REST API REST or Representational state transfer API defines a set of functions which developers can perform requests and receive responses via HTTP protocol such as GET and POST. It providing interoperability between computer systems on the Internet. REST-compliant Web services allow requesting systems to access and manipulate textual representations of Web resources using a uniform and predefined set of stateless operations. Other forms of Web service exist, which expose their own arbitrary sets of operations such as WSDL and SOAP. The REST used by browsers. REST technology is generally preferred to the more robust SOAP technology because REST leverages less bandwidth, making it more suitable for internet usage. An API for a website is code that allows two software programs to communicate with each another. With cloud use on the rise, APIs are emerging to expose web services. REST is a logical choice for building APIs that allow users to connect and interact with cloud services. This APIs are commonly used by popular sites like Amazon, Google, LinkedIn and Twitter. The REST architectural style describes six constraints. The six constraints are: Uniform Interface The uniform constraint interface is fundamental to the design of any REST service. It simplifies and decouples the architecture, which enables each part to evolve independently. Client-server Separation of concerns is the principle behind the client-server constraints. By separating the user interface concerns from the data storage concerns, it improves the portability of the user interface across multiple platforms and improve scalability by simplifying the server components. Stateless The client–server communication is constrained by no client context being stored on the server between requests. Each request from any client contains all the information necessary to service the request, and session state is held in the client. The session state can be transferred by the server to another service such as a database to maintain a persistent state for a period and allow authentication. The client begins sending requests when it is ready to make the transition to a new state. Cacheable As on the World Wide Web, clients and intermediaries can cache responses. Responses must therefore, implicitly or explicitly, define themselves as cacheable or not to prevent clients from reusing stale or inappropriate data in response to further requests. Layered system A client cannot ordinarily tell whether it is connected directly to the end server, or to an intermediary along the way. Intermediary servers may improve system scalability by enabling load balancing and by providing shared caches. They may also enforce security policies. Code on demand Servers can temporarily extend or customize the functionality of a client by transferring executable code. How does it work? A REST API breaks down a transaction to create a series of small modules. Each module addresses a particular underlying part of the transaction. This modularity provides developers with a lot of flexibility. A REST API explicitly takes advantage of HTTP methodologies defined by the RFC 2616 protocol. They use GET to retrieve a resource; PUT to change the state of or update a resource, which can be an object, file or block; POST to create that resource; and DELETE to remove it. With REST, networked components are a resource you request access to a black box. The presumption is that all calls are stateless. Because the calls are stateless, REST is useful in cloud applications. Stateless components can be freely redeployed if something fails, and they can scale to accommodate load changes. This is because any request can be directed to any instance of a component; there can be nothing saved that has to be remembered by the next transaction. That makes REST preferred for web use, but the RESTful model is also helpful in cloud services.