CONCURRENT AND DISTRIBUTED SYSTEMS

Definition•A distributed system is a system consisting of a collection of autonomous machines connected by communication networks and equipped with software systems designed to produce an integrated and consistent computing environment.

•Distributed systems enable people to cooperate and coordinate their activities more effectively and efficiently.

Motivations for Distributed Systems•resource sharing, •openness, •concurrency,• scalability, •fault-tolerance, and•transparency

Motivations contd.• Resource sharing. In a distributed system, the resources - hardware, software

and data can be easily shared among users. For example, a printer can be sharedamong a group of users.

• Openness. The openness of distributed systems is achieved by specifying thekey software interface of the system and making it available to softwaredevelopers so that the system can be extended in many ways.

• Concurrency. The processing concurrency can be achieved by sending requeststo multiple machines connected by networks at the same time.

• Scalability. A distributed system running on a collection of a small number ofmachines can be easily extended to a large number of machines to increase theprocessing power.

• Fault-tolerance. Machines connected by networks can be seen as redundantresources, a software system can be installed on multiple machines so that inthe face of hardware faults or software failures, the faults or failures can bedetected and tolerated by other machines.

Motivations Contd.• Transparency. Distributed systems can provide many forms of

transparency such as:1. Location transparency, which allows local and remote

information to be accessed in a unified way;2. Failure transparency, which enables the masking of

failures automatically; and 3. Replication transparency, which allows duplicating

software/data on Multiple machines invisibly.

Network Architecture

• In order to utilize the full potential of computer networks, international standards were created to ensure that any system could communicate with any other system anywhere in the world.

• This gives rise to Open System Interconnection (OSI) reference model or OSI Architecture

• It is a seven-layer model for inter-process communication.• Its architecture is comprised of application, presentation,

session, transport, network, data link and physical layers

The OSI Architecture

TCP/IP reference model

• It is a four-layer architecture: application, transport, Internet, and network interface

• The current Internet based on ARPANET uses this architecture

• In this model, the network interface (or access) layer relies on the data link and physical layers of the network, and the application layer corresponds to application and presentationlayers of the OSI model, since there is no session layer in the TCP/IP model.

TCP/IP reference model

Network Fault Tolerance

• The term network fault tolerance refers to how resilient the network is against the failure of a component.

• Network reliability refers to the reliability of the overall network to provide communication in the event of failure of a component or components in the network.

Characteristics of Networks that makes them vulnerable

• Unreliable communications;• Unreliable resources (computers, storage,

software, etc.);• Highly heterogeneous environment;• Potentially very large amount of resources:

scalability;• Potentially highly variable number of

resources.

Network Reliability Issues

• Communication network reliability depends on the sustainability of both hardware and software

• network reliability nowadays encompasses more than what was traditionally addressed through network availability

• Basic techniques used in dealing with network failures include: retry (retransmission), complemented retry with correction, replication (e.g., dual bus), coding, special protocols (single handshake, double handshake, etc.), timing checks, rerouting, and retransmission with shift (intelligent retry), etc..

Protocols

• Generally speaking, a protocol is an agreement between the communication parties on how communication is to proceed.

• Network software is arranged in a hierarchy of layers. Each layer presents an interface to the layers above it that extends the properties of the underlying communication system. One layer on one machine carries on a conversation with the same layer on another machine.

• The rules and conventions used in this conversation are collectively known as the protocol of this layer.

Protocols Contd.

• The definition of a protocol has two important parts:

a. A specification of the sequence of messages that must be exchanged; and

b. A specification of the format of the data in the messages.

Protocols Contd.

• A protocol is implemented by a pair of software modules located in the sending and receiving computers.

• Each network layer has one or more protocols corresponding to it so that it can provide a service to the layer above it and extend the service provided by the layer below it.

• Hence, these protocols are arranged in a hierarchy of layers as well.

Quality of Service (QoS)

• Quality of Service (QoS) is a somewhat vague term referring to the technologies that classify network traffic and then ensure that some of that traffic receives special handling.

• The special handling may include attempts to provide improved error rates, lower network transit time (latency), and decreased latency variation (jitter).

• It may also include promises of high availability, which is a combination of mean (average) time between failures (MTBF) and mean time to repair (MTTR).

Software for Distributed Computing

• Traditional Client-Server Model: The client-server model has been a dominant model for distributed

computing since the 1980s. The development of this model has been sparked by research and the

development of operating systems that could support users working at their personal computers connected by a local area network.

The issue was how to access, use and share a resource located on another computer, e.g., a file or printer, in a transparent manner.

In the 1980s computers were controlled by monolithic kernel basedoperating systems, where all services, including file and naming services, were part of that huge piece of software.

In order to access a remote service, the whole operating system must be located and accessed within the computer providing the service.

• There was the need to distinguish from a kernel based operating system --

• that part of software which only provides a desired service -- and embody it into a new software entity. This entity is called a server.

• Thus, each user process, a client, can access and use that server, subject to possessing access rights and a compatible interface.

• Hence, the idea of the client-server model is to build at least that part of an operating system which is responsible for providing services to users as a set of cooperating server processes.

Web-Based Distributed Computing Models

• To meet the requirements of quick development of the Internet, distributed computing may need to shift its environment from LAN to the Internet.

