Software Engineering of Distributed Systems

1. Software Engineering of Distributed Systems University of Colorado Boulder ECEN5053

2. Course Logistics • Introductions • http://ece.colorado.edu/~swengctf • http://ece.colorado.edu/~swengctf/distributed • Format • Calendar • Exams -- final exam only • Homework -- in teams of 2 to 3 • Phone number for late arrival • Contact information • Text web site: www.cdk3.net -- see key pts. University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

3. Outline for this session • Definition of distributed systems • Purposes • Demands/challenges • Hardware concepts • Software concepts • An example model University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

4. Definition of a Distributed System • A distributed system is a collection of independent computers that appears to its users as a single coherent system. Andrew Tanenbaum • A distributed system is one in which components located at networked computers communicate and coordinate their actions only by passing messages. Coulouris et al (your text) • concurrency of components • lack of a global clock • independent failures of components University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

5. Alternative definition of a distributed system • “You know you have one when the crash of a computer you’ve never heard of stops you from getting any work done.” Leslie Lamport University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

6. If true, implied characteristics? • Computer heterogeneity & the user • Communication paths from user’s perspective • User interaction with system from various locations • User interaction with applications • Scalability • Availability • Addition or temporary removal of certain components University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

7. Examples? • internet -- • Not quite there -- some internet applications more so than others • Some applications, user must be very aware of which computer is being accessed • and what else? University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

8. Timeline of what had to happen first 1945 ~1985 powerful microprocessors mainframes high speed networks University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

9. Necessary Developments • Take an historical view • 1945 - 1985 • Computers are large & expensive • Most organizations had only a few • lacked a way to connect them • operated independently from one another • By mid-80’s ... powerfulmicroprocessors with power of a then-contemporary mainframe • High speed networks! • Result: Easy to combine large numbers of computers via a high-speed network. University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

10. Purposes -- what problems are solved? • Easily connect users to remote resources • Share resources with remote users in a controlled way • Hide the fact that the resources are physically distributed over a network -- transparency • Should be an open system • Offers services by standard rules that describe the syntax and semantics of those services • Should be scalable • size, geography, and administration University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

11. Purpose 1: Access and sharing remotely • Why share? • economics • ease of collaboration -- virtual organizations • ease of info exchange • commerce • Connectivity and sharing lead to security issues • Currently, inadequate protection University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

12. Purpose 2: Transparency University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

13. Degree of Transparency • Hiding all distribution aspects not always good idea • Some times desirable to remain fixed • Messages between processes that are thousands of miles apart will take hundreds of milliseconds • Trade-off between high degreeof transparency and performance -- why? • The degree of desirable transparency should be considered in context with other issues such as performance and cost University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

14. Purpose 3: Openness • Offers services according to standard rules describing syntax and semantics of the services. • Rules are formalized in protocols • Services generally specified through interfaces • using Interface Definition Language (IDL) • specify syntax only • natural language used to describe semantics • allows arbitrary process that needs an interface to talk to another process that provides it • proper interfaces are complete and neutral University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

15. Goals of Openness • Interoperability and portability • completeness and neutrality are prerequisites • Flexible • easy to configure the system out of different components from different developers • easy to add new components without impact • easy to replace existing ones without impact • i.e. extensible • easier said than done University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

16. Purpose 4: Flexibility -- Policy and Mechanism • System must be organized as a collection of relatively small and easily replaceable or adaptable components • Need for change: component does not provide optimal policy for a specific user or app • Example: differing caching policies • Need to be able to separate policy & mechanism University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

17. Purpose 5: Scability Challenges -- Size • Size • Limitations of centralized services, data, and algorithms -- become bottleneck • Unlimited processing power and storage cannot overcome communication limitations • Decentralization introduces some kinds of uncertainty University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

18. Purpose 6: Scalability Challenges -- Geography • Existing distributed systems designed for LANs are based on synchronous communication • Communication in WANs is inherently unreliable and almost always point-to-point • LANs provide reliable comm based on broadcasting -- WAN needs special location services • Centralized components prevent geographic scale University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

19. Purpose 7: Scalability Challenges -- Administration • How to scale across multiple independent administrative domains • Conflicting policies • usage (payment) • management • security • protect against malice from the new domains • protect against malice from the distributed system -- e.g. downloaded programs University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

20. Scaling Techniques • Scalability problems appear as performance ones • hide communication latencies • avoid waiting for responses as much as possible • i.e. construct the requestor to use asynchronous comm as much as possible • reduce overall communication • distribution -- spreading component parts across the system, e.g. DNS (see next slide) • replication across the distributed system • increases availability (helps hide latency) • helps balance the load between components University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

21. Example: Dividing DNS name space into zones Generic Countries int com edu gov mil org ... Z1 colorado Z2 cs ece ... Z3 University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

22. Outline • Definition • Purposes • Demands/challenges • Hardware concepts • Software concepts • An example model University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

