Is it possible for server to act as a client? Yes or no?

In most business –oriented application and environment, data-level organized as object-oriented databases? Yes or No? 

Is that true, that many shared data-space systems use SQL-like interface to share repository? Yes or no?

Correct definition for DHT is:

What mechanisms provide us with distribution in client-server architecture?

Client-server applications can be represented as these tree levels: 

Centralized architecture, where the client-server interacts, we can use two types of protocols?

What is the first goal of client software:

In large software systems, two basic architecture styles are:

In Layered architecture, control information flow