نظم موزعة Distributed systems slides.01.pdf
- 1. Distributed Systems (3rd Edition) Chapter 01: Introduction Version: February 25, 2017
- 2. Introduction: What is a distributed system? Distributed System Definition A distributed system is a collection of autonomous computing elements that appears to its users as a single coherent system. Characteristic features Autonomous computing elements, also referred to as nodes, be they hardware devices or software processes. Single coherent system: users or applications perceive a single system ⇒ nodes need to collaborate. 2 / 56
- 3. Introduction: What is a distributed system? Characteristic 1: Collection of autonomous computing elements Collection of autonomous nodes Independent behavior Each node is autonomous and will thus have its own notion of time: there is no global clock. Leads to fundamental synchronization and coordination problems. Collection of nodes How to manage group membership? How to know that you are indeed communicating with an authorized (non)member? 3 / 56
- 4. Introduction: What is a distributed system? Characteristic 1: Collection of autonomous computing elements Organization Overlay network Each node in the collection communicates only with other nodes in the system, its neighbors. The set of neighbors may be dynamic, or may even be known only implicitly (i.e., requires a lookup). Overlay types Well-known example of overlay networks: peer-to-peer systems. Structured: each node has a well-defined set of neighbors with whom it can communicate (tree, ring). Unstructured: each node has references to randomly selected other nodes from the system. 4 / 56
- 5. Introduction: What is a distributed system? Characteristic 2: Single coherent system Coherent system Essence The collection of nodes as a whole operates the same, no matter where, when, and how interaction between a user and the system takes place. Examples An end user cannot tell where a computation is taking place Where data is exactly stored should be irrelevant to an application If or not data has been replicated is completely hidden Keyword is distribution transparency The snag: partial failures It is inevitable that at any time only a part of the distributed system fails. Hiding partial failures and their recovery is often very difficult and in general impossible to hide. 5 / 56
- 6. Introduction: What is a distributed system? Middleware and distributed systems Middleware: the OS of distributed systems Local OS 1 Local OS 2 Local OS 3 Local OS 4 Appl. A Application B Appl. C Distributed-system layer (middleware) Computer 1 Computer 2 Computer 3 Computer 4 Same interface everywhere Network What does it contain? Commonly used components and functions that need not be implemented by applications separately. 6 / 56
- 7. Introduction: Design goals What do we want to achieve? Support sharing of resources Distribution transparency Openness Scalability 7 / 56
- 8. Introduction: Design goals Supporting resource sharing Sharing resources Canonical examples Cloud-based shared storage and files Peer-to-peer assisted multimedia streaming Shared mail services (think of outsourced mail systems) Shared Web hosting (think of content distribution networks) Observation “The network is the computer” (quote from John Gage, then at Sun Microsystems) 8 / 56
- 9. Introduction: Design goals Making distribution transparent Distribution transparency Types Transparency Description Access Hide differences in data representation and how an object is accessed Location Hide where an object is located Relocation Hide that an object may be moved to another location while in use Migration Hide that an object may move to another location Replication Hide that an object is replicated Concurrency Hide that an object may be shared by several independent users Failure Hide the failure and recovery of an object Types of distribution transparency 9 / 56
- 10. Introduction: Design goals Making distribution transparent Degree of transparency Observation Aiming at full distribution transparency may be too much: Degree of distribution transparency 10 / 56
- 11. Introduction: Design goals Making distribution transparent Degree of transparency Observation Aiming at full distribution transparency may be too much: There are communication latencies that cannot be hidden Degree of distribution transparency 10 / 56
- 12. Introduction: Design goals Making distribution transparent Degree of transparency Observation Aiming at full distribution transparency may be too much: There are communication latencies that cannot be hidden Completely hiding failures of networks and nodes is (theoretically and practically) impossible You cannot distinguish a slow computer from a failing one You can never be sure that a server actually performed an operation before a crash Degree of distribution transparency 10 / 56
- 13. Introduction: Design goals Making distribution transparent Degree of transparency Observation Aiming at full distribution transparency may be too much: There are communication latencies that cannot be hidden Completely hiding failures of networks and nodes is (theoretically and practically) impossible You cannot distinguish a slow computer from a failing one You can never be sure that a server actually performed an operation before a crash Full transparency will cost performance, exposing distribution of the system Keeping replicas exactly up-to-date with the master takes time Immediately flushing write operations to disk for fault tolerance Degree of distribution transparency 10 / 56
- 14. Introduction: Design goals Making distribution transparent Degree of transparency Exposing distribution may be good Making use of location-based services (finding your nearby friends) When dealing with users in different time zones When it makes it easier for a user to understand what’s going on (when e.g., a server does not respond for a long time, report it as failing). Degree of distribution transparency 11 / 56
- 15. Introduction: Design goals Making distribution transparent Degree of transparency Exposing distribution may be good Making use of location-based services (finding your nearby friends) When dealing with users in different time zones When it makes it easier for a user to understand what’s going on (when e.g., a server does not respond for a long time, report it as failing). Conclusion Distribution transparency is a nice a goal, but achieving it is a different story, and it should often not even be aimed at. Degree of distribution transparency 11 / 56
