What went wrong with Horizon: learning from the Post Office Trial

As the Post Office (Horizon/Fujitsu) scandal is getting more coverage this week, I thought this accountancy blog (which talks about an accompanying video, for those who like video) might be of interest. I’ve been following this story for years but this is the first thing I’ve read that gets into the detail of what went wrong with the software.

The Post Office trial is one of the few cases where an in-depth examination of system failures is made public and so it’s a valuable lesson to learn from. Even simple problems like maintaining a stock balance become complex when part of a distributed system. Techniques like ACID transactions can reduce the likelihood of errors but real implementations will sometimes fail. When a system processes a large number of transactions, this small probability of failure can add up to frequent errors. I hope that the presumption that computers operate correctly is revisited, and the factors revealed by the Post Office trial are taken into account when doing so.