I was looking at potential concurrency issues in DB so i went to read up. I found http://publib.boulder.ibm.com/infocenter/db2luw/v8/index.jsp?topic=/com.ibm.db2.udb.doc/admin/c0005267.htm and it mentions access to uncommitted data.
Access to uncommitted data.
Application A might update a value in
the database, and application B might
read that value before it was
committed. Then, if the value of A is
not later committed, but backed out,
the calculations performed by B are
based on uncommitted (and presumably
invalid) data.
What… i thought other sessions (same app and even same thread) can read data that has not been committed yet? I thought only the connection/session (i am not sure of my terminology) that wrote the data into the uncommitted transaction can read uncommitted data.
Can other threads really read data that hasnt been committed?
I plan to use mysql but i may use sqlite
What other sessions can read depends on how you set up your database. In MySQL it also depends on what database engine you use. The term you’re looking for (in ANSI SQL terms) is “isolation level”.
Many databases will default to an isolation level where reads on uncommitted data will block. So if transaction A updates record 1234 in table T and then transaction B tries to select record 1234 before A commits or rolls back then B will block until A does one of those things.
See MySQL Transactions, Part II – Transaction Isolation Levels.
One serious downside of this is that batch update operations that live in long-running transactions (typically) can potentially block many requests.
You can also set it so B will see uncommitted data but that is often ill-advised.
Alternatively you can use a scheme called MVCC (“Multiversion concurrency control”), which will give different transactions a consistent view of the data based on the time the transaction started. This avoids the uncommitted read problem (reading data that may be rolled back) and is much more scalable, especially in the context of long-lived transactions.
MySQL supports MVCC.