Services rarely operate in isolation. They interact with one another over the network to enable macro behavior. Using a specific (often standardized) protocol, data is exchanged at specific endpoints. There are two forms to this communication:

• Using an Application Programming Interface (API)—a Request/Response model over a network protocol such as Hypertext Transfer Protocol (HTTP)
• Using messaging—where services exchange messages to exchange data

Messaging is covered in Chapter 6, Messaging. This chapter focuses on the first model of communication.