Hacker News Reader

Comment by hueving | Hacker News Reader

The basic idea of event sourcing is to store the actions rather than the end state of the action. This allows actions to interleave and systems calculate the endstate from all of the actions.

Think of ATMs. They don't update the balance of your bank account directly. They just record a debit against it and then the sum of all of your credits and debits is your balance. This avoids it having to have some kind of lock on your account during the transition and even allows significant delays from various transaction sources.

View on HN

In my view most of the problems with event sourcing that people run into boils down to overdoing it and modelling large parts of their systems as sets of events instead of maintaining an event log just for those things that falls out naturally as discrete events from the rest of your design.

Having certain critical events, and especially complex ones, logged as events that can be replayed and reconstructed and model transformations as operations over the state of those logs can be invaluable. Building the system around modelling all changes as events is amazingly painful.

The first thing ATMs (at least over here) do is check if you have enough balance for your transaction. If you just stored events, you'd need to collapse all the previous delta operations at that point, which would be arbitrarily slow if you never persisted data.

Storing event records vs materializing changes is sort of an old hat trick in databases. People did that in the early 90s for better TPC-C scores. It has its uses, and in some contexts storing deltas can have huge advantages (e.g. bigtable with a log structured file system), but it's no silver bullet.

Databases that try to only reflect the entire truth as-is are like everyone working from a shared whiteboard.

Databases setup to event source, ie reflect the truth as it happened at each step, are like everyone working from a shared spreadsheet with change tracking.

It's an entire extra dimension that makes reconciliating problems of conflicting actions in disparate systems possible.

Reply on news.ycombinator.com