible, with a simple example.

Answer 1

Both of these things are related to data consistency and concurrent access, but they are two different mechanisms.

Locking prevents concurrent access to some object. For example when you attempt to update a todo list item, with pessimistic locking database places a row lock on the record until you either commit or rollback the transaction, so that no other transaction is allowed to update the same record. Optimistic locking is application-side check whether the timestamp/version of a record has changed between fetching and attempting to update it. This is regardless of transaction isolation level.

Transaction isolation is about read consistency.

• Read uncommitted level allows session to see other session's uncommitted changes
• Read committed level allows session to see other session's committed changes only
• Serializable level allows session to see only changes committed before the transaction began

Take a look at below example, I indicated the query results that differ between transaction isolation levels.

SESSION 1                                  SESSION 2 --------------------------------           -------------------------------------- SELECT count(*) FROM test; => 10                                            INSERT INTO test VALUES ('x');  SELECT count(*) FROM test; => 10 with read committed/serializable => 11 with read uncommited (dirty read)                                            COMMIT;  SELECT count(*) FROM test; => 10 with serializable => 11 with read uncommitted/read committed 

There are four ANSI specified transaction isolation levels (one not mentioned in the example above is "repeatable read"), all of them except serializable are subjects to some anomalies. Note it has nothing to do with locking.

You can take a look at Oracle documentation on this here, the concepts are quite universal.

Finally, your approach to use optimistic locking seems sensible for a web application. Most probably you fetch a list item and update it in two different HTTP requests. It is impossible (or unwise at least) to keep transaction open with explicit lock on the record after the fetch (how do you know whether the second request will arrive at all?) Optimistic locking handles this gracefully.