Slides for Chapter 2: Architectural Models From Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edition 4, © Pearson Education 2005

Architecture Model Architecture model is concerned with the placement of its parts, namely how components are mapped to underlying network and the relationship between them, that is, their functional roles and patterns of communication between them. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Architectural Elements What are the entities that are communicating in the distributed system? From system perspective, the entities are typically processes and sometimes supplemented by threads. From a programming perspective: Objects: objects are accessed via interface, with an associated interface definition language providing a specification of the methods defined on an object. Components: there are many problems with objects, so components has emerged as a direct response to such weakness. Components resemble objects in that they offer problem-oriented abstractions for building system and are also accessed through interface. It requires a strict and explicit assumption for other components, namely dependency and contract are clear. Its enable third-party development. Web service: encapsulate behavior and access through interface. It uses web standards to represent and discover services. XML-based message exchanges via Internet-based protocols. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Architectural Elements What are the communication paradigm is used in the distributed system? Inter-process communication Low-level support for communication between processes, including message-passing primitive, socket programming, multicast communication Remote invocation The most common communication paradigm based on a two-way exchange between entities and resulting in the calling of a remote operation, procedure or method. Request-reply protocol is underlying message-passing service to support client-server computing. A pairwise exchange of message from client to server and from server back to client. Remote procedure call Remote method invocation is similar to remote procedure call but in a word of distributed objects. Indirect communication through a third entity allowing a strong degree of decoupling between senders and receivers. In particular, senders do not need to know who they are sending to (space uncoupling). Senders and receivers do not need to exist at the same time ( time uncoupling). Group communication, publish-subscribe systems, message queues, Tuple space and distributed shared memory Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Architectural Elements What are the Roles and Responsibilities for Communication Entities? It has major implications for the performance, reliability and security of the resulting system. Client and Server: The most often cited when distributed systems are discussed. The next figure illustrates the simple structure in which client processes interact with individual server processes in separate host computers. Server may in turn be clients of other servers. For instance, web server is often a client of a local file server that manages the files in which the web pages are stored. Web servers are clients of DNS service. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Clients invoke individual servers Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Architectural Elements Peer-to-peer system: all of the processes involved in a task or activity play similar roles, interacting cooperatively as peers without any distinction between client and server processes. While client and server model is simple but it scales poorly. The centralization of service provision and management does not scale well beyond the capacity of the computer that hosts the service and bandwidth of its connection. The next figure illustrates the form of P2P. Applications are composed of large number of peer processes running on separate computers and the pattern of communication depends entirely on application requirements. Processing and communication loads are distributed across many computers and links. Some objects are even duplicated to further distribute the load and increase the error resilience when some nodes fail. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

A distributed application based on peer processes Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Model Variations Several variations on the above models can be derived from the consideration of the following factors: The user of multiple servers and caches to increase performance and resilience. The user of mobile code and mobile agents. Thin client: User’s need for low-cost computers with limited hardware resources that are simple to manage. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

A service provided by multiple servers The servers may partition the set of objects on which the service is based and distributed them between them, or they may maintain replicated copies of them. Web provides a common example of partitioned data Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Web proxy server Cache is a store of recently used data objects that is closer than the object themselves. Proxy server and caches: web proxy server provides a shared cache of web resources for the client machines at a site or across several sites. The purpose is to increase the availability and performance of the service by reducing the load on the wide-area network and web server. . Proxy server can take on other roles. For example, they may be used to access remote web server through a firewall. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Mobile code and Mobile agent Mobile code can be downloaded from one node to the other. Applet is one example. The advantages of applet is good interactive response since it does not suffer from the delays due to the changing network bandwidth. Mobile agent is a running program including both code and data that travels from one computer to another carrying out a task on someone’s behalf, eventually returning with the results. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Thin clients and compute servers Network computer or PC network Application Thin Process Client Thin client refers to a software layer that supports a window-based user interface on a computer that is local to the user while executing application programs on a remote computer. Instead of downloading the code onto local computer, it runs them on the remote powerful server, which has the capacity to run large number of applications simultaneously. It is usually a cluster or at least multi-processor machine. The main drawback is in highly interactive graphical activities such as CAD and imaging processing, where the delay are increased by the need to transfer image and vector information between the link. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Architectural Patterns: 1: Software and hardware service layers in distributed systems Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Software and Hardware service layers Platform: the lowest-level hardware and software layers are often referred to as a platform, which provides services to the layers above them. Intel x86/Windows, Intel 86/Linux, PowerPC/Mac OS X are such examples. Middleware: a layer of software whose purpose is to mask heterogeneity and to provide a convenient programming model to application programmers. Java RMI, web services and CORBA are such examples Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Architectural Patterns: 2: Two-tier and three-tier architectures Complementary to layering. Tiering is a technique to organize functionality of a given layer and place this functionality into servers and on to physical node. The presentation logic, which is concerned with handling user interaction and updating the view of the applications as presented to the user. The application logic, which is concerned with the detailed application-specific processing associated with the application. The data logic, which is concerned with the persistent storage of the application, typically in a database management system. Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012

