As your application becomes popular and more users start using it, you will have to scale your application to meet the increased usage. Application scaling can be done in either of the following two ways: vertical scaling (or scaling up) and horizontal scaling (scaling out). Vertical scaling is about adding more power to a single machine, that is, faster CPU, more RAM and SSD, and so on. Vertical scalability has a limit and the cost increases exponentially. Horizontal scaling, on the other hand, is about handling more requests and load by adding more machines.

With most PaaS solutions such as OpenShift, you will soon hit the vertical scaling limit. Currently, in OpenShift, you can't get a bigger gear size than 2 GB of RAM space (that is, large gear), so it is recommended that you design your application for horizontal scalability. There are many good books, such as Scalability Rules, Martin L. Abbott and Michael T. Fisher, Addison-Wesley Professional, written...