Concurrency

As processes are separated, they should support concurrency to allow for horizontal scaling. In contrast to monoliths which generally scale vertically by increasing CPU or memory, twelve-factor apps can scale horizontally for just the processes needing more resources.

Monoliths require more resources granted to the entire application, whereas a twelve-factor app can scale part of the application without affecting the rest of it. Concurrency is critical to a distributed systems’ reliability. The ability to horizontally scale also provides availability since each process can be deployed in pairs to protect against outages.