API Testing Tutorial Part 3 – Understand Terms URN , URL ,URI & API

This is a frequently asked interview question for API testing. All these terms are confusing to beginners as well as experienced.  I will try to explain it well in this post. 

I will start with the line – ” All above terms are same at a final point. These words are the stages representing evolution of final point. FInal point we will get to know at end of this post. “

Let’s see abbreviations of each terms: 

URN – Uniform Resource Name
URL – Uniform Resource Locator 
URI – Uniform Resource Identifier 

In all abbreviations above, do you notice common term – “Resource”? Let’s see what is resource first.

What is Resource?

Resource is a physical or virtual component, which supports/extends/represents/constructs a system. Consider a real time example of Motor Bike. Wheels, breaks, headlight, fuel tank etc all are resources of a Motor bike. All resources have names, location and identifiers.

Take another example of Google. Now append “/maps” in last of base url “https://www.google.com/” as “https://www.google.com/maps“. Google will launch Google Maps. Try same with “/news”. Google will launch Google News. Here “Maps” and “News” are actually resources which has its names and locator.

Hopefully, you will have understanding of term “Resources”.

As per RFC-3986:

A Uniform Resource Identifier (URI) is a compact sequence of characters that identifies an abstract or physical resource. Each URI begins with a scheme name, as defined in Section 3.1, that refers to a specification for assigning identifiers within that scheme.

The following example URIs illustrate several URI schemes and variations in their common syntax components: ftp://ftp.is.co.za/rfc/rfc1808.txt
http://www.ietf.org/rfc/rfc2396.txt
ldap://[2001:db8::7]/c=GB?objectClass?one
mailto:John.Doe@example.com
news:comp.infosystems.www.servers.unix
tel:+1-816-555-1212
telnet://192.0.2.16:80/
urn:oasis:names:specification:docbook:dtd:xml:4.1.2

A URI can be further classified as a locator, a name, or both. The term “Uniform Resource Locator” (URL) refers to the subset of URIs that, in addition to identifying a resource, provide a means of locating the resource by describing its primary access mechanism (e.g., its network “location”). The term “Uniform Resource Name” (URN) has been used historically to refer to both URIs under the “urn” scheme [RFC2141], which are required to remain globally unique and persistent even when the resource ceases to exist or becomes unavailable, and to any other URI with the properties of a name.

An individual scheme does not have to be classified as being just one of “name” or “locator”. Instances of URIs from any given scheme may have the characteristics of names or locators or both, often depending on the persistence and care in the assignment of identifiers by the naming authority, rather than on any quality of the scheme. Future specifications and related documentation should use the general term “URI” rather than the more restrictive terms “URL” and “URN” [RFC3305].

The generic URI syntax consists of a hierarchical sequence of components referred to as the scheme, authority, path, query, and fragment.

URI = scheme “:” hier-part [ “?” query ] [ “#” fragment ]

The scheme and path components are required, though the path may be empty (no characters). When authority is present, the path must either be empty or begin with a slash (“/”) character. When authority is not present, the path cannot begin with two slash characters (“//”). These restrictions result in five different ABNF rules for a path (Section 3.3), only one of which will match any given URI reference.

The following are two example URIs and their component parts:

         foo://example.com:8042/over/there?name=ferret#nose
         \_/   \______________/\_________/ \_________/ \__/
          |           |            |            |        |
       scheme     authority       path        query   fragment
          |   _____________________|__
         / \ /                        \
         urn:example:animal:ferret:nose

Can we say now URI, URN and URL all are same. Latest term to be used is URI.

And an URI is an API and this is a final point.

More about API Testing in upcoming posts. Stay tuned.

If you have any doubt, feel free to comment below.
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