Types of Session Affinity
In system design, session affinity, also known as sticky sessions, ensures that all requests from a specific user during a session are directed to the same server. Various methods can achieve session affinity, each with unique advantages and limitations. Here are the main types in detail:
- IP-based Session Affinity:
- The load balancer uses the client’s IP address to maintain session affinity. It tracks the IP address of incoming requests and routes them to the same server.
- Easy to implement and does not require changes to the application or client-side configurations. Works even if cookies are disabled in the client’s browser.
- Cookie-based Session Affinity:
- The load balancer sets a cookie in the client’s browser that includes a unique identifier for the server handling the session. Subsequent requests from the client contain this cookie, allowing the load balancer to route the request to the correct server.
- Works well even when clients are behind proxies or NAT. Can handle large numbers of users effectively as each session is individually tracked.
- URL Rewriting:
- The session ID is embedded in the URL, and the load balancer uses this ID to route requests to the appropriate server.
- Suitable for clients that do not support cookies or have cookies disabled. Provides a straightforward way to map requests to sessions.
- Application-controlled Session Affinity:
- The application itself manages session persistence, often by storing session data in a centralized location accessible by all servers, such as a database or distributed cache.
- Can handle complex session data requirements and server failures gracefully. Session management is decoupled from the load balancer, allowing more sophisticated session handling logic.
- Database-based Session Affinity:
- Session data is stored in a centralized database accessible by all servers. Each server retrieves and updates session data as needed.
- Ensures session data is consistent and not lost if a server fails. Supports complex session data and large-scale applications.
- Token-based Session Affinity:
- Similar to cookie-based affinity but uses tokens that can be stored in various ways, such as in HTTP headers or local storage.
- Can be used across different platforms and not tied to browser cookies. Tokens can be encrypted and have a shorter lifespan, enhancing security.
What is Session Affinity in Load Balancing?
Session affinity in load balancing means directing a user’s requests to the same server throughout their session. This helps maintain a consistent user experience, especially for applications that store user data temporarily on the server, like shopping carts in online stores. Without session affinity, each new request might go to a different server, causing potential issues or data loss.
Important Topics for Session Affinity in Load Balancing
- What is Session Affinity?
- Importance of Session Affinity
- Types of Session Affinity
- Techniques to Implement Session Affinity
- Example
- Pros and Cons
- Use Case Scenarios