The application tier collects user requests from the web tier and performs the heavy lifting of calculating business logic and talking to the database. When user requests increase, the application tier needs to scale to handle them and shrink back as demands decrease. In such scenarios, users are tied to the session, where they may be browsing from their mobile and purchasing from their desktop. Performing horizontal scaling without handling user sessions may cause a bad user experience as it will reset their shopping progress.

Here, the first step is to take care of user sessions by decoupling them from the application server instance, which means you should consider maintaining the user session in an independent layer such as a NoSQL database. NoSQL databases are key-value pair stores, where you can store semi-structured data.

Once you start storing your user session in NoSQL databases such as Amazon DynamoDB or MongoDB, your instance can scale horizontally without impacting the user experience. You can add a load balancer in front of a fleet of application servers, which can distribute the load among instances; with the help of auto scaling, you can automate adding or removing instances on demand.