- 16. Introduction: Design goals Being open Openness of distributed systems What are we talking about? Be able to interact with services from other open systems, irrespective of the underlying environment: Systems should conform to well-defined interfaces Systems should easily interoperate Systems should support portability of applications Systems should be easily extensible Interoperability, composability, and extensibility 12 / 56
- 17. Introduction: Design goals Being open Policies versus mechanisms Implementing openness: policies What level of consistency do we require for client-cached data? Which operations do we allow downloaded code to perform? Which QoS requirements do we adjust in the face of varying bandwidth? What level of secrecy do we require for communication? Implementing openness: mechanisms Allow (dynamic) setting of caching policies Support different levels of trust for mobile code Provide adjustable QoS parameters per data stream Offer different encryption algorithms Separating policy from mechanism 13 / 56
- 18. Introduction: Design goals Being open On strict separation Observation The stricter the separation between policy and mechanism, the more we need to make ensure proper mechanisms, potentially leading to many configuration parameters and complex management. Finding a balance Hard coding policies often simplifies management and reduces complexity at the price of less flexibility. There is no obvious solution. Separating policy from mechanism 14 / 56
- 19. Introduction: Design goals Being scalable Scale in distributed systems Observation Many developers of modern distributed systems easily use the adjective “scalable” without making clear why their system actually scales. Scalability dimensions 15 / 56
- 20. Introduction: Design goals Being scalable Scale in distributed systems Observation Many developers of modern distributed systems easily use the adjective “scalable” without making clear why their system actually scales. At least three components Number of users and/or processes (size scalability) Maximum distance between nodes (geographical scalability) Number of administrative domains (administrative scalability) Scalability dimensions 15 / 56
- 21. Introduction: Design goals Being scalable Scale in distributed systems Observation Many developers of modern distributed systems easily use the adjective “scalable” without making clear why their system actually scales. At least three components Number of users and/or processes (size scalability) Maximum distance between nodes (geographical scalability) Number of administrative domains (administrative scalability) Observation Most systems account only, to a certain extent, for size scalability. Often a solution: multiple powerful servers operating independently in parallel. Today, the challenge still lies in geographical and administrative scalability. Scalability dimensions 15 / 56
- 22. Introduction: Design goals Being scalable Size scalability Root causes for scalability problems with centralized solutions The computational capacity, limited by the CPUs The storage capacity, including the transfer rate between CPUs and disks The network between the user and the centralized service Scalability dimensions 16 / 56
- 23. Introduction: Design goals Being scalable Formal analysis A centralized service can be modeled as a simple queuing system Queue Process Requests Response Assumptions and notations The queue has infinite capacity ⇒ arrival rate of requests is not influenced by current queue length or what is being processed. Arrival rate requests: λ Processing capacity service: µ requests per second Fraction of time having k requests in the system pk = 1− λ µ λ µ k Scalability dimensions 17 / 56
- 24. Introduction: Design goals Being scalable Formal analysis Utilization U of a service is the fraction of time that it is busy U = ∑ k0 pk = 1−p0 = λ µ ⇒ pk = (1−U)Uk Average number of requests in the system N = ∑ k≥0 k ·pk = ∑ k≥0 k ·(1−U)Uk = (1−U) ∑ k≥0 k ·Uk = (1−U)U (1−U)2 = U 1−U Average throughput X = U · µ |{z} server at work +(1−U)·0 | {z } server idle = λ µ · µ = λ Scalability dimensions 18 / 56
- 25. Introduction: Design goals Being scalable Formal analysis Response time: total time take to process a request after submission R = N X = S 1−U ⇒ R S = 1 1−U with S = 1 µ being the service time. Observations If U is small, response-to-service time is close to 1: a request is immediately processed If U goes up to 1, the system comes to a grinding halt. Solution: decrease S. Scalability dimensions 19 / 56
- 26. Introduction: Design goals Being scalable Problems with geographical scalability Cannot simply go from LAN to WAN: many distributed systems assume synchronous client-server interactions: client sends request and waits for an answer. Latency may easily prohibit this scheme. WAN links are often inherently unreliable: simply moving streaming video from LAN to WAN is bound to fail. Lack of multipoint communication, so that a simple search broadcast cannot be deployed. Solution is to develop separate naming and directory services (having their own scalability problems). Scalability dimensions 20 / 56
- 27. Introduction: Design goals Being scalable Problems with administrative scalability Essence Conflicting policies concerning usage (and thus payment), management, and security Examples Computational grids: share expensive resources between different domains. Shared equipment: how to control, manage, and use a shared radio telescope constructed as large-scale shared sensor network? Exception: several peer-to-peer networks File-sharing systems (based, e.g., on BitTorrent) Peer-to-peer telephony (Skype) Peer-assisted audio streaming (Spotify) Note: end users collaborate and not administrative entities. Scalability dimensions 21 / 56
- 28. Introduction: Design goals Being scalable Techniques for scaling Hide communication latencies Make use of asynchronous communication Have separate handler for incoming response Problem: not every application fits this model Scaling techniques 22 / 56
- 29. Introduction: Design goals Being scalable Techniques for scaling Facilitate solution by