1DT066 D ISTRIBUERADE I NFORMATIONSSYSTEM Distribuerade System Karaktäristik och Design 1

O UTLINE 1. What is a Distributed System 2. Examples of Distributed Systems 3. Common Characteristics 4. Basic Design Issues 5. Summary 2

W HAT IS A D ISTRIBUTED S YSTEM ? Definition: A distributed system is one in which components located at networked computers communicate and coordinate their actions only by passing messages. This definition leads to the following characteristics of distributed systems: Concurrency of components Lack of a global ‘clock’ Independent failures of components ‘acceptable’ 3 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 17

C ENTRALIZED S YSTEM C HARACTERISTICS One component with non-autonomous parts Component shared by users all the time All resources accessible Software runs in a single process Single point of control Single point of failure 4 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 24-30

D ISTRIBUTED S YSTEM C HARACTERISTICS Multiple autonomous components Components are not shared by all users Resources may not be accessible Software runs in concurrent processes on different processors Multiple points of control Multiple points of failure 5 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 24-30

E XAMPLE O N D ISTRIBUTED S YSTEM : L OCAL A REA N ETWORK 6 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 24-30

C OMMON C HARACTERISTICS What are we trying to achieve when we construct a distributed system? Certain common characteristics can be used to assess distributed systems Heterogeneity Openness Security Scalability Failure Handling Concurrency Transparency 7 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 30-40,

H ETEROGENEITY Variety and differences in Networks Computer hardware Operating systems Programming languages Implementations by different developers Middleware as software layers to provide a programming abstraction as well as masking the heterogeneity of the underlying networks, hardware, OS, and programming languages (e.g., CORBA). Mobile Code to refer to code that can be sent from one computer to another and run at the destination (e.g., Java applets and Java virtual machine). 8 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 30-40

O PENNESS Openness is concerned with extensions and improvements of distributed systems. Detailed interfaces of components need to be published. New components have to be integrated with existing components. Differences in data representation of interface types on different processors (of different vendors) have to be resolved. 9 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 30-40

S ECURITY In a distributed system, clients send requests to access data managed by servers, resources in the networks: Doctors requesting records from hospitals Users purchase products through electronic commerce Security is required for: Concealing the contents of messages: security and privacy Identifying a remote user or other agent correctly (authentication) New challenges: Denial of service attack Security of mobile code 10 CDKB, pp Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012

S CALABILITY Adaptation of distributed systems to accommodate more users respond faster (this is the hard one) Usually done by adding more and/or faster processors. Components should not need to be changed when scale of a system increases. Design components to be scalable! 11 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 30-40

F AILURE H ANDLING (F AULT T OLERANCE ) Hardware, software and networks fail! Distributed systems must maintain availability even at low levels of hardware/software/network reliability. Fault tolerance is achieved by recovery redundancy 12 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 30-40

F AILURE T YPES IN D ISTRIBUTED S YSTEMS 13 Class of FailureAffectsDescription ClockProcess Process ’ s local clock exceeds the bounds on its rate of drift from real time. PerformanceProcessProcess exceeds the bounds on the interval between two steps. PerformanceChannel A message ’ s transmission takes longer than the stated bound. Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 83-87

F AILURE T YPES IN D ISTRIBUTED S YSTEMS 14 Class of failure AffectsDescription Fail-stop ProcessProcess halts and remains halted. Other processes may detect this state. CrashProcessProcess halts and remains halted. Other processes may not be able to detect this state. OmissionChannelA message inserted in an outgoing message buffer never arrives at the other end ’ s incoming message buffer. Send-omissionProcessA process completes asend, but the message is not put in its outgoing message buffer. Receive-omissionProcess 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. Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 83-87

C ONCURRENCY Components in distributed systems are executed in concurrent processes. Components access and update shared resources (e.g. variables, databases, device drivers). Integrity of the system may be violated if concurrent updates are not coordinated. Lost updates Inconsistent analysis 15 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 38-40

T RANSPARENCY Distributed systems should be perceived by users and application programmers as a whole rather than as a collection of cooperating components. Transparency has different aspects. These represent various properties that distributed systems should have. 16 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 39-40

B ASIC D ESIGN I SSUES General software engineering principles include rigor and formality, separation of concerns, modularity, abstraction, anticipation of change, … Specific issues for distributed systems: Naming Communication Software structure System architecture Workload allocation Consistency maintenance 17 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 56-57

N AMING A name is resolved when translated into an interpretable form for resource/object reference. Communication identifier (IP address + port number) Name resolution involves several translation steps Design considerations Choice of name space for each resource type Name service to resolve resource names to comm. id. Name services include naming context resolution, hierarchical structure, resource protection 18 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 56-57

C OMMUNICATION Separated components communicate with sending processes and receiving processes for data transfer and synchronization. Message passing: send and receive primitives synchronous or blocking asynchronous or non-blocking Abstractions defined: channels, sockets, ports. Communication patterns: client-server communication (e.g., RPC, function shipping) and group multicast 19 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 58-66

C ONSISTENCY AND O RDERING OF E VENTS Messages take different times to reach the other components. When a message reach the destination it may be invalid at the sender There is normally no global consistent state in a distributed system. A component may send its state to all other components. There is no guarantee that the state has reached all component The sender state may change its state before the old state has arrived. Two messages from different components may arrive in different order at other component. Two components cannot “see the same time”. When consistent states are needed protocols for synchronization is used. Quiz – what is the crucial shared state of TCP? How is it kept consistent? 20 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 77-83

O RDERING OF E VENTS 21 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 77-83

S OFTWARE S TRUCTURE Layers in centralized computer systems: 22 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 67-68

S OFTWARE S TRUCTURE Layers and dependencies in distributed systems: Applications Distributed programming support Open services Open system kernel services Computer and network hardware 23 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 67-68

SYSTEM ARCHITECTURES Client-Server Peer-to-Peer Services provided by multiple servers Proxy servers and caches Mobile code and mobile agents Network computers Thin clients and mobile devices 24 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012

C LIENTS I NVOKE I NDIVIDUAL S ERVERS 25 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 58-66

P EER - TO - PEER S YSTEMS 26 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp58-66

A S ERVICE BY M ULTIPLE S ERVERS 27 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp 58-66

W EB P ROXY S ERVER 28 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp58-66

W EB A PPLETS 29 HelloWorld_example.html A Java applet example Here it is: This is where HelloWorld.class runs. Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDBK 58-66

T HIN C LIENTS AND C OMPUTE S ERVERS Thin Client Application Process Network computer or PC Compute server network 30 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp

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S UMMARY Definitions of distributed systems and comparisons to centralized systems. The characteristics of distributed systems. The seven forms of transparency. The basic design issues. 32 Adapted from: Instructor ’ s Guide for CDKB, Distributed Systems: Concepts and Design Edn. 5 © Pearson Education 2012 CDKB, pp