Distributed Systems Architecture

1. Distributed Systems Architecture Presenters Rose Kit & Turgut Tezir

2. Outline • Introduction to Distributed System • Characteristics of Distributed System • Advantages of Distributed System • Disadvantages of Distributed System • Loosely-coupled Systems • Tightly-coupled system • Array Processor

3. Introduction to Distributed System • Centralized System • A single computer with one ore more CPUs processes all incoming request • Problems with cost, reliability • Specification and implementation are defined within a single system • Distributed System • Opposite of centralized system • Set of separate computers that are capable of autonomous operation, link by a compute network. • Enable individual computers (different location) to share resources in the network • Server implementation for the same interface located in different servers.

4. Distributed and Centralized Computer System

5. Characteristics of Distributed System • Resource sharing • Resource provided by a computer which is a member of a distributed system can be shared by clients and other members of the system via a network. • “Resource Manager” is a software module (based on set of management policies) that provides interfaces which enables resource to be manipulated by clients, Openness • Openness • Resource sharing services can be incorporated without disruption or duplication of existing services • Concurrency • All concurrent access must be synchronised to avoid problems such as lost update, dirty read, incorrect summary & unrepeatable read.

6. Characteristics of Distributed System • Scalability • flexible to grow in size. • Fault tolerance • the system appropriately handles errors • hardware redundancy, software recovery • Transparency • hide all unnecessary details from users • Example: Location transparency - clients do not need to know the location of the servers

7. Advantages of Distributed System • Shareability • Allows systems to use each other’s resources • Expandability • Permits new systems to be added as members of the overall system • Local Autonomy • Manage local resources • Improved performance • Resource replication • Combined processing power of multiple computers provides much more processing power than a centralised system with multiple CPUs • Improved reliability and availability • disruption would not stop the whole system from providing its services as resources spread across multiple computers • Potential cost reductions

8. Disadvantages of Distributed System • Network reliance • problems on network would disrupt activities in the system as a whole • Complexities • must be able to deal with errors from all computers that make up the distributed system • Security • private resources would be exposed to a wider range of potential hackers, with unauthorised accesses from any computers connected to the system • centralised system is more secure

9. General Types of Distributed Systems • Loosely-Coupled Systems • Tightly-Coupled Systems • Array Processor Systems

10. Loosely-coupled Systems • Loosely-coupled distributed systems are single-user computers or workstations that are locally networked enabling them to access shared data and resources located in other servercomputers • Most distributed systems links two types of computers, workstations and servers (also known as client/server). • An early example of the client/server model is the Xerox distributed system • Systems with different hardware and software can be networked together • Advantages of loosely coupled systems over tightly coupled systems: • More scalable. • Generally more reliable/fault-tolerant. • Allows sharing of widely separated resources.

11. Loosely-coupled Systems

12. A Loosely-coupled System Local Network Memory Memory Memory Memory Prg A Prg B Prg C Prg D Processor Processor Processor Processor I/O I/O I/O I/O

13. Tightly-coupled system • Tightly-coupled system connects a number of processor into an integrated hardware system under the control of a single operating system. • The operating system assigns processor and memory space to users’ tasks and allow them to run at the same time. • The hardware environment includes a shared memory or a high-speed connection between several separate processor / memory systems with a unified virtual addressing system. This enables users’ tasks to communicate with each other and with the operating system. • Advantage of tightly coupled systems over loosely coupled systems: • Easier to program but expensive (Limited by the memory bandwidth)

14. A Tightly-coupled Multiprocessor System Shared Memory Program A Program B Program C Program D Cache Memory Cache Memory Cache Memory Cache Memory Processor Processor Processor Processor Input / Output

15. Array Processor • An array processor is similar to a personal computer with a large number of arithmetic and logic units linked in a regular array. They can be used to perform operations in parallel on arrays of data. • The distinguishing characteristics of such a machines is that the entire array of processors obeys a single stream of instructions, some of the instructions being applied to many data items distributed throughout the array of processing unit. • There are also other tightly–coupled distributed systems that are designed to make calculations faster by processing them in parallel. • Advantages: • Useful for achieving high processing speed when processing large regular set of data • Disadvantages • Geographical distribution • Many independent tasks need to executed at the same time.

16. Array Processor M M M M M M M M M P P P P P P P P P Input / Output Program M: Memory P: Processor