Load Balancing: Persistence
In load balancing, "persistence" (also called "session persistence" or "sticky sessions") refers to a feature where a load balancer directs all requests from a single user to the same backend server throughout their session, ensuring that a user interacts with the same server for consistent experience, especially when an application relies on storing session data locally on the server, like items in a shopping cart or login information; this is achieved by tracking a unique identifier associated with the user, commonly through cookies or their source IP address.
Key points about persistence in load balancing
Benefits:
- Improved user experience: By keeping a user on the same server throughout a session, it avoids the need to re-establish the session state on a different server, leading to smoother interactions, particularly for complex applications with multiple steps.
- Efficient use of server resources: When a server already has information about a user's session cached, sending subsequent requests to the same server can improve performance.
How it works:
- Identifying the user: The load balancer uses a specific attribute, like their source IP address or a cookie set in their browser, to identify a user.
- Mapping to a server: Once identified, the load balancer associates the user with a particular backend server and routes all their requests to that server for the duration of the session.
Persistence methods:
- Source IP-based persistence: The simplest method uses the user's source IP address to identify them.
- Cookie-based persistence: The load balancer sets a cookie on the user's browser, and subsequent requests include this cookie to identify the user.
Considerations:
- Scalability concerns: If many users are actively using a service, relying heavily on persistence can strain individual servers as all requests from a user are directed to the same server.
- Session timeout: It's important to set a session timeout to automatically release a user from a server after a period of inactivity.
This is covered in Security+.