The consistency guarantees of databases are very well documented. For starter, we have good old standard SQL isolation levels. Recently, with the advent of NoSQL databases, the consistency of data got even more attention. Unfortunately for many use cases this data-centric point of view is not sufficient as modern systems frequently use message-based asynchronous communication in which messages are durably stored and delivered multiple times, until acknowledged.

In this talk we are going to explore consistency violations that arise from anomalies in handling of asynchronous communication between two database-backed components. In the case study we are going to analyze the most common example of a user agent sending a HTTP request that results in a database update and a message being sent. That message is processed by a backend component that updates its database and sends another message.