Recovery from "bad" user transactions

Abstract

User written transaction code is responsible for the "C" in ACID transactions, i.e., taking the database from one consistent state to the next. However, user transactions can be flawed and lead to inconsistent (or invalid) states. Database systems usually correct invalid data using "point in time" recovery, a costly process that installs a backup and rolls it forward. The result is long outages and the "de-commit" of many valid transactions, which must then be re-submitted, frequently manually. We have implemented in our transaction-time database system a technique in which only data tainted by a flawed transaction and transactions dependent upon its updates are "removed". This process identifies and quarantines tainted data despite the complication of determining transactions dependent on data written by the flawed transaction. A further property of our implementation is that no backup needs to be installed for this because the prior transaction-time states provide an online backup.