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CSE5306 Distributed Systems

CSE5306 Distributed Systems. Mohan Kumar Preliminaries and Models: Week 2. System models. Architectural models Client-server model Peer-to-peer model Functional models Interaction model Failure model Security model. Architecture. Structural organization of various components

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CSE5306 Distributed Systems

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  1. CSE5306 Distributed Systems Mohan Kumar Preliminaries and Models: Week 2

  2. System models • Architectural models • Client-server model • Peer-to-peer model • Functional models • Interaction model • Failure model • Security model

  3. Architecture • Structural organization of various components • Simple of abstraction of components • Two main objectives • Placements • Network topology • Data distribution • Interrelationships • Patterns of communications • Relationships between data objects • Data access patterns, dependencies

  4. Basic Processes • Server • Accepts inputs from other processes • Performs a service • Returns outcomes • Client • User/application level • Makes requests, receives results The roles of server and client may change with time • Peer • All are equal

  5. Layered representation Applications and services Middleware Mask Heterogeneity Provide abstraction, transparency Uniformity Operating System Communications Network Hardware PLATFORM

  6. Client/Server Client Server Client Server Server

  7. Peer-to Peer and Client/server variations • Authors’ slides for figures • Peer-top-peer • No distinction among peers • Excellent scalability compared to C-S • Resources are utilized in a distributed network, and more efficiently. • Minimize bottleneck points • Variations • Multiple servers • Each server specializes in a providing a particular service • E.g., web servers, DNS server, authentication etc. • Proxy servers • Enhance availability • Reduce latency • Caches • Objects cached to reduce latency • Mobile code and mobile agents • Mobile code (e.g., applet) downloaded to client’s site • Local interactions, fast response as there are no communication delays • Mobile agents include code and data • Go around execute on different processors

  8. Multiple servers

  9. Basic applications • Remote login • Keyboard and display interface • Virtual terminal support • telnet, rlogin • File transfer • File, file structures, file attributes • E.g., FTP • Messaging • Send and receive • Email, SMTP • Browsing • Information retrieval • Remote execution • Execute a program on a remote server • E.g, MIME – multipurpose Internet mail extension

  10. Goals • Efficiency • Propagation delays, communications • Overlapped computation/communication • Efficient distributed processing and load sharing • Flexibility • User friendly • Ability to evolve and migrate • Modularity, scalability, portability, and interoperability • Consistency • Predictability and uniformity in system behavior • Integrity in concurrency control, failure handling and failure handling • Robustness • Ability to handle exceptional situations and errors • Change in topology, lost message, crashed system etc. • Reliability, protection and access control • Secure and privacy preserving

  11. Design requirements • Performance • Responsiveness • Access to shared resources • Communication delays • Server loads, scheduling, wait periods • Control switching • Load balancing • Caching and replication • Combined computation/communication scheduling

  12. Transparency This is in contrast to the meaning of transparency in English – open, visible, see through etc. • Ability to hide/mask all system details from users/application developers • System details are irrelevant to users/developers • System details are very relevant to system managers • Creation of an illusion of a model that it is supposed to be

  13. Goals and transparencies • Efficiency • Concurrency • Parallelism • Performance • Flexibility • Access • Location • Migration • Size • Consistency • Access • Replication • performance • Robustness • Failure • Replication • Size

  14. Major issues Interaction and control transparency Performance transparency Resource transparency Failure transparency Communication Synchronization Distributed algorithms Process scheduling Deadlock handling Load balancing Resource scheduling File sharing Concurrency control Failure handling Configuration Redundancy

  15. Services • Primitive • Send, receive, synchronize • System servers • Name server, directory server, network server, time server, file server, print server, security server • Value-added • Web server, conferencing server, • Distributed • A service is composed of several subservices distributed in the network • Services are invoked one-by-one

  16. Processes • A process is a program in execution • Sequential • A single control block regulates the execution • A control block contains state information – program counters, register contents, stack pointers, communication ports, file descriptors etc. • Process control block (PCB) • Concurrent • Simultaneously interacting sequential processes are said to be concurrent • Asynchronous • Separate address space and PCBs • Components may interact through communication/synchronization PCB PCB PCB Process Process Process

  17. Threads • A lightweight process • Threads of a process share the same address space, but have their own registers • A thread control block (or TCB) is local to a thread • Typically, • Threads have their own PC, SP and register set. • Threads share address space, communication ports and file descriptors • Multiple threads are spawned by a process • A PCB is shared among interacting threads • Context switching among threads is lightweight compared to context switching among processes PCB PCB PCB Thread TCB| TCB| TCB Thread Thread Thread Thread TCB| TCB Thread Thread run-time library support Operating System Support

