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WWW and its capabilities


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WWW and its capabilities In order to understand how HTTP and HTML work, it is necessary to explain how the communication on the World Wide Web looks. The WWW works on the client-server model. URL (Uniform Resource Locator) Looks like this: the schema identifies a service through which information can be obtained (ftp, http, telnet, ...) specific part has the following form:: @: / not all parts are mandatory, depending on the schema used HTTP (HyperText Transfer Protocol) One of the main features of HTTP is that it is undeclared (= each requirement is judged separately regardless of previous actions), which avoids a number of problems but some others appear Definition of a document placement on the Internet is done using the following URL: http: //: /? ..

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WWW and its capabilities

In order to understand how HTTP and HTML work, it is necessary to explain how the communication on the World Wide Web looks. The WWW works on the client-server model.

URL (Uniform Resource Locator)

Looks like this:

  • the schema identifies a service through which information can be obtained (ftp, http, telnet, ...)
  • specific part has the following form:: @: /
  • not all parts are mandatory, depending on the schema used

HTTP (HyperText Transfer Protocol)

One of the main features of HTTP is that it is undeclared (= each requirement is judged separately regardless of previous actions), which avoids a number of problems but some others appear

Definition of a document placement on the Internet is done using the following URL:

http: //: /?

- computer is the specified domain name or IP address of the computer

- port is for default 80 for http, but can be set differently

- The job specifies the location of the document within the server, if it is missing, you will usually get the home page of the server

The original text was originally meant to search for the text only, but today it has a wider meaning if the URL does not designate a static document, it is passed through the parameters that affect the program's activity, such as PHP, CGI.

The standard extension for www documents is .html, but servers running on different versions of MS-Windows with short (3-character) extensions use the .htm extension, for pages that the server modifies when they are sent, they have the suffix .shtml

HTML (HyperText Markup Language)

It is the language in which static (unchanging) websites are created. Contains special commands to indicate different parts of a page (e.g.,) by which the client displaying the page knows that it should highlight, underline, etc.

- The same page displayed by different clients may not look the same, depending on the configuration!

- You can also use scripts (such as CGI), so pages can also be dynamic.

CGI (Common Gateway Interface)

Also called gateways are programs that create an HTML document as a result of their work

Can be essentially any program if it meets 2 conditions:

  1. Can get information as defined by CGI
  2. The result of the activity is an HTML document that sends to its standard output

The programs used for this purpose are Perl, Unix shell, C ++, C, PHP

Translation to run the CGI script looks like this:

  1. The client sends a query whose URL points to a CGI script. It can also pass additional data, such as searched text
  2. The server determines that the request has to be serviced by an external program, runs it, and passes through the CGI interface the necessary information.
  3. The script runs, processes the data, and creates an HTML document based on the standard output that passes the server as a result.
  4. The server sends the result of the CGI script to the client.

It is relatively slow because each process must be run for each request

Proxy (or proxy cache)

It is a server that represents another computer (more precisely a group of computers) relative to the WWW server.

They were due to overflowing of the lines (one information was transferred to each request separately, which is quite uneconomical)

If the proxy receives a request for a particular website, it not only sends it to the applicant, but recognizes the HTTP header if it can save it (it can not be dynamic, with static time in the header it can store) and does so. Once the proxy is asked for the same page, it does not contact the WWW server, but passes the page that it has stored so it does not burden the line unnecessarily.

Of course, there is a problem with the possible age of the document, but if you put CTRL + reload, the page will be requested from the WWW server and you will get it updated.

Can work as Parent or Sibling.

Parent always asks for a higher level (whether it's another proxy or source server)

Sibling is at the same level more, and when asked, first examine their siblings to find out if they have the required document and look for a higher level when they find negative.

It also has a security function, because both the packet of requests and the answer to it is ending its path and the proxy creates new packets, which it sends further (meaning they are the same)

Forms

Propably the best way to get data from the user for processing

It has two parts:

  1. The HTML page, which is a visible part and through the input fields, provides information. It contains a link to the CGI script that will process the information you enter.
  2. A CGI script that processes information from the form and sends the resulting HTML page

It is important to know that HTTP communication is intercepted and data does not go encrypted, for HTTP Secured (HTTPS)

PHP scripting language

The code is written to the HTML of the page, but it is surrounded by special characters. It is often used to process specified data from forms and to handle databases

Dynamic pages

This are any pages that contain, in addition to HTML, another part, such as PHP, CGI script,

linking to SQL databases

Database is the place to store all sorts of information. Access to the data contained in it is provided by DBMS (DataBase Management System)

DBMS includes: Oracle, MS SQL Server, Informix, PostgreSQL, MySQL, and more

The way of communicating with the DBMS is similar to communicating with the WWW server. Most often, DBMS is continuously running as a daemon on Unix or as a Windows NT service on a particular port. At this port, it expects requests from other apps, and then responds.

The most commonly used SQL language (Structured Query Language)

In combination with PHP scripts with SQL statements, you can access databases over the Web.

Various SQL servers have different protocols. The client should therefore be able to communicate multiple protocols, which is impractical. That's why ODBC was created on Windows platforms. This is the interface between the client and the SQL server, so the ODBC client, whose driver transfers the request to the so-called native protocol understood by all SQL servers, communicates it to the client after receiving the response. Currently, ODBC is implemented on most versions of Unix.

Most of the DBMS used is a relational data model (based on relational algebra, which is the mathematical apparatus on which the relational data model builds), such data being arranged in tables

P. number           Name                   Address               Salary

1024                     Peter Borg          La Guardia 256 16384

1354                     John Nash           Tall road 5          12150

1684                     Jenny Mitchell    Cm street 16      13504

...                          ...                          ...                          ...

You can see an example of the table above. Name, address, salary, and personal number are attribute names. The column or attribute is highlighted under the name. The highlighted line is called a record.

We need something to work with information in the spreadsheet so we can select the record you need. This is what the primary key is. This must of course be unique (no 2 entries must have an entry in the same key attribute as the same). In our example of the table it would be a personal number.

Data database can contain multiple tables, each identified by its name. Information from different tables can be related, they may have different relationships among themselves. In this case, one attribute (column), a so-called foreign key, is added to the table associated with another, which determines which records are related.

Table can have more foreign keys, but the primary one only!



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