Issues can be addressed by the following three models: Fundamental Models It deals with a more formal description of the properties that are common in all of the architectural models. Since no global time in a distributed system, so the clocks on different computers do not necessarily give the same time as one another.. Messages communications can be affected by delays and suffer from a variety of failures and vulnerable to security attacks. Issues can be addressed by the following three models: The interaction model deals with performance and with difficulty of setting time limits. The failure model attempts to give a precise specification of the faults that can be exhibited by processes and communication channels. The security model discusses the possible threats to processes and communication channels. It introduces the concept of a secure channel, which is secure against those threats. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Fundamental Models All system models have some common fundamental properties. The following models are used to capture these: Interaction: Processes communicate with messages and coordinate via synchronization and ordering of activities. The message delays are often of considerable duration, the coordination between processes is limited by lack of global clock. Failure: The correct operation is threatened whenever a fault occurs in any of the computers and network. We should define types of faults in order to tolerate them for the system to continue to run correctly. Security: The modular feature of distributed system and their openness eposes them to attack by both eternal and internal agents. Security model defines and classifies the forms of attack in order resist them. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Two variations of interaction model It is hard to set time limits on the time taken for process execution, message delivery or clock drift. Two opposing extreme positions provide a pair of simple models: the first has a strong assumption of time an the second makes no assumption about time. Synchronous distributed system: Lower and upper bounds are defined for the time to execute each step of a process; each message transmitted over a channel; each process has a local clock whose drift rate from real time has a bound. Asynchronous distributed system including Internet: no bounds can be defined. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Figure 2.8 Real-time ordering of events Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Figure 2.9 Processes and channels Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Figure 2.10 Omission and arbitrary failures Class of failure Affects Description Fail-stop Process Process halts and remains halted. Other processes may detect this state in synchronous system using timeout. . Crash not be able to detect this state. Omission Channel A message inserted in an outgoing message buffer never arrives at the other end’s incoming message buffer. Send-omission A process completes a send, but the message is not put in its outgoing message buffer. Receive-omission A message is put in a process’s incoming message buffer, but that process does not receive it. Arbitrary (Byzantine) Process or channel Process/channel exhibits arbitrary behaviour: it may send/transmit arbitrary messages at arbitrary times, commit omissions; a process may stop or take an incorrect step. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Figure 2.12 Objects and principals Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Security The security of a distributed system can be achieved by securing the processes and the channels used for their interactions and by protecting the objects against unauthorized access. Access right specify who is allowed to perform the operations of an object. So we include users as the beneficiaries of access right. The invocation and result should have an authority which is called principle. The server checks the identify of principle for each invocation and see if it has the access right. The client may check the identify of the principle behind the server to ensure the results come from the required server. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

m m’ p q Figure 2.13 The enemy Copy of The enemy Process Communication channel Copy of m Process p q The enemy m’ Enemy can copy, alter or inject messages as they travel across the network. Such Attack present a threat to the privacy and integrity of information and the integrity Of the system. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005

Figure 2.14 Secure channels Principal B Principal A Process p Secure channel Process q Secure channel can be used to defeat the threat to communication channel based on cryptography and authentication. Instructor’s Guide for Coulouris, Dollimore and Kindberg Distributed Systems: Concepts and Design Edn. 4 © Pearson Education 2005