Tuesday, October 11, 2011
clemens vasters discusses lack of dtc on the azure platform
clemens vasters responded to complaints on the azure nda list this week. he addresses the lack of a distributed transaction coordinator on the azure platform. i thought his response was interesting, told him that, and he turned it into a public blog post.
he still got skewered by the mvps, but he gets an "a" for effort.
below is my favorite part:
"The grand canonical example for 2PC transactions is a bank account transfer. You debit one account and credit another. These two operations need to succeed or fail together because otherwise you are either creating or destroying money (which is illegal, by the way). So that’s the example that’s very commonly used to illustrate 2PC transactions. The catch is – that’s not how it really works, at all. Getting money from one bank account to another bank account is a fairly complicated affair that touches a ton of other accounts. More importantly, it’s not a synchronous fail-together/success-together scenario. Instead, principles of accounting apply (surprise!). When a transfer is initiated, let’s say in online banking, the transfer is recorded in form of a message for submission into the accounting system and the debit is recorded in the account as a ‘pending’ transaction that affects the displayed balance. From the user’s perspective, the transaction is ’done’, but factually nothing has happened, yet. Eventually, the accounting system will get the message and start performing the transfer, which often causes a cascade of operations, many of them yielding further messages, including booking into clearing accounts and notifying the other bank of the transfer. The principle here is that all progress is forward. If an operation doesn’t work for some technical reason it can be retried once the technical reason is resolved. If operation fails for a business reason, the operation can be aborted – but not by annihilating previous work, but by doing the inverse of previous work. If an account was credited, that credit is annulled with a debit of the same amount. For some types of failed transactions, the ‘inverse’ operation may not be fully symmetric but may result in extra actions like imposing penalty fees. In fact, in accounting, annihilating any work is illegal – ‘delete’ and ‘update’ are a great way to end up in prison."