  18. Graph models

  19. Time-space model for interaction Processes P1 P2 P3 P4 TIME Event Communications

  20. Communication in C-S Client Server Client and server communication model RPC Communication Message Passing Communication TCP/UDP Transport Service

  21. Functional models • Why? • Main entities • Interaction among entities • Characteristics – individual and collective • Purpose? • Assumptions • Generalizations • Algorithms • Logical analysis • Mathematical analysis • Models • Interaction • Message passing • Shared objects • Failure • Faults • Detection • Recovery • Security • Security vs. openness • Privacy vs. efficiency

  22. Interaction model • Process interact ions • C-S, P2P, message passing, shared space, synchronous, asynchronous • Single process/thread, multiple threads • Distributed algorithms • Behavior of multiple processes • Includes message transmissions • Each process • Has own its PCB and inaccessible by other processes • Likely on different systems in the network • Difficult to coordinate • Two significant factors • Communication performance • Maintenance of global state • Computer locks drift • Clock drifts differ from one another

  23. Performance of communication channels • Latency • Time taken for the first string of bits to arrive at the destination • Delay in accessing the network • Delays (processing times at)OS communication services at both ends • Bandwidth • Frequency, interference, number of channels sharing • Jitter • Variation in times to deliver different components of a message

  24. Two variants • Synchronous • Process execution time is bounded • Message latency over a channel is bounded • Process’ local clock drift is bounded • Though difficult to build, very useful as a model • Time outs • Detect failures • Asynchronous • Blue bullets (Assumptions) above are NOT true • Most systems are asynchronous

  25. Event ordering • See author’s slide • Lamport’s logical ordering • X sends m1 before Y receives m1 • Y sends m2 before X receives m2 • Because we know replies are sent after receiving messages • That is m2 is a reply to m1 • Y receives m1 before sending m2

  26. Failure model • Omission failures • Processor/process crash • Communication failure/message drops • Arbitrary failures • Process setting wrong values in data • Data corruption during transmission • Timing failures • Synchronous systems • Real-time systems • Clock, process, channel • Masking failures • Replication • Service to mask failures

  27. Security model • Protecting objects • Who is allowed to access to access what data • Check access rights, verify identity • Securing process and interactions • Processes • Server, client, peer • Communication channel • Copy/alter messages; inject harmful messages • Encryption, authentication, time stamping • Denial of service • Mobile code, mobile agents

  28. 01/26/11

  29. Time services • Global time consensus is needed to • Coordinate distributed activities • File backup • Expiration time of a received message/data • Event related activities • When an event occurs or occured • How long did it take • Which event occurred first

  30. Clocks • Physical clock • Approximation of realtime • Logical clock • Preserves ordering of events

  31. Physical Clocks • Physical clocks in computers synchronize and schedule hardware activities • Software timers depend on physical clock interrupts to manage software activities • In distributed systems • There are many such clocks • Need for a time server to synchronize clocks • Delay associated with receiving time server inputs

  32. Time server • Signal propagation delay • Network communication delay • Noise and other delays (TCP) • Suppose • Ts: Time for client-to-server communication • Tr: time for server-to-client communication and • Tp: time to process the request at the time server • Time= Timeserver(time) + (Ts+Tr+Tp)/2 • Tp/2 is included to account for service to transport layer chores

  33. Logical Clocks • Leslie Lamport’s logical clock is the basis for • Ordering processes and events in distributed systems • Process Pi maintains a logical clock Ci •  is the happens before relation • Two Rules: • Rule1 : If event a occurs before event b within a single process then logical clocks C(a) and C(b) are assigned such that C(a) < C(b). • Rule 2: Suppose - a is a sending event of Piand b is the corresponding receiving event of Pjthen Ci(a) < Cj(b)

  34. Logical clocks (contd.) C(b) = C(a) +  Cj(b) = max [TSa+ , Cj(b)] Tsa is the timestamp of the sending event and  is a positive number • Rule 1 is easy to enforce as both a and b are on the same process • How to enforce Rule 2 •  describes the causality between two events and it is transitive • If a  b and b  c, then a c. • If neithera bnorb athen the two events are disjoint and can be run concurrently.

  35. Happens-before relation b,35 a,25 27 41 e,44 c,24 d,36 38 28 40 f,27 g,42 39  = 1

  36. Synchronization

  37. Networking review Please read up chapter 4 or a networking book I will cover only mobile and wireless networking

  38. jec

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