02 - Distributed Systems

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1 02 - Distributed Systems Definition Coulouris 1 (Dis)advantages Coulouris 2 Challenges Saltzer_84.pdf Models Physical Architectural Fundamental

2 2/60 Definition Distributed Systems Distributed System is a collection of independent computers that appear to its users as a single coherent system Examples of Distributed Systems: Computer cluster in a university Air lines database and reservation system Web Cloud

3 3/60 Network vs Distributed Remote components FileZilla Communication sshfs Addressing Network system Explicit Distributed system Mostly implicit

4 4/60 Parallel vs Distributed Systems A concurrent system could be Parallel or Distributed: Two possible Views to make the distinction View 1: Parallel System : A particular tightly-coupled form of distributed computing Distributed System: A loosely-coupled form of parallel computing View 2: Parallel System:processors access a shared memory to exchange information Distributed System: uses a distributed memory. Massage passing is used to exchange information between the processors as each one has its own private memory.

5 5/60 Parallel vs Distributed Systems

6 Over Centralized Systems Economics: Speed: May have a more total computing power than a centralized system Enhanced performance through load distributing. Inherent Distribution: lower (price/performance) ratio Some applications are inherently distributed Availability and Reliability: No single point of failure. The system survives even if a small number of machines crash Incremental Growth: Can add computing power on to your existing infrastructure 6/60 Advantages

7 7/60 Advantages vs networked PCs Computation: Shared management of system: many users can access the same common database Resources Sharing: backups & maintenance... Data Sharing: can be shared over multiple machines can share expensive peripherals Flexibility: Spreading workload over the system CPUs.

8 8/60 Disadvantages Software: Developing a distributed system software is hard Creating OSs / languages that support distributed systems concerns Network: Security : When network is overloaded/messages lost, rerouting/rewiring the network is costly/difficult more sharing leads to less security especially in the issues of confidentiality & integrity Incremental growth is hard in practice due to changing of hardware and software

9 9/60 Distributed Systems Challenges Heterogeneity Openness Security Scalability Failure handling Concurrency Transparency Quality of service

10 10/60 Heterogeneity Applies to the following elements: Networks Hardware Operating Systems Programming languages Multiple implementations by different developers

11 11/60 Openness Capability of a system to be: Extended Implemented in various ways Determined by degree of How new services can be added How can be accessed by multiple clients Open systems Have interfaces published Are based on uniform communication mechanisms Can be built from heterogenous components But components must be conform published standards

12 12/60 Security Security for resources has three components: Confidentiality Integrity Protection agains disclosure to unauthorized individuals Protection agains alteration and corruption Availability Protection agains interference with the means to access the resources

13 13/60 Sclability A system is scalable if Remain effective when there is an increase number of users Challenges Control the cost of physical resources Control performance lost Prevent SW resources starvation Avoid performance bottlenecks

14 14/60 Failure handling Fails produce incorrect results or stop services Failures handling techniques: Detecting failures Masking failures Tolerating failures Recovering from failures Redundancy

15 15/60 Concurrency Resources can be accessed simultaneously By multiple clients Serialization of requests limits throughput Concurrent processing should be allowed Shared resources should operate correctly in concurrent environment Server Services Objects Operations should be guarded

16 16/60 Transparency Concealment Of separation/distributions of components From the user and programmer System is perceived as a whole Rather than a collection of components

17 17/60 Transparency Access transparency Location transparency Local and remote resources are accessed using the same operations Resources can be accessed without knowledge of physical and network location Concurrency transparency Processes can operate concurrently using shared resources without interference between them

18 18/60 Transparency Replication transparency Multiple instances of a resource can be used without knowledge of the replicas by the users or application programmers Failure transparency Faults should be concealed Users and programs should complete their tasks despite failures of HW or components Mobility transparency Resources and clients can move within the system without affecting the operation of users and programs

19 19/60 Transparency Performance transparency Scaling transparency Systems can be reconfigured to improve performance as loads vary System and application can scale without change to the system structure and algorithms Network transparency Access Local vs remote Location Location independent addresses

20 20/60 Quality of service Users are provided with a functionality with a certain quality level Quality of service is affected by non-functional properties: Reliability Security Performance Adaptability to changing configuration and resources Performance Important aspect to Quality of service Usually defined in terms of responsiveness and throughput QoS Capability of a system to to meet pre-defined deadlines Reliability, security or performance

21 21/60 Physical models Early Distributed Systems Late 70/early 80s nodes Local network / Homogeneous / Few services Internet scale 90s Internet based / Network of networks / Static Global / Heterogeneous (but server or client) / Open Contemporary Mobile nodes (Wifi, GSM) Ubiquitous (embedded in objects and environment) Systems of systems (cloud) New level of heterogeneity (architecture and capabilities)

22 22/60 Physical models

23 23/60 Architectural models What entities are communicating in a DS? How they communicate? Communication entities Communication paradigms What roles and responsibilities they have? How are they mapped on the physical infrastructure?

