1. The Client/Server Model

1. The Client/Server Model
	Chapter 4. Distribution Programming

The Client/Server Model
Distributed System:
A distributed computer system contains software programs and data resources dispersed across independent computers connected through a communication network.
Advantages:
It allows users of individual, networked computers to share data and processing power, often over long distances.
Distribution can also enhance availability, reliability and performance.
Figure 4.1. Heterogeneous Network
Coordination Models:
A coordination model establishes logical roles and associated behaviours (for applications that assume such roles) for executing distributed interactions.
Client/Server Model
The client: requests an operation or service that some other application, the server, provides.
The server: receives a client request, performs the requested service and returns to the client any results.
Advantages:
- It offers simplicity in closely matching the flow of data with the control flow.
- It promotes modular, flexible, and extensive system designs.
- Data resources and computing services can be organized, integrated and used as a service.
- Widely used in distributed systems.
Communication:
- Servers communicate over a network.
- In Unix and Windows machines the network uses: TCP/IP & UDP.
Networking Basics
TCP and UDP protocols:
Figure 4.2. Network Layers
- TCP
  Definition:
  TCP is a connection-based protocol that provides a reliable flow of data between two computers.
  Examples: HTTP, ftp, Telnet
- UDP
  Definition:
  UDP is a protocol that sends independent packets of data, called datagrams from one computer to another with no guarantees about arrival.
  Examples: ping
- Ports
  Definition:
  The TCP and UDP protocols use ports to map incoming data to a particular process running on a computer.
  - Computers are identified by a 32-bit IP address.
  - IP uses to deliver data to the right computer on the network.
  - Ports are identified by a 16-bit number.
  - TCP and UDP use to deliver the data to the right application.
    Figure 4.3. Ports
- Port Numbers
  Range: from 0 to 65535
  Reserved: 0 - 1023 are restricted--they are reserved for use by services such as HTTP and ftp and other system services.
  Figure 4.4. Port Numbers
Sockets
- Are the basic components of interprocess communication.
- Provide access to the network transport protocols.
- Are an endpoint of communication to which a name can be bound.
Stream Socket:
A stream socket provides bidirectional, reliable, sequential, and unduplicated flow of data with no record boundaries. It uses TCP/IP protocol.
Datagram Socket:
A datagram socket provides bidirectional flow of data, the data is divided in packages, the receiver can receive them in a different order from the sending sequence and may receive duplicated messages. It uses UDP protocol.