I just got prompted to write this in an email reply and I think it’s worth sharing.
My personal definition for ACID’s Durability tenet is as follows:
“The outcome of the transaction outlasts the transaction, meaning that the result of the transaction is published to other consumers as the transaction completes. ‘I’ resolves into ‘D’.”
People seem to think that there’s an implied guarantee on the outcome of the transaction lasting forever. I don’t think that’s true and also believe that even the implication is out of scope of the transaction mechanism. To me, all that ‘D’ says is that the result of the transaction is published to subsequent consumers with the same reliability assurances that apply for the same kind of data within the same system. That is to say that if the system holds its operational data in volatile memory, committing to and publishing in memory is sufficient. If the system stores operational data in a replicated ring store in memory or if the system stores it on nonvolatile media, the transaction outcome must be verifiably published following the common storage strategy to satisfy ‘D’. It doesn’t have to be a spindle.