Hermitage: Testing the “I” in ACID

Re:Hermitage: Testing the “I” in ACID by jeffdavis on Wednesday November 26, 2014 @02:49AM (#8661460)

I know, in general, about various kinds of serialization anomalies, and I know (in moderate detail) how all of the isolation modes are implemented in postgres.

What I don't know is about the particular problem the exchange encountered, and why postgres's isolation would not have prevented it. Based on the very brief descriptions of the problem that I've seen, it seems like postgres would have prevented it, but I don't have enough information to say for sure.

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Re:Hermitage: Testing the “I” in ACID by jasonwatkinspdx on Wednesday November 26, 2014 @01:53PM (#8663963)

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.

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Re:Hermitage: Testing the “I” in ACID by jeffdavis on Thursday November 27, 2014 @11:26AM (#8667856)

Did you actually try in postgres? If so, please provide an example.

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Re:Hermitage: Testing the “I” in ACID by jasonwatkinspdx on Sunday November 30, 2014 @01:33AM (#8676192)

Read the papers.

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