DISTRIBUTED SYSTEMS Concepts and Definition By G Coulouris

DISTRIBUTED SYSTEMS Concepts and Definition By G. Coulouris, J. Dollimore and T. Kindberg 1

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Topics § § Research Scope Defining Distributed Systems Resource sharing and the Web Design Challenges of Distributed Systems 2

Table 3. 1. Some NDC research and development areas Research Scope cluster concepts collaborative computing distributed storage grid computing peer-to-peer pervasive computing dependable systems languages real time and embedded distributed agents massively parallel systems security distributed algorithms middleware Semantic Web distributed databases mobile and wireless spaces computing distributed filesystems network protocols ubiquitous computing distributed media operating systems Web Services 3

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Defining Distributed Systems § Our definition of distributed systems has the following significant consequences: Ø Concurrency v Tasks carry out independently Ø No global clock v Tasks coordinate their actions by exchanging messages Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 4

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Defining Distributed Systems Ø Independent Failures v Faults in the network result in the isolation of the computers that are connected to it. v Each component of the system can fail independently, leaving the others still running. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 5

CHARACTERIZATION OF DISTRIBUTED SYSTEMS General Examples of Distributed Systems § § Internet/Intranets/extranets Mobile networks DNS : Distributed Database system Grid, Cloud, Fog computing Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 6

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Design Challenges of Distributed Systems n Designers of distributed systems need to take the following challenges into account: Ø Heterogeneity v Heterogeneous components must be able to interoperate. Ø Openness v Interfaces should allow components to be added or replaced. Ø Security v The system should only be used in the way intended. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 7

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Design Challenges of Distributed Systems Ø Scalability v System should work efficiently with an increasing number of users. v System performance should increase with inclusion of additional resources. Ø Failure handling v Failure of a component (partial failure) should not result in failure of the whole system. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 8

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Design Challenges of Distributed Systems Ø Transparency v Distribution should be hidden from the user as much as possible. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 9

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Heterogeneity § Heterogeneous components that must be able to interoperate, apply to all of the following: Ø Networks Ø Hardware architectures Ø Operating systems Ø Programming languages Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 10

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Heterogeneity § Examples that mask differences in network, operating systems, hardware and software to provide heterogeneity are Ø Middleware Ø Internet protocols Ø Mobile code Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 11

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Heterogeneity Ø Middleware v Middleware applies to a software layer. v Middleware provides a programming abstraction. v Middleware masks the heterogeneity of the underlying networks, hardware, operating systems and programming languages. v The Common Object Request Broker (CORBA) is a middleware example. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 12

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Heterogeneity Ø Mobile code v Mobile code is the code that can be sent from one computer to another and run at the destination. v Java applets are the example of mobile codes. Ø Virtual machine v Virtual machine provides a way of making code executable on any hardware. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 13

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Openness § Distributed systems must be extensible. § Openness of a computer system is the characteristic that determines whether the system can be extended and reimplemented in various ways. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 14

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Openness § The first step in openness is publishing the documentation of software components and interfaces of the components to make them available to software developers. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 15

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Security § Security of a computer system is the characteristic that the resources are accessible to authorized users and used in the way they are intended. § Security for information resources has three components: Ø Confidentiality v Protection against disclosure to unauthorized individual. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 16

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Security Ø Integrity v Protection against alteration or corruption. Ø Availability v Protection against interference with the means to access the resources. § Security Mechanisms are: Ø Encryption Ø Authentication Ø Authorization Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 17

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Scalability § Scalable distributed systems operate effectively and efficiently at many different scales, ranging from a small Intranet to the Internet. § Scalable distributed systems remain effective when there is a significant increase in the number of resources and the number of users. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 18

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Scalability § Challenges of designing scalable distributed systems are: Ø Controlling the cost of physical resources v Cost should linearly increase with the system size. Ø Controlling the performance loss v For example, in hierarchically structured data, search performance loss due to data growth should not be beyond O(log n), where n is the size of data. Couloris, Dollimore and Kindberg Distributed Systems: Concepts & Design Edn. 4 , Pearson Education 2005 19

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Scalability Ø Preventing software resources running out v An example is the numbers used as Internet addresses (IP)(32 bit->128 -bit) v Y 2 K like problem. Ø Avoiding performance bottlenecks v Using decentralized algorithms to avoid having performance bottlenecks. § Caching and replication in Web are examples of providing scalability. 20

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Failure handling § Failures in distributed systems are partial, that is some components fail while others continue to function. § Techniques for dealing with failures: Ø Detecting failures v E. g. Checksums Ø Masking failures v E. g. Retransmission of corrupt messages v E. g. File redundancy 21

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Failure handling Ø Tolerating failures v E. g. Exception handling v E. g. Timeouts Ø Recovery from Failure v E. g. Rollback mechanisms Ø Redundancy v E. g. Redundant components 22

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Concurrency § With concurrency, services and applications can be shared by clients in a distributed system. § For an object to be safe in a concurrent environment, its operations must be synchronized in such a way that its data remains consistent. § Concurrency can be achieved by standard techniques such as semaphores, which are used in most operating systems. 23

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Transparency § Transparency is defined as the hiding of the separation of components in a distributed systems from the user and the application programmer. § With transparency the system is perceived as a whole rather than a collection of independent components. 24

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Transparency § Forms of transparencies: Ø Access transparency v Enables local and remote resources to be accessed using identical operations. Ø Location transparency v Enables resources to be accessed without knowledge of their physical or network location (for example, which building or IP address). 25

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Transparency Ø Concurrency transparency v Enables several processes to operate concurrently using shared resources without interference between them. Ø Replication transparency v Enables multiple instances of resources to be used to increase reliability and performance without knowledge of the replicas by users or application programmers. 26

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Transparency Ø Failure transparency v Enables the concealment of faults, allowing users and application programs to complete their tasks despite the failure of hardware or software components. Ø Mobility transparency v Allows the movement of resources and clients within a system without affecting the operation of users or programs. 27

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Transparency Ø Performance transparency v Allows the system to be reconfigured to improve performance as loads vary. Ø Scaling transparency v Allows the system and applications to expand in scale without change to the system structure or the application algorithms. 28

CHARACTERIZATION OF DISTRIBUTED SYSTEMS Transparency § The two most important transparencies are access and location transparency referred to together as network transparency. § Presence or absence of network transparency most strongly affects the utilization of distributed resources. 29