High availability versus scalability

High availability is different to scalability, although both are serious architectural concerns. Scalability refers to the flexibility and elasticity to add more resources or reduce resources to existing deployment to accommodate more users than normal without comprising application performance. Scalability indirectly helps in making an application highly available. However, it does not mean scalability eventually leads to high availability. High availability is an architectural concern that is not dependent on the number of users, while scalability rules are determined by a number of users consuming the service. High availability could be a requirement even if there were very few users. High availability is about services being present and operational as and when users demand its consumption. It is a function of consumption based on the SLA.