The network based services takes on the form of RPCs, (Remote Procedure Calls). For example, a client requesting a currency conversion from a server is a form of RPC. This term was commonly used to indicate communication between two end points, despite being internal or external. RPCs are used in Machine-to-Machine communication. The various technologies below are a form of RPC.
MOM, (Messaging Oriented Middleware), was introduced as another form of RPCs. Using MOM has a disadvantage of creating mesh connectivity; because all end points that need to communicate have to create a one-to-one communication path amongst them. An example of this messaging system is provided by JMS, (Java Message Service), Message-queues, (MSMQ), by Microsoft, and MQSeries by IBM. Maintenance of such messaging system can be more complicated due to the number of connections created between communication end points.
The mesh connectivity was solved by the SOA architecture, (Service Oriented Architecture), which uses SOAP, (Simple Object Access Protocol) where a Registry was created for entities to publish their services using WSDL, (Web Service Definition Language) and other entities can subscribe to these services. Information is then exchanged in the form of XML, (eXtensible Machine Language) using SOAP as the protocol. The WSDL is considered a document that describes the contract for the service. The main communication methodology is a Request/Response type between the client and the server. SOA and SOAP are known to provide Web Services. The term Web APIs is commonly attached to the term Web Services.
In the recent years, a new architecture has been adopted. ROA, (Resource Oriented Architecture), where operations are performed on Resources vs. services in the SOA domain. REST, (REpresentational State Transfer), is an architectural style which recommends the use of HTTP between communicating entities, and in a Request/Response fashion. This is considered a competing technology to SOA and is gaining high momentum in the industry. REST is considered to be simpler than SOAP and has been adopted by GSMA for its OneAPI initiative.