Jenkins is a good example of a stateful service that forces us to preserve its state. Moreover, it is incapable of sharing or synchronizing state between multiple instances. As a result, it cannot scale. There cannot be two Jenkins masters with the same or replicated state. Sure, you can create as many masters as you want but each will be an entirely separate service without any relation to other instances.
The most obvious negative side-effect of Jenkins inability to scale horizontally is performance. If a master is under heavy load, we cannot create a new instance and thus reduce the burden from the original.
There are only three types of services that can be scaled. They need to be stateless, stateful and capable of using shared state, or stateful and capable of synchronizing state. Jenkins is none of those and, therefore, it cannot be scaled horizontally. The only thing we can do to increase Jenkins capacity is to add more resources (for...