Mission

Mission critical applications need to be highly available with minimal scheduled or unscheduled downtime. Availability is expressed as a percentage, for instance as 99.95%. In order to create an application that is highly available, redundancies must be built into the system at multiple levels to avoid single points of failure. Techniques like clustering and replication are used to achieve redundancy. Higher availability is associated with added complexities to the system that results in higher operational and development costs.
Scalability is a measure of how the application responds when the load on the system increases. If the application response time remains constant in spite of the increase in the number of concurrent users or growth in the volume of data, the application can said to be scalable. In practice, this does not happen because of physical limitations of systems like available memory and processing power. Application response degrades with increase in load and at some point will become totally unresponsive. However, by properly architecting the application, scalability can be achieved. One common method is adding more machines to handle more load. This is called scaling out. However, this needs proper and careful architecting and addressing of issues like session data sharing.