24 24/60 Architectural models

25 Communication entities Processes DS = Processes + IPC Some systems don't have processes Most systems have threads the real communication endpoint Objects Migrations of OO to DS OO Designs and OO programming Natural decomposition unit for a problem Accessed by and interface 25/60 Architectural models

26 Communication entities Components Provide interface like objects Specify assumption Dependencies are explicit and used to pair components Contracts Web services Implementation of Objects and Components On the WEB Identified by a URL Defined/described/discovered by XML 26/60 Architectural models

27 Communication paradigms Remote invocation Request-reply protocols Low level Programmer creates/sends messages Example: HTTP 27/60 Architectural models

28 Communication paradigms Remote invocation Remote procedure calls Attributed to Birrel and Nelson (84) RPC system hides Supports client/server Distribution Encoding/decoding messages Passing of message Semantic of the procedure call Server offer set of operations (by interfaces) Clients call those operations directly as if local Access and location transparency (minimal) 28/60 Architectural models

29 Communication paradigms Remote invocation Remote method invocation Resembles RPC Client objects invoke methods on remote objects Underling details are hidden May support But in a world of objects Object identity Pass objects as parameter Tight integration to OO languages 29/60 Architectural models

30 Communication paradigms Remote invocation Two-way relationship (sender receiver) Receiver identity is know Both parties exist simultaneously at the same time Direct communication Indirect communication Space uncoupling Sender does not know who is sending to Time decoupling Senders and receiver do not need to exist at the same time 30/60 Architectural models

31 Communication paradigms Indirect communication Group communication Delivery of messages to groups of recipients One-to-many communication Communication relies on groups abstraction Recipients join groups Publish-subscribe Information dissemination/ distributed events Producers(publishers) distribute Information items of interest (events) Consumers register the interest or events to receive 31/60 Architectural models

32 Communication paradigms Indirect communication Message Queues Point-to-point communication channel Producer places message on Queue Consumer retrieves message Consumers are notified of message availability Tuples space Communication of performed by the access to shared structured data (tuples) Add tuples to the persistent tuple space Consumers can read or deleted existing tuples Can be client server or P2P 32/60 Architectural models

33 Communication paradigms Indirect communication Distributed Shared Memory DSM systems provide a view of a shared memory space Programmers are presented with the abstraction of reading/writing local memory Composed of data on multiple remote nodes All accesses are to local address space Although data can be on a remote node Infrastructure guarantees Copies of data are provided in a timely manner Synchronization and consistency of data 33/60 Architectural models

34 Roles and responsibilities Client-Server Client processes interact with individual servers (pottenctially) In separate hosts To access shared resources Servers can also be clients 34/60 Architectural models

35 Roles and responsibilities Peer-to-peer All processes have similar roles Interacting cooperatively as peers No distinctions between client or server Resources owned by users Can be put to use to support the service Resources increase with the number of users Data objects are shared and distributed Distribution and replication increases complexity 35/60 Architectural models

36 Placement Where to place entities on the physical model? Distribute service among several servers Data may be partitioned Data may be replicated Caching Store of recently used data Locally to the client On a separate server Objects are retrieved from cache If available 36/60 Architectural models

37 Placement Mobile code Code is downloaded from the server Executed on the client No network delays Mobile agents Code (and data) roams on computers Executes on behalf of other Invokes local services Lower execution time Code transfer + local invocation vs remote invocation 37/60 Architectural models

38 38/60 Architectural models Architectural patterns Layering Related to abstraction Each layer offers an abstraction Higher layers not aware of lower implementations Applications, services Middleware Operating system Platform Computer and network hardware

39 39/60 Architectural models Architectural patterns Tiered Related to composition Complements layerings Layering: vertical organization of services into layers Tiering: Distribution of a given layer into appropriate servers Presentation logic Application logic Data logic

40 Architectural patterns 2-Tiered` 40/60 Architectural models

41 Architectural patterns 3-Tiered 41/60 Architectural models

42 42/60 Architectural models Architectural patterns Thin-Client Moves complexity away from end-user Client has no logic only presentation Compute server Network computer or PC Thin Client network Application Process