• At the execution level, distributed computing /applications may rely on the following parts:

• Processes: A typical computer operating system on a computer host can run several processes at once.

• While it is running, a process has access to the resources of the computer (CPU time, I/O device and communication ports) through the operating system.

Web-Based Distributed Computing Models contd.

• Threads: Every process has at least one thread of control. Some OS support the creation of multiple threads of control within a single process.

• Each thread in a process can run independently from the other threads.

• The threads may share some memory such as heap or stacks. Usually the threads need synchronization.

• Distributed Objects: Distributed Object technologies are best typified by the Object Management Group’s Common Object Request Broker Architecture (CORBA), and Microsoft’s Distributed Component Object Model (DCOM)

Web-Based Distributed Computing Models contd.

• Agents: An agent could be defined as a program that assists users and acts on their behalf.

• From the perspective of end - to - end users, agents are active entities that obligate the following mandatory behavior rules:

• R1:Work to meet designer’s specifications;• R2: Autonomous: has control over its own actions provided

this does not violate R1.• R3: Reactive: senses changes in requirements and

environment, being able to act according to those changes provided this does not violate R1.

Web-Based Distributed Computing Models contd.

• From the perspective of systems, an agent may possess any of the following orthogonal properties:

• Communication: able to communicate with other agents.

• Mobility: can travel from one host to another.• Reliability: able to tolerate a fault when one

occurs.• Security: appear to be trustful to the end user.

Web-based Client-Server Computing

• Web-based client-server computing systems could be categorized into four types: the proxy computing, code shipping, remote computing and agent-based computing models.

Web-based Client-Server Computing contd.

• The proxy computing (PC) model is typically used in Web-basedscientific computing. According to this model a client sends data and programs to a server over the Web and requests the server to perform certain computing. The server receives the request, performs the computing using the

• programs and data supplied by the client and returns the result back to the client.

• Typically, the server is a powerful high-performance computer or it has some special system programs (such as special mathematical and engineering libraries) that are necessary for the computation. The client is mainly used for interfacing with users.

Web-based Client-Server Computing Contd.

• The code shipping (CS) model is a popular Web-based client-server computing model.

• A typical example is the downloading and execution of Java applets on Web browsers, such as Netscape Communicator and Internet Explorer.

• According to this model, a client makes a request to a server, the server then ships the program (e.g., the Java applets) over the Web to the client and the client executes the program (possibly) using some local data.

• The server acts as the repository of programs and the client performs the computation and interfaces with users.

Web-based Client-Server Computing Contd.

• The remote computing (RC) model is typically used in Web-based scientific computing and database applications [Sandewall 1996].

• According to this model, the client sends data over the Web to the server and the server performs the computing using programs residing in the server. After the completion of the computation, the server sends the result back to the client.

• Typically the server is a high-performance computing server equipped with the necessary computing programs and/or databases. The client is responsible for interfacing with users. The

• NetSolve system [Casanova and Dongarra 1997] uses this model

Web-based Client-Server Computing Contd.

• The agent-based computing (AC) model is a three-tier model. According to this model, the client sends either data or data and programs over the Web to the agent.

• The agent then processes the data using its own programs or using the received programs.

• After the completion of the processing, the agent will either send theresult back to the client if the result is complete, or send the data/program/midium result to the server for further processing. In the latter case, the server will perform the job and return the result back to the client directly (or via the agent). Nowadays, more and more Web-based applications have shifted to the AC model [Chang and Scott 1996] [Ciancarini et al 1996].

The Agent-Based Computing Models

• The basic agent-based computing model has many extensions and variations.

• However, there are two areas of distinction among these models, which highlight their adaptability and extensibility: one is whether the interactions among components are preconfigured (hard-wired) and the other is where the control for using components or services lies (e.g., requester/client, provider/server, mediator etc.).

The Agent-Based Computing Models contd: Conversational Agent Model

• Conversational agent technologies model communication and cooperation among autonomous entities through message exchange based on speech act theory. The best-known foundation technology for developing such systems is the Knowledge Query and Manipulation Language (KQML) [http://www.cs.umbc.edu/kqml/], which is often used in conjunction with the Knowledge Interchange Format (KIF) [http://www.cs.umbc.edu/kqml/].

• In these systems, service access control also lies with a client, which requests a service from a service broker or name server, and then initiates peer-to-peer communication with the provider at an address provided by the broker.

• Although language-enriched interchanges occur, conversational agents suffer from the same restriction as distributed objects in that the interactions among components are hard-coded in the requester, thus making services inflexible and difficult to reuse and extend.

Summary

• The key purposes of distributed systems can be represented by: resource sharing,openness, concurrency, scalability, fault-tolerance and transparency.

• Distributed computing systems comprise the three fundamental components: computers,networks, and operating systems and application software.

MODELLING FOR DISTRIBUTED NETWORK SYSTEMS: THE CLIENT SERVER MODEL

Figure 2.1: The basic client-server model

• A more detailed client-server model has three components:

• Service: A service is a software entity that runs on one or more machines. It provides an abstraction of a set of well-defined operations in response to applications’ requests.

• Server: A server is an instance of a particular service running on a single machine.

• Client: A client is a software entity that exploits services provided by servers. A client can but does not have to interface directly with a human user.