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The birth of the open source enterprise stackThe birth of the open source enterprise stackPosted Jul 5, 2006 19:48 UTC (Wed) by pimlott (guest, #1535)In reply to: The birth of the open source enterprise stack by nix Parent article: The birth of the open source enterprise stack
it's amazing how often huge systems can get away without using transactionsIt is amazing, but it doesn't make it right! When something finally does go wrong, it's extremely hard to debug and fix. Yes, I know, sometimes the cost of cleaning up the occasional mess is lower than the cost of doing it right. But given the ubiquity of DBMSs with transactions, it's hard to see why you'd risk it.
The PostgreSQL manual makes the point that perfect isolation isn't possible without giving the database what amounts to a theorem prover *and* complete knowledge of your app's control flowI'm curious what you're talking about, because I can't find anything like that in the PostgreSQL manual. I think perfect isolation is straightforward using read and write logs, and that PostgreSQL does exactly that. What am I missing?
that perfect isolation isn't possible without giving the database what amounts to a theorem prover *and* complete knowledge of your app's control flow: but most databases, including major expensive ones with clowns with E in their surnames as CEOs of their controlling company, don't even try: you have to *ask* for half-decent isolation, and when you do your transaction goes read-only!I agree that they don't even try. It's a travesty that the isolation level required for reliability in most applications--that the SQL standard says must be the default--is not the default in any DBMS I know. And when you do turn it on, many systems perform badly; PostgreSQL, with multi-version concurrency control, does quite well. But what do you mean about transactions going read-only?
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The birth of the open source enterprise stack Posted Jul 7, 2006 0:38 UTC (Fri) by nix (subscriber, #2304) [Link] See section 12.2.2.1 of the PostgreSQL manual, and the discussion of predicate locking. (Note, when it says `the details of every query', it means the details. In the presence of random-access cursors I suspect this reduces to solving the halting problem.)The PostgreSQL manual makes the point that perfect isolation isn't possible without giving the database what amounts to a theorem prover *and* complete knowledge of your app's control flowI'm curious what you're talking about, because I can't find anything like that in the PostgreSQL manual. I think perfect isolation is straightforward using read and write logs, and that PostgreSQL does exactly that. What am I missing? But what do you mean about transactions going read-only?If you turn on serializable isolation in some transaction in said expensive proprietary RDBMS, you can no longer carry out INSERTs or UPDATEs in that transaction. It makes it really very useful :/
The birth of the open source enterprise stack Posted Jul 7, 2006 1:24 UTC (Fri) by pimlott (guest, #1535) [Link] See section 12.2.2.1 of the PostgreSQL manual, and the discussion of predicate locking.Thank you, I am enlightened. I guess this could be solved by logging at the table level: when the where clause is non-trivial, mark the entire table read, and mark the table written on insert. Then, when you commit, you would see (in one of the transactions) that the table has been modified since you queried it. If you turn on serializable isolation in some transaction in said expensive proprietary RDBMS, you can no longer carry out INSERTs or UPDATEs in that transaction.Hmm... I've used serializable in (if memory serves) Informix, Oracle, and Sybase, reading and writing. Have I not gone expensive enough?
The birth of the open source enterprise stack Posted Jul 8, 2006 22:58 UTC (Sat) by nix (subscriber, #2304) [Link] My experience of Oracle is that at least in some releases, it had (and was documented as having!) the nasty read-only semantics I described :(
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