Event Sourcing as Replacement for CRUD

Most developers are familiar withCRUD(create-read-update-delete) model of data storage in business applications. It is a simple model that has served us well so far. But the problem withCRUDis that it allows users (or developers) to modify history (change past). That might be okay for someapplications (a discussion board, say), but not for all, such as financial accounting. In this article is a very gentle introduction to an alternative model,Event Sourcing. There are more detailed introductionselsewhere, but they may not be as gentle(Subjective,I know).

For example, one of SpinSpire’s financial clients hasan application that currently uses CRUD model for accounting financial transactions. The problem is that the application allows users to go back and adjust (modify) financial transactions that have already happened, reconciled and reported to the relevant authorities (regulators). So, in effect, you’re changing the past. Now, you may say, that “Hey, we need to be able to update the data as new and more accurate information becomes available!”. True. Butthe right way would be if the database had two entries - one entered originally, and another one entered later that adds the adjustment amount. And both the entries have separate timestamps on them. So if you queried the database with an “as-of” time, you’ll get one answerbeforethe second entry, and a different answerafterit.

The above is what Event Sourcing (ES) accomplishes. In CRUD, the database simply stores that transaction amounts as they standnow. But in ES, the database is ajournal of transactionentries with a timestampthat can be sequenced up to any “as-of” time to reflect the situation as it was at that time. Implementingthis (ES) is not easy. But for mission-critical systems where historical accuracy is paramount, there’s much of an alternative.

In a later update, I’ll try to go into the details of ES and how to implement it. In the meanwhile, checkout some of the resources below …

• EventStore, the event sourcing database
• ES Basics for developers
• ES Discussion on HN