What is a database transaction?

Question

Can someone provide a straightforward (but not simpler than possible) explanation of a transaction as applied to computing (even if copied from Wikipedia)?

Answer 1

Here's a simple explanation. You need to transfer 100 bucks from account A to account B. You can either do:

accountA -= 100;
accountB += 100;

or

accountB += 100;
accountA -= 100;

If something goes wrong between the first and the second operation in the pair you have a problem - either 100 bucks have disappeared, or they have appeared out of nowhere.

A transaction is a mechanism that allows you to mark a group of operations and execute them in such a way that either they all execute (commit), or the system state will be as if they have not started to execute at all (rollback).

beginTransaction;
accountB += 100;
accountA -= 100;
commitTransaction;

will either transfer 100 bucks or leave both accounts in the initial state.