What is the difference between web APIs and web services? Let’s start off with an analogy: all tigers are cats but not all cats are tigers. In a similar way, all web services are APIs, but not all APIs are web services. Web APIs and web services are often confused with each other; however, web APIs are an evolution of web services. Both facilitate information transfer, but web APIs are more dynamic than web services are. Let’s see how.
What is a web service?
By definition, a web service is any piece of software that makes itself available over the Internet and standardizes its communication via XML encoding. A client invokes a web service by sending a request (usually in the form of an XML message), and the service sends back an XML response. Web services invoke communication over a network, with HTTP as the most common means of connectivity between the two systems. For many, web services are synonymous with SOA (Services Oriented Architecture) and primarily rely on standards such as XML-RPC and SOAP (Simple Object Access Protocol). One of the primary criticisms of the web services approach is the degree to which the client and server are coupled to one another; an inherent issue whenever a client is remotely calling a procedure (“RPC”) on a distant system.
Above image visually describes the difference between an API and a web service.
Why is a web API not a web service?
In contrast, a typical Web API specifies how software components should interact with each other using the web’s protocol (HTTP) as the go-between. The client doesn’t need to know what procedure to call on the server. Instead, it uses a set of commands (called “verbs”) that are built into HTTP and when the command arrives on the other end, it’s up to the receiving system to know what to do with it. For example, the HTTP verb that’s typically used to retrieve data is “GET”. When HTTP is used to abstract systems from one another, the systems are considered to be more loosely coupled (when compared to web services) and therefore the entire system is considered less brittle. Another advantage of web APIs (often referred to as RESTful APIs) is flexibility. The client system (usually called the “consumer”) and the serving system (the “provider”) are so independent of one another, that they can each use different languages (Java, Python, Ruby, etc.) for their part of the overall implementation. Additionally, the data payloads can be of multiple types such as JSON or XML. RESTful APIs most typically use the web’s communication protocol (again, HTTP), but are not limited in the same way a web service is. For example, CoAP, an HTTP-like protocol that’s common to the Internet of Things, is also considered to be RESTful.
What is an API?
In contrast, an API specifies how software components should interact with each other. It is a set of protocols and routines, and it’s responses are generally returned as JSON or XML data. APIs can use any type of communication protocol, and are not limited in the same way a web service is.
What do APIs and web services have in common?
Both web APIs and web services serve as a means of communication between consumers and providers. Both support XML-based data payloads, but JSON is the more common payload type for web APIs. When comparing web services to web APIs, the significance lies in the amount of work that has to be done by the consumers and providers to package and unpackage the data; this is a process known as serialization and deserialization, respectively. Serializing and deserializing JSON in a web API scenario typically requires far less work which, in turn, equates to better performance and fewer compute cycles. This is one reason why web APIs are great for information transfer on mobile devices and tablets; as opposed to on desktops and services, where they have restricted processing environments.
Conversely, web services facilitate interactions between two systems, and almost always depend on an XML-RPC-like interface to communicate with each other. SOAP, the successor to XML-RPC, defines the XML-based exchange mentioned above and is more bound to a client/server architecture.
In terms of what web APIs and web services have in common, both are essentially a means to an end and the same problems can be solved by both. And both can be configured to operate over a network or within a machine.But the approaches of each come with their own pros and cons.
For now, web services are a service from one device to another; they communicate over the Internet and are optimized for machine-to-machine communication, meaning that machine-readable files and formats (like XML) are easily transferable. APIs are software-to-software interfaces with an abstract set of verbs instructions for accessing web-based applications. Whatever you wanted to accomplish with a web API can be accomplished with a web service; web services are merely predecessors in the evolution of web integration methodologies.
I will acknowledge that this article glosses over the relationship between web APIs and RESTful APIs. For further reading on, look at ProgrammableWeb’s series on creating RESTful APIs and turning web pages into RESTful APIs! In the meanwhile, we defer to this differentiation between web APIs and web services: web APIs are most-often JSON-based and web-services are almost always XML-RPC or SOAP (XML). As web integration methodologies progress, the gaps between each integration pattern will get smaller and smaller.
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