What is a Server – Server Types – Client Server Model
What is Web Server ?
- Client asks for file. The request and response for the file will be in HTTP format.
- Server finds appropriate file.
- Server sends back a response header (HTTP Response) followed by the requested file’s data.
- Server closes connection.
How internet works ?
- The client machine will make the request for some resource or service, and the server machine (the server process) handles the request and sends the response (result) back to the requested client.
- In case of database server, the client machine queries the server for some specific records from the database (Ex: Select * from table), and the server looks up the records and responds to the client.
- The user on the client machine may not even know what machine in the network actually has the database. For example, in gmail.com, we are accessing our Gmail account and we don’t have
- In fact, the database may be distributed across several machines on the network.
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Note :
- The Client needs to know the existence and the address of the server. Considering, I want to access my Gmail. In that case, the website www.gmail.com will be accessed by the user.
- However, the server does not need to know the existence or address of the client prior to the connection.
- Once a connection is established, both sides can send and receive information.
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What is HTTP (Hypertext Transfer Protocol)
- HTTP is a set of rules, or protocols which governs the transfer of hypertext between two or more computers through internet /network.
- The World Wide Web encompasses the universe of information that is available via HTTP.
- HTTP is based on the client/server Model.
- HTTP allows "computer A" (the client) to establish a connection to "computer B" (the server) and make a request. The server responds by accepting the connection initiated by the client.
- An HTTP request identifies the resource that the client is interested in and tells the server what "action" to take on the resource.
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How DNS works
- A DNS server maintains the name to IP address mapping of the domain for which it is the name server (Ex : www.gmail.com will have the IP address 176.45.64.36 The ip address of the websites can be extracted using DNS server).
- The DNS server for a domain is registered with the domain registrar and the entry is maintained by the Internet Root-Servers (13) or Country Level Root-Servers.
- Whenever a server is queried, if doesn’t have the answer, the root servers are contacted.
- The root servers refer to the DNS server for that domain (in case the domain is a top level domain like .com, .org, .co.in) or the Country Root Server (in case the domain is country level domain).
HTTP Message formats
- HTTP Request - Message Format - Message from the client machine to the server
GET /index.html HTTP/1.0\r\n
Connection: Keep-Alive\r\n
User-Agent: Mozilla/4.72 [en] (X11..)\r\n
Host: 127.0.0.1:31415\r\n
Accept: image/gif, image/jpeg, */*\r\n
Accept-Encoding: gzip\r\n
Accept-Language: en\r\n
Accept-Charset: iso-8859-1,*,utf-8\r\n
\r\n
- HTTP Response - Message Format - Response from the server back to the client
HTTP/1.0 200 OK\r\n
Date: blah-blah-blah\r\n
Server: blah-blah-blah\r\n
Content-Type: important\r\n
Content-Length: 12345\r\n
Last-Modified: blah-blah-blah\r\n
\r\n
Raw data
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- HTTP Response code 200 - Format - Http Request success and the client will get the response from the server.
HTTP/1.0 200 OK\r\n
Content-Type: stuff\r\n
\r\n
Data
- HTTP Response code 404 - Format - File not found error. Client requests for a file and its not available in the server.
HTTP/1.0 404 Not Found\r\n
\r\n
- HTTP Response code 302 - Format
HTTP/1.0 302 Moved\r\n
Location: newurl\r\n
\r\n
Client Server model Vs Peer to peer model :
- In client server model, the client will request for a file or some operations and the server will process the same whereas in the peer to peer model, the every machine will act both as a client and also a server.
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Remote Procedure Call :
- RPC is another way of networking where local client can connect remote machine and process in the remote machine.
- By definition, the process of executing a particular instruction or command in a remote machine is called as RPC.
- The request may have arguments to be passed It can be key board clicks or commands.
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How website works
- The Web works on a client / server model in which client software (Browser) runs on a local computer.
- The Server software runs on a web host.
- To use the web you must make an Internet connection and then launch your web browser.
- Type the URL you want to visit
- Browser makes a connection to a server using HTTP protocol
- The Server then
- Accepts the connection
- Sends the file contents back to browser, and
- Closes the connection
- Web browsers interpret the html pages and display them on screen, they can also display
- Programs,
- Animation,
- Graphics,
- Video-conferencing, download files etc., Browsers also allow you to create and publish web pages.
- Entire suites built around browsers including modules for reading newgroups, for sending/receiving e-mail
- Multimedia files - helper applications (plug-ins)
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Common Errors with Browsers
- Server Does Not Have A DNS Entry
- 503 Service Unavailable
- 403.9 Access Forbidden - Too Many Users are Connected
- Spinning Hourglass
- 404 Not Found
- 401 Unauthorised and 402 Forbidden
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Client Server Model Types :
- Client (Dumb) Server Model
- True Client Server Model
- Distributed Client Server Model
Client (Dumb) Server Model :
- Client will provide inputs/informations in the network. Entire processing and calculations will be happening on the server side.
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True Client Server Model
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Distributed Client Server Model
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Types of Server :
File Server
- File Servers manage a work group’s application and data files, so that they may be shared by the group.
- Very I/O oriented.
- Pull large amount of data off the storage subsystem and pass the data over the network.
- Requires many slots for network connections and a large-capacity, fast hard disk subsystem.
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Compute Server
- Performs Application logic processing.
- Compute Servers requires.
- Processors with high performance capabilities.
- Large amounts of memory.
- Relatively low disk subsystems.
- By separating data from the computation processing, the compute server’s processing capabilities can be optimized.
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Computation can be achieved through a very different concept of Multithreading through which multi-client can access the server and the server process the client individually.
Multithreaded Server
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Data Server
- Data-oriented; used only for data storage and management
- Since a data server can serve more than one compute server, compute-intensive applications can be spread among multiple severs
- Does not prefer any application logic processing
- Performs processes such as data validation, required as part of the data management function.
- Accepts requests for data (through Select statement in tables), retrieves the data from its database (or requests data from another node) and passes the results back.
- Requires fast processor, large amount of memory and substantial Hard disk capacity.
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Communication Server
- Provides gateway to other LANs, networks & Computers.
- E-mail Server & internet server.
- Modest system requirements
- Multiple slots
- Fast processor to translate networking protocols
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Internet Server
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Mail Server
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How email works
- Simple Mail Transfer Protocol (SMTP) is used to transfer mail between Mail Servers over Internet.
- Post Office Protocol (PoP) and Interactive Mail Access Protocol (IMAP) is used between Client and Mail Server to retrieve mails.
- The mail server of a domain is identified by the MX record of that domain.
- Popular Mail Servers
- Send mail/Postfix
- Microsoft Exchange Server
- IBM Lotus
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