Statefulness
Statefulness & Stickiness
Statefulness
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 (Sticky Sessions)
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.
Methods of Implementing Stickiness
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.
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