How do you handle session persistence and affinity in load balancing?

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Load balancing is a technique to distribute incoming requests across multiple servers or nodes, improving performance, availability, and scalability of web applications. However, load balancing also introduces some challenges, such as how to maintain session persistence and affinity. Session persistence means that a user's requests are consistently handled by the same server or node during a session, while session affinity means that a user's requests are preferably handled by the same server or node, but not necessarily. In this article, you will learn how to handle session persistence and affinity in load balancing, and what are the advantages and disadvantages of different methods.

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Hamouda Yassine

DevOps Engineer - Certified Azure | CKAD | CKA | Openshift

1 Why session persistence and affinity matter

Session persistence and affinity matter because they affect the user experience and the application logic. For example, if a user logs in to a web application and then gets redirected to a different server or node, they may lose their authentication status, their shopping cart, or their preferences. Similarly, if a web application relies on session variables or local cache to store data or state, switching servers or nodes may cause inconsistency or errors. Therefore, session persistence and affinity ensure that a user's requests are handled by the same or a similar server or node throughout a session, preserving the continuity and functionality of the web application.

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2 How to implement session persistence and affinity

Implementing session persistence and affinity in load balancing requires an understanding of the level and mechanism of the load balancer. The level refers to whether the load balancer operates at the network layer (L4) or application layer (L7), while the mechanism is how the load balancer determines which server or node to route a request to. Common methods for achieving session persistence include IP hashing, cookie-based, SSL session ID, and custom header. IP hashing uses a hash function to map the source IP address of a request to a server or node, while cookie-based inserts a cookie in the response header of the first request from a user. SSL session ID uses the SSL session ID of a request to identify the server or node that handled the previous request from the same user. Lastly, custom header uses a custom header in either the request or response to indicate which server or node should handle the request. Each method has its own benefits and drawbacks; for example, IP hashing may not work well if users share an IP address, while custom header may not be compatible with some intermediaries.

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Hamouda Yassine

DevOps Engineer - Certified Azure | CKAD | CKA | Openshift
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An example i've seen is, sticky sessions and persistance volumes are necessary to achieve affinity sessions. In a case where an external LB is in place sticky sessions won't address the problem of affinity session therefore a mechanism of saving the session state could be applied either on the app level or the cluster.

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3 How to choose the best method

When selecting a session persistence and affinity solution for load balancing, there is no one-size-fits-all approach. Several factors must be taken into account, such as the type and configuration of the load balancer, the architecture and design of the web application, the characteristics and behavior of the user traffic, and the performance and security requirements of the web application. It is important to evaluate the pros and cons of each method in your specific context, testing them in different scenarios. Additionally, alternative or complementary approaches can be considered, such as stateless or distributed sessions, sticky sessions, or consistent hashing. Stateless or distributed sessions reduce dependency on session persistence and affinity while improving scalability and resilience. Sticky sessions achieve session affinity but not persistence, allowing flexibility and fault tolerance in load balancing. Consistent hashing achieves session persistence or affinity while distributing requests evenly across servers or nodes.

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How do you handle session persistence and affinity in load balancing?

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1 Why session persistence and affinity matter

2 How to implement session persistence and affinity

3 How to choose the best method

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