Question

What are stateless servers

Answer 1

State is what networking people call memory. One of the important design principles in the Internet has always been to minimise the number of places that need to keep track of who is doing what.

In the case of stateless information servers this means that they do not keep track of which clients are accessing them. In other words, between one access and the next, the server and protocol are constructed in such a way that they do not care who, why, how, when or where the next access comes from.

This is essential to the reliability of the server, and to making such systems work in very large scale networks such as the Internet with potentially huge numbers of clients: if the server did depend on a client, then any client failure, or network failure would leave the server in the lurch, possibly not able to continue, or else serving other clients with reduced resources.

Another use of the term stateless is to describe whether or not the server keeps note of the actual data from each access by a client (irrespective of whether it notes who the client was). This is called server caching.

Server Caching is a way of improving the response time of a server. Usually, servers keep data on disk. If they keep a copy of all the most frequently or most recently accessed data in memory, they may be able to respond to new (or repeating) clients more quickly.

Answer 2

Let us start by understanding what is State full application. An application is considered 'Stateful' when it remembers details specific to a user, such as user details, profile, preferences and user's previous actions. This information is considered 'state' of a system. Think of your shopping cart when using any website that lets you shop. Each time you select an item and put it your cart, it needs to add it to items you previously added, and eventually needs to navigate to payment page.

In case of web applications that have server-side logic implmented as 'Stateful', all of the above details for each current user, will have to be remembered (in some kind of memory) by the server. As the number of concurrent users grow, the memory used by the application for storing a large number of users increases, which adversely affects the performance of the application. It is then known as an application that 'scales poorly'.

As the volume of concurrent users grow, more servers running the applications can be added and the load can be distributed evenly between those servers using a load-balancer. But because each server 'remembers' each logged-in user's state, it becomes necessary to configure this load balancer in 'sticky-mode'. In otherwords, even when distributing load across servers, the load-balancer is required to send each user's request to the same server that responded to that user's previous request. This defeats the purpose of load balancing, because load is not distributed in a true round-robin fasion.

To get around this issue of 'Stateful' applications, the server-side logic needs to be coded in such a way that it does not depend on 'previously stored state'. Which means, the state information is sent along with each request, to the server using which server can proceed with servicing the request. This way, the load-balancer need not worry about routing requests to the same server and truly uniform load balancing can be achieved. Application servers where previously stored state is no longer a concern we call them stateless servers