In a distributed system with multiple servers or nodes, a load balancer is used to distribute incoming requests across these servers to achieve optimal resource utilization and ensure high availability. Sticky sessions, also known as session affinity, is a technique used with load balancers to ensure that all requests from a particular client are routed to the same server or node in the cluster.
Sticky sessions are useful when a user’s session state is stored on a specific server and the user needs to access that state for each request. For example, in a web application where a user logs in and their session is maintained in memory or on a specific database server, the user’s requests should be routed to the same server that has the session data.
Without sticky sessions, each request from a client could be routed to a different server in the cluster, resulting in the user losing their session state and being logged out of the application. With sticky sessions, the load balancer uses a unique identifier, such as a cookie or session ID, to associate a client with a particular server. As a result, all requests from that client are sent to the same server, ensuring that their session data is always available.
Overall, sticky sessions improve the user experience by ensuring that session data is always available, and can also improve the performance of the system by reducing the overhead of constantly transferring session data between servers. However, sticky sessions can also lead to uneven load distribution across servers and may not be ideal for all types of applications.
When using a Classic Load Balancer, requests are typically routed independently to the registered instance with the smallest load. However, the load balancer can be configured to use Sticky Sessions, also known as session affinity, which binds a user’s session to a specific instance. This ensures that all requests from the user during the session are sent to the same instance, improving performance and user experience.
To manage Sticky Sessions effectively, it is essential to determine how long the load balancer should consistently route the user’s request to the same instance. If the application has its session cookie, Elastic Load Balancing can be configured to follow the duration specified by the application’s session cookie. Otherwise, Elastic Load Balancing can create a session cookie with a specified stickiness duration.
During the Sticky Sessions process, Elastic Load Balancing generates a cookie called AWSELB, which is used to map the session to the instance.
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