Statefulness
Statefulness & Stickiness
Last updated
Statefulness & Stickiness
Last updated
The server stores information about the client's current session in a stateful system. This is common in traditional web applications. Here's what characterizes a stateful system:
Session Memory: The server remembers past interactions and may store session data like user authentication, preferences, and other activities.
Server Dependency: Since the server holds session data, the same server usually handles subsequent requests from the same client. This is important for consistency.
Resource Intensive: Maintaining state can be resource-intensive, as the server needs to manage and store session data for each client.
Example: A web application where a user logs in, and the server keeps track of their authentication status and interactions until they log out.
In this diagram:
Initial Request: The client sends the initial request to the load balancer.
Load Balancer to Server 1: The load balancer forwards the request to Server 1.
Response with Session ID: Server 1 responds to the client with a session ID, establishing a sticky session.
Subsequent Requests: The client sends subsequent requests with the session ID.
Load Balancer Routes to Server 1: The load balancer forwards these requests to Server 1 based on the session ID, maintaining the sticky session.
Server 1 Processes Requests: Server 1 continues to handle requests from this client.
Server 2 Unused: Server 2 remains unused for this particular client due to the stickiness of the session with Server 1.
Stickiness or sticky sessions are used in stateful systems, particularly in load-balanced environments. It ensures that requests from a particular client are directed to the same server instance. This is important when:
Session Data: The server needs to maintain session data (like login status), and it's stored locally on a specific server instance.
Load Balancers: In a load-balanced environment, without stickiness, a client's requests could be routed to different servers, which might not have the client's session data.
Trade-off: While it helps maintain session continuity, it can reduce the load balancing efficiency and might lead to uneven server load.
Cookie-Based Stickiness: The most common method, where the load balancer uses a special cookie to track the server assigned to a client.
IP-Based Stickiness: The load balancer routes requests based on the client’s IP address, sending requests from the same IP to the same server.
Custom Header or Parameter: Some load balancers can use custom headers or URL parameters to track and maintain session stickiness.
