Wenbo, are you talking about what do you want to see in DBMS or are you trying to explain how SQLite works? If the latter then you're wrong. In SQLite 'read lock' is designed for transaction that _made_ any reads, 'write lock' - for transaction that _made_ any writes.
Pavel On Sat, Sep 19, 2009 at 12:18 AM, Wenbo Zhao <zha...@gmail.com> wrote: > This is not a good example i think. > If a transaction is intent to update after the select, it should start > a write lock before the select. > And as described in previous 'dead lock' example, the update in this > example could fail due to 'dead lock' > I believe the 'read lock' is designed for a 'read only' transaction, > and the 'write lock' is for a transaction that 'may write something'. > > 2009/9/19 Igor Tandetnik <itandet...@mvps.org> > >> Angus March <an...@uducat.com> wrote: >> > Yes, I see. So what is key to the problem is that someone tries to >> > change their read lock to a write lock. I guess I just thought that >> > the kernel that manages fcntl() would have a way of dealing with >> > this. Can this situation not be averted if at step 3, transaction A >> > releases its read lock before requesting a write lock? >> >> Then it wouldn't be much of a transaction, now would it? Imagine the >> classic example, where a transaction first verifies that the balance in >> a bank account is sufficient, then performs a withdrawal. If it >> relinquishes all locks between these two steps, then somebody else may >> record a withdrawal from that account, so that the write operation would >> then make the balance negative, thus violating an invariant. >> >> Of course, if that's what the application wants, it can simply perform >> the read and the write operations in two separate transactions. >> >> Igor Tandetnik >> >> >> >> _______________________________________________ >> sqlite-users mailing list >> sqlite-users@sqlite.org >> http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users >> > > > > -- > > Best Regards, > ZHAO, Wenbo > > ======================= > _______________________________________________ > sqlite-users mailing list > sqlite-users@sqlite.org > http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users > _______________________________________________ sqlite-users mailing list sqlite-users@sqlite.org http://sqlite.org:8080/cgi-bin/mailman/listinfo/sqlite-users
