Abstract Server that handles the business logic of the system like Authentication Stateful Compute Server Remembers client data like Session-Cookie Authentication (state) from one request to the next From the above diagram, user A’s session data and profile image are stored in Server 1 To authenticate User A, HTTP requests must be routed to Server 1 If a request is sent to other servers like Server 2, authentication would fail because Server 2 does not contain User A’s session data Similarly, all HTTP requests from User B must be routed to Server 2; all requests from User C must be sent to Server 3 Limitations Every request from the same client must be routed to the same server. This can be done with Sticky Session with an overhead Adding and removing servers is much difficult with this approach, challenging to handle server failures Thus, less Scalability (可扩展性) and Fault Tolerance (容错性) Stateless Compute Server Move the state li Session-Cookie Authentication out of the Compute Server, and store them in an external Database. And all the compute servers fetch state data from a shared database Requests from Client can be sent to any compute servers, resolves the limitations of stateful compute server Thus more Scalability (可扩展性) and Fault Tolerance (容错性)
