So far, we have focused on a Continuous Delivery process that was applied to a web service. A simple factor in this was that web services are inherently stateless. This fact means that they can easily be updated, restarted, cloned in many instances, and recreated from the given source code. A web service, however, is usually linked to its stateful part: a database that poses new challenges to the delivery process. These challenges can be grouped into the following categories:
- Compatibility: The database schema, and the data itself, must be compatible with the web service all the time
- Zero-downtime deployment: In order to achieve zero-downtime deployment, we use rolling updates, which means that a database must be compatible with two different web service versions at the same time
- Rollback: A rollback of a database can be difficult, limited, or sometimes even impossible, because not all operations are reversible (for example, removing a column that contains data)
- Test...