Types of Session Persistence

In system design, session persistence, also known as session affinity or sticky sessions, can be implemented in various ways to ensure that client requests are consistently routed to the same backend server. Here are some common types:

1. Cookie-based Persistence

When a client makes an initial request to a server behind the load balancer, the server responds by setting a unique session identifier (a cookie) in the client’s browser. Subsequent requests from the same client include this cookie, allowing the load balancer to direct them back to the same server.

• Cookie-based persistence is widely supported and works well for web applications where cookies can be reliably set and maintained.
• It may not work if cookies are disabled on the client’s browser, and there can be security concerns related to session hijacking or cookie tampering.

2. IP-based Persistence

The load balancer uses the client’s IP address to determine which server to route requests to. Requests from the same IP address are consistently directed to the same server.

• IP-based persistence is simple to implement and doesn’t rely on client-side features like cookies.
• It can be less effective in scenarios where clients are behind proxies or use dynamic IP addresses. Additionally, it can lead to uneven load distribution if clients with different session states share the same IP address.

3. URL Rewriting

Session information is encoded into the URL itself, typically as a query parameter. The load balancer examines the URL and forwards requests to the appropriate server based on the encoded session data.

• URL rewriting is versatile and can be used in environments where cookies are not feasible or desirable.
• It can result in longer and less user-friendly URLs, and there are potential security risks associated with exposing session data in the URL.

4. SSL Session ID Persistence

For HTTPS connections, the SSL handshake generates a unique session ID. The load balancer uses this session ID to maintain session affinity, ensuring that requests with the same SSL session ID are routed to the same backend server.

• SSL session ID persistence is transparent to the application layer and can be effective for HTTPS traffic.
• It may not work for non-HTTPS traffic, and there can be compatibility issues with certain SSL configurations or implementations.

5. Custom Headers

Session information is included in custom HTTP headers by the client or the load balancer itself. The load balancer inspects these headers and routes requests based on the session data.

• Custom headers provide flexibility and can be tailored to specific application requirements.
• It requires additional configuration and may not be supported by all load balancers or client applications.

Load Balancer Session Persistence explores how load balancers manage user sessions in web applications. It explains how these systems evenly distribute incoming traffic across multiple servers to maintain stability and prevent overload. The article will help to understand the concept of session persistence, where load balancers ensure that a user’s requests consistently reach the same server during a session.