How does a load balancer maintain session persistence across backend servers?

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This question explores Networking and addresses practical considerations related to: How does a load balancer maintain session persistence across backend servers?

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Load balancers maintain session persistence, also known as "sticky sessions," by ensuring that requests from a client are consistently directed to the same backend server. This is crucial for applications that store session data locally on the server. Load balancers achieve this through methods such as cookie insertion, source IP affinity, or URL hashing, depending on the specific technology and configuration used.

Example Concept: Session persistence is typically implemented by inserting a cookie into the client's session that uniquely identifies the backend server handling the session. This cookie is then used by the load balancer to route subsequent requests from the same client to the same server. Alternatively, source IP affinity can be used, where the client's IP address is hashed to determine the backend server. These methods ensure that all requests from a session are handled by the same server, maintaining state consistency.

ADDITIONAL COMMENT:

Session persistence is essential for applications with stateful interactions, such as shopping carts or user profiles.
Cookie-based persistence is widely used in HTTP/HTTPS environments and is supported by most load balancers like F5, NGINX, and AWS ELB.
Source IP affinity might not be suitable for environments with many clients behind a NAT, as it can lead to uneven load distribution.
Ensure that session persistence is configured according to application requirements to avoid potential performance bottlenecks.

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How does a load balancer maintain session persistence across backend servers?

Asked on Nov 12, 2025

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