Can a server make a request?

Question

I have solid understanding of request and response between browser and server; but, can a server make a request as well. I heard that when computer interact with api, the server is the one making the request. How is this possible?

Answer 1

This is a great question.

In short, the server-client relationship is relative. That is, a process can sometimes act as a server and other times as a client.

Imagine a web app where a user logs in to see their email. The first thing a user must do is authenticate themselves with the email server so that the server can decide to grant them access or not.

So:

[ User ] -- | send username/password | -> [ Email Server ]

Now, the Email Server doesn't handle authentication directly. It actually consults an authentication server. So, what happens next is:

[ Email Server ] -- |forwards username/password| --> [ Authentication Server ]

This process unwinds with the authentication server sending a response to the email server, and the email server either granting or denying access to the user based on the authentication servers response.

Now, back to the original question:

Step 1: The Email Server was acting as a server to the user.

Step 2: The Email Server was acting as a client to the Authentication Server.

So, in summary, the server-client relationship is relative and should be assessed from the perspective of the action. A client makes a request, and a server services that request. Some processes can be dedicated clients that only consume information from servers. And, some processes can be dedicated servers, only servicing requests from clients. But, increasingly, a given server is likely to both act as a server and a client.