Sticky Sessions Load Balancer

In the world of Software Engineering, efficiently managing incoming requests across multiple servers is a critical aspect of building robust and high-performing applications. Load balancers serve as indispensable tools to achieve this goal. By evenly distributing incoming traffic among available backend servers, load balancers help prevent server overload and ensure a seamless user experience. This article examines “sticky sessions” a load balancing feature that can affect request handling and load distribution. We’ll explore its concept, assess implications on load balancers, and explore alternative strategies for improved system performance.

What Are Sticky Sessions?

Sticky sessions, also referred to as session affinity or persistence, is a load balancing technique where a load balancer routes a user’s subsequent requests to the same back-end server they initially connected to. This is achieved by storing session information in the form of cookies or other mechanisms. The idea behind sticky sessions is to maintain user state and prevent issues with sessions being lost or disrupted when users interact with a dynamic web application.

In Layman terms, Sticky sessions are like having a favorite waiter in a restaurant. When you visit, the manager assigns you a specific waiter. You had a great experience, the next time you come back, they make sure you get the same waiter who already knows your preferences, making your dining experience enjoyable again.

Load balancers mainly use round-robin algorithms for even traffic distribution, but sticky sessions break this method by directing specific clients to the same server, sacrificing load balancing fairness.

When you might go for a Sticky Sessions approach?

In scenarios involving personalized data or services like shopping carts, banking, or customized pagination, also at times it is difficult to manage eventual consistency or availability of data to all servers. Sticky sessions help maintain continuity and personalized experiences for users throughout their interactions.

Modern load balancers like Nginx offer straightforward configuration options for enabling sticky sessions, ensuring seamless user experiences and improved application performance.

We just need to modify our nginx config file by adding the ip_hash directive in the upstream block to enable sticky sessions based on the client’s IP address.

http {
    # Define the upstream backend servers
    upstream my_backend_servers {
        ip_hash; # This enables sticky sessions
        server backend_server1_ip:backend_server1_port;
        server backend_server2_ip:backend_server2_port;
    }
 # other configuration
}

There may be different ways to support this feature in different load balancers but… Hey! you get the idea. So lets look at the pros and cons of using Sticky Sessions.

Pros

Sticky sessions offer seamless user experience, session persistence, improved performance, support for stateful applications, simplified load balancer operations, less data consistency challenges to deal with and aid in troubleshooting by isolating issues to a single server.

Cons

Sticky sessions can lead to an uneven distribution of traffic across back-end servers. If a specific server gets more requests from sticky sessions, it might become overloaded while other servers remain underutilized. This can affect the overall performance and efficiency of the load balancing setup.

Sticky sessions may not make the most efficient use of server resources. Some backend servers might be idle or have minimal traffic while others are heavily loaded. This inefficient resource allocation can increase operational costs and waste hardware resources.
Sticky sessions can complicate server maintenance and failover processes. If a backend server fails, the users associated with that server’s sticky sessions will lose their connection, and their sessions will be interrupted. Implementing failover mechanisms that handle sticky sessions requires careful planning and synchronization between servers.

So the purpose of load balancer to balance the traffic evenly may at times be uneven with sticky sessions and we may end up overload and destroy the servers.

Conclusion

Sticky sessions can offer benefits for specific scenarios, but they can also introduce issues related to load balancing, scalability, and failover. When designing load balancing strategies, it’s crucial to weigh the pros and cons of sticky sessions against alternative approaches. Each application is unique, and the choice of load balancing technique should align with the specific requirements and performance goals.