23. Hardware Concepts • Introduction to how distributed systems can be organized • how they are interconnected • how they communicate Memory Interconnection University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

24. Shared Memory & Private Memory • Multiprocessors (not multicomputers) • Single physical address space shared by all CPUs • CPU A writes 37 to address 1000 • CPU B then reads from address 1000 and gets 37 • e.g., multiple processors on a board with shared memory • Multicomputers • Every machine has its own private memory • CPU A writes 37 to its address 1000 • CPU B reads from its address 1000 and gets whatever happens to be there; not affected by the other write • For example, PCs connected by a network University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

25. Bus-based & Switch-based • Bus architecture of the interconnection network • single network, backplane, bus, cable or other medium that connects all the machines • For example, cable television • Switched architecture • Individual wires from machine to machine with many different wiring patterns in use • Msgs move along wires with an explicit switching decision made at each step to route the message along one of the outgoing wires. • e.g., worldwide public telephone system University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

26. Divide & conquer -- select and explain • Performance Impacts • bus, shared memory • switched, shared memory • not quite shared memory • homogeneous multicomputers • private memory, bus-based network • private memory, switch-based network • heterogeneous multicomputer systems University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

27. Performance Impacts--bus, shared memory • Bus-based multiprocessor, shared memory • Coherent memory • Bus contention • If cache memory for each CPU has a high hit rate, bus traffic drops dramatically • but introduces serious problem -- what is it? • Caching and memory coherence is an issue for distributed systems • Limited scalability University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

28. Performance impacts -- switched, shared memory • 1. Divide memory into modules; connect them to CPU’s with a matrix of switches called a crossbar switch • Allows multiple CPU’s to access shared memory simultaneously • One still has to wait if both want to access same module • 2. Network of switches to route any input to any output • May be several switching stages in-between • Need extremely fast switching to reduce latency=$$ University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

29. Performance impacts--not quite shared memory • Reduce cost of switching with hierarchical system • SOME memory associated with each CPU (not shared) • Access to own local memory is quick • Accessing anybody else’s memory is available but slower • NUMA - “Non Uniform Memory Access” • better average access times than switched nw’s • what’s the problem? University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

30. Performance impacts-- homogeneous multicomputers (SANs) • System of individual computers. Therefore... • Each CPU has direct connection to its own local memory • Challenges surround communication between the CPUs • Traffic volume will be orders of magnitude lower than when interconnection network is also used for CPU-to-memory traffic University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

31. Performance impacts - private memory, bus-based network (SANs) • Processors connected thru shared multiaccess network such as Fast Ethernet • Limited scalability -- performance degrades with 25-100 nodes depending on amt of communication University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

32. Performance impacts - private memory, switch-based network (SANs) • Messages are routed through an interconnection network instead of broadcast as in bus-based • Interconnection networks vary • Grid -- suitable to 2-dimensional problems • Hypercube -- n-dimensional cube • MPPs - massively parallel processors (1000’s) • high-performance proprietary interconnection network designed for low latency, high bandwidth • COWs - clusters of workstations • Std wkstns connected by off-the-shelf communication components; no special measures for high bandwidth or reliability --> ?? University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

33. Performance impacts - heterogeneous multicomputer systems • Most distributed systems are these • Computers are heterogeneous w.r.t. processor type, memory size, I/O bandwidth, etc. • Interconnection networks can be heterogeneous, too • Many large-scale heterogeneous multicomputers lack a global system view • cannot assume same performance or services are available everywhere • THEREFORE sophisticated software is needed • shield application developers from what is going on at hardware level (transparency) University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

34. Software Concepts • Distributed systems software • acts as resource manager(s) for the underlying hardware • Hide intricacies and heterogeneity of underlying hardware • The issues that this software faces are the core of distributed systems principles we will study this semester University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

35. When is a distributed system not a distributed system? • Distributed operating system: • Not intended to handle a collection of independent computers • Network operating system: • Does not provide a view of a single coherent system • “true” distributed system • Goal: scalability and openness of network o.s. andtransparency and ease of use of distributed o.s. • Additional layer called middleware University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

36. Various middleware models (paradigms) • A particular paradigm is a set of decisions about how to describe distribution and communication • Distributed file systems • Remote procedure calls • Distributed objects • Distributed documents • See table University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

37. Sample Paradigms University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

38. Each paradigm must address these issues: • Communication • Processes & their synchronization • Processes & their interaction • Naming • Consistency and replication • Fault tolerance • Security University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction

39. Software Engineering of Distributed Systems • Requirements specification of these issues in distributed systems -- how to recognize, analyze, specify, trace, and manage • Design -- how to choose, represent, and verify • Implementation -- tools, language support • Testing -- static and dynamic University of Colorado ECEN5053 Software Engineering of Distributed Systems Week 1 Introduction