What is statelessness in REST API in layman term?

Question

I have gone through various REST API documentation. However, I don't have a clear understanding of it. Can someone help me to understand it in layman's terms? How is it different from API being stateful? Why today all the APIs we develop are restful APIs?

Answer 1

Term stateless/stateful is a common term and has the same meaning for Rest API as for anything else.

1. Stateless means that when you make a call to some entity the response (output) will always depend on your input only. I.e. for any number of calls with the same input you will ALWAYS get the same response. Example: a request of "what is 2 + 2 equals to?" will always get you an answer 4 no matter how many times you ask and regardless if there were other queries in-between your query requests.
2. Stateful means that the output will depend on your input and some internal state. Example: "Please add 2 to current number that you hold. what is the new number?" (Assuming that the internal state (the current number) is 0). after the first query you will get the answer 2 but after the same query for the second time you will get answer 4 and so forth.

Answer 2

Each request from client to server must contain all of the information necessary to understand the request, and cannot take advantage of any stored context on the server. -- Fielding, 2000

Note that this is a constraint on semantics: all of the information that you need to understand the meaning of the request is in the request itself.

Counter example: the LIST command in FTP, where a null argument for pathname "implies the user's current working or default directory" is not stateless -- current working directory is data of session state that the server needs to remember from previous requests.