Hermitage: Testing the “I” in ACID

But that will not happen in postgres on any version in any configuration with any isolation level. It just can't happen.

I strongly rebut the following claim, which is central to the article:

"Internet commenters, in their infinite wisdom, were quick to point out that if you’re dealing with money, you had better use an ACID database. But there was a major flaw in their argument. Most so-called ACID databases — for example Postgres, MySQL, Oracle or MS SQL Server — would not have prevented this race condition in their default configuration."

I hesitate because I don't really understand the details of the situation the exchange faced. But, going by the linked references here:

https://bitcointalk.org/index.php?topic=499580 http://www.reddit.com/r/Bitcoin/comments/1wtbiu/how_i_stole_...

it appears that the pattern in question, if translated very unnaturally to SQL, is something like:

    CREATE TABLE account(id int8, balance numeric);
    ...
    BEGIN;
    SELECT balance FROM account WHERE id = 123;
    -- application sees 100, subtracts 90, sees that
    -- it's still positive and does:
    UPDATE account SET balance = 10 WHERE id = 123;
    COMMIT;

Technically speaking, running that in postgres in the default configuration (read committed) is prone to a race, and you'd need to use repeatable read or serializable mode to protect you.

But that's ridiculous. Anyone using SQL would instead do:

    CREATE TABLE account(id int8, balance numeric, check(balance >= 0));
    ...
    UPDATE account SET balance = balance - 90 WHERE id = 123;

And that is not prone to a race. Try it in any version of postgres, in any configuration. You can't get double withdrawls (where only one takes effect), and you can't get it to go below zero.

So, the author is technically right: (a) if you translate the NoSQL-isms into SQL in an unnatural way; and (b) don't bother to use SERIALIZABLE mode, which costs very little in most situations.

I agree with the author that isolation is tricky, and developers should not be expected to understand the nuances. And I applaud the development of a testing framework to really understand the various kinds of isolation and how they apply to different products. But the example is a bad one, because it actually does work just fine in postgres, and probably many other systems.

Your first ("very unnatural") example is what I had in mind. And since read committed is the default isolation level in most RDBMS, it is prone to the lost update anomaly. (In MySQL, repeatable read is the default, but its implementation of repeatable read doesn't prevent lost updates.) Note I did point out that I'm referring to the default configuration, not the strongest supported isolation level.

Although your second example is probably what a human would write, an ORM framework would very likely generate a transaction looking like your first example.

Another example would be inserting a transaction into a table, and summing the transactions in the account in order to calculate the account balance. Making that safe requires preventing phantom reads, which means requiring serializability.

You say serializable costs very little in most situations. I can't claim to know what most situations are like, but all I know is that I've seen many people who have tried serializable and found it too slow for them. User a-priori gives an example elsewhere on this thread: http://www.michaelmelanson.net/2014/03/20/transactions/

My point is that weak isolation is very subtle, easy to get wrong, and you don't know that you got it wrong until it's too late. We need better understanding and better tools so that concurrency is less easy to screw up.

An interesting read.

I don't know first hand, but could take a guess:

Think of a transaction that inserts a row representing a withdrawal, updates a materialized total balance, and checks that it's positive. Under snapshot isolation, two concurrent instances of this transaction could commit. The materialized balance would reflect only one of the debits however, and would be inconsistent vs queries that recompute the aggregate in full.

Postgres pre version 9.2-ish would allow this situation even in "serializable" mode. Later versions wouldn't.