43 Architectural patterns Proxy Offers the same interface as the server Located on the local client Is contacted and redirect calls to remote note Offers location transparency Can encapsulate other functionalities Placement policies of replication Caching 43/60 Architectural models

44 Architectural patterns Brokerage Supporting interoperability 44/60 Architectural models

45 45/60 Fundamental models Describe the general and fundamental characteristics of a DS Not how it is implemented Define the assumptions about the system Allows the generalization about (imp)possibilities General purpose algorithms Desirable properties that are guaranteed Interaction Failure Security

46 Interaction Assumptions about the communication channels Latency: delay between start of message transmission and receiving Time taken on transmission Delay assessing network Time of message processing (on the OS) Bandwidth: Amount of information that can be transmitted on a give time Jitter: variation in time take to deliver a series of messages 46/60 Fundamental models

47 Interaction Assumptions about computer clocks Each computer has local clock Simultaneous clock read render different values Used to obtain current time Clocks on computers drift at different rates 47/60 Fundamental models

48 Interaction Synchronous Distributed Systems Time to execute each step Messages Are received within a known bounded time Process clocks Has lower bound Has higher bound Drift rate has known bound Can be built if It is possible to guarante previous bounds 48/60 Fundamental models

49 Interaction Asynchronous Distributed Systems There are no bounds for Internet Process execution speed Message transmission delays Clock drift rates Good example No limits to server or network load May take days to arrive Servers can have drifted clock 49/60 Fundamental models

50 Failure In a DS processes and communication can fail Failure model defines how failures can occur In order to provide understanding of their effects Omission failures Process omission failures: Communication omission failures process does not execute the task (crashes) Can be detected using timeouts A sent message is not delivered to the receptor Both benign 50/60 Fundamental models

51 Failure Arbitrary (Bynzatine) failures Any error can occur A process set wrong values in data items A process answers a wrong values A process arbitraly fails No way to distinguish Process fail from no answer :( A message gets corrupted A bogus messages is created A messages is intercepted from delivery A messages is delivered twice 51/60 Fundamental models

52 Failure Class of failure Fail-stop Affects Description Process Process halts and remains halted. Other processes may detect this state. Crash Process Process halts and remains halted. Other processes may 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 Process A process completes a send, but the message is not put in its outgoing message buffer. Receiveomission Process A message is put in a process s incoming message buffer, but that process does not receive it. Arbitrary (Byzantine) Process Process/channel exhibits arbitrary behavior: it may or send/transmit arbitrary messages at arbitrary times, Channel commit omissions; a process may stop or take an incorrect step. 52/60 Fundamental models

53 Failure Timing failures Applicable to synchronous systems With limits to execution, delivery times and clock drifts Class of failure Affects Description Clock Process Process s local clock exceeds the bounds on its Performance Process Process exceeds the bounds on the interval between two steps. Performance Channel A message s transmission takes longer than the stated bound. 53/60 Fundamental models

54 Masking failures A reliable system can be composed of Knowledge of each component failures Allows a service to mask them Hiding a failure Unreliable components that exhibit failures e.g. retrying Converting to a more acceptable failure Recovering an old version of the file 54/60 Fundamental models

55 Reliability on one-to-one communication Basic communication channels can exhibits failures (e.g. omission) But can be used to build a service that masks some failures Provides reliable communication Reliable communication Validity: any message is eventually delivered Integrity: the received message is identical to the sent one 55/60 Fundamental models

56 56/60 Network Platform Networking issues Performance Scalability Reliability Security Mobility QoS Types of networks Personal Area (wireless) Local Area (wireless) Wide Area (wireless) Metropolitan Area Handled by the platform OS + HW Low level protocols

57 57/60 Middleware SW layer that Provides a programming abstraction Masks underlying heterogeneity Network, HW, OS, programming languages Provides a uniform computational model To be used by programmers of distributed applications RMI, remote event notification, distributed transactions,...

58 58/60 Middleware

59 Limitations Some application rely only on middleware If suitable for client-serves can use RPC Not all issues can be handled by middleware Name/Address database Some dependability aspects Large file transfer over unreliable link TCP offers some error detection and correction TCP does not recover from major network interruption If service offers a new level of fault tolerance Must maintain a progress level Must resumes transmission with a new TCP connection 59/60 Middleware

60 Limitations End-to-end argument Some communication functions can only be completely and reliably implemented Saltzer et al (84) With knowledge and help of the application standing at the end points of the communication system In the previous case: TCP does not how to restart a file transmission Client must know where to restart Server must receive information about restart 60/60 Middleware

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