[ajmurmann]

You got some good, but short answers. Since you asked me, I'll give you my definition: It means that your app servers are homogenous. They are all running the same server code in similar configuration. How many instances you are running has nothing to do with it being a monolith or not. You need more than one server just to get some fault tolerance. In practice your scalability bottleneck will typically be your database as all servers share that and your servers themselves ideally are stateless (they typically are for Rails apps, as Zendesk is using Rails).

Microservices on the other hand in theory have a number of very small services that at most share some libraries, but can be deployed independently and could each even be written in different languages if you want. This mostly is an advantage if you know that different portions of your app are gonna scale very differently from each other and rely on disparate data models as well and only share a small interface. In practice, things frequently don't end up as clear cut as you expected on the outset and you end up having to heavily coordinate many of your deploys across services.