Reading guide : I'm learning XML When , It's mentioned in many places in the book URL And so on , When I was working on projects before , When redirecting or forwarding , I also used this URL, I'm learning Ajax when ,ajax Undermining the unified resource positioning (URN) They are more or less exposed to these concepts , But I didn't know about it , Now to sum up .

One 、 Basic concepts

1.1,URI(Uniform Resource Identifier)

In information technology, a Uniform Resource Identifier (URI) is a string of characters used to identify a resource. Such identification enables interaction with representations of the resource over
a network, typically the World Wide Web, using specific protocols. Schemes specifying a concrete syntax and associated protocols define each URI. The most common form of URI is the Uniform Resource Locator (URL), frequently referred to informally as a web
address. More rarely seen in usage is the Uniform Resource Name (URN), which was designed to complement URLs by providing a mechanism for the identification of resources in particular namespaces.

In Information Technology , A uniform resource identifier is a string used to mark a resource . This kind of mark , Make resources interact on the network . A typical application is the world wide web , By using special protocols , Specify a specific syntax scheme and related protocols to define each URI. The most common URI The form is URL( Uniform resource locator ), It's often called informal web Address . A rare usage is URN( Unified resource name ), It tries to provide a mechanism for resource definition by naming , To make URL Become more complete .

1.2,URN(Uniform Resource Name)

A URN is a URI that identifies a resource by name in a particular namespace. A URN can be used to talk about a resource without implying its location or how to access it.

One URN yes URI A kind of , It defines a resource by the name of a common namespace . One URN You can describe a resource , But it doesn't include the location of the resource and how to access it .

1.3,URL(Uniform Resource Locator)

A URL is a URI that, in addition to identifying a web resource, specifies the means of acting upon or obtaining the representation of it, i.e. specifying both its primary access mechanism and network
location. For example, the URL http://example.org/wiki/Main_Page refers to a resource identified as /wiki/Main_Page whose representation, in the form of HTML and related code, is obtainable via HyperText Transfer Protocol (http) from a network host whose domain
name is example.org.

One URL yes URI A form of , In addition to defining a web Beyond resources , It's more about acquiring and representing resources . That is to specify the access mechanism and access location of resources at the same time . for example :http://example.org/wiki/Main_Page Represented as Wiki's home page , It is from HTML Related resources composed of code , Host domain name :example.org, And pass HTTP Hypertext protocol for data transmission .

Two 、 comparative analysis

On the whole ,URI Is a uniform resource identifier , Used to uniquely identify a resource .URL and URN It's two main forms of expression ,URL It not only identifies a resource , It also indicates the access address of the resource ;URN, It is to identify a resource by naming it , It's not about the address .

URN perhaps URL It must be URI, however URI It doesn't necessarily mean URN or URL.

Actually , I think this can be URN and URL The name of a person ( No double name ) And home address . Go to a specific home address to find a specific person , By name , You can also find specific people . Combined with recent learning , I think the clustered index and nonclustered index shared with my classmates last time are very similar ,URL It's like a nonclustered index , By finding the address where the resources are stored , Then find the corresponding resources . and URN It's like a clustered index , We use the name of the resource itself , You can find resources .( Understanding is based on their own actual learning , There's something wrong with it , How understanding )

3、 ... and 、 summary

If URL If you also specify the access address of the resource , So it might mean that once the resources are URL identification , Then its physical location will not be allowed to change . I have a deep understanding of this from my own project , When we visit the home page of a project , If we change the physical location of this home page , Then we will have the error that the resource cannot be found or deleted .

But , if URN If you define resources by naming , Does it mean that , The location of resources can be changed , As long as the name of the unique identification resource does not change . Here also suddenly thought of the computer room charging system , The reflection that everybody's using , Why can we change the database through reflection without changing other code , Why use reflection , If our class name is mismatched, an error will be reported ? This is probably the same reason as the unified resource name . The above things , It's all personal opinions , There is still a long way to learn ....

Make complaints : Sometimes when looking up foreign language materials , There is an interesting but painful phenomenon : It's easy to understand the meaning of the article , But when it comes to translating into Chinese , All kinds of difficulties . Damn it , This kind of translation work is really not easy to do . Next time I will post English directly , Or just use simple English words , To translate Advanced English words , I think it will be easier .

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