Stateless

Essentially, stateless means that the necessary state during the request is contained within the request and it is not persisted in any hypothesis that could be recovered further. Basically, the URI is the unique identifier to the destination and the body contains the state or changeable state, or the resource. In other words, after the server handles the request, the state could change and it will send back to the requestor with the appropriate HTTP status code:

In comparison to the default session scope found in a lot of existing systems, the REST client must be the one that is responsible in providing all necessary information to the server, considering that the server should be idempotent.

Stateless allows high scalability since the server will not maintain sessions. Another interesting point to note is that the load balancer does not care about sessions at all in stateless systems.

In other words, the client needs to always pass the whole request in order to get the resource because the server is not allowed to hold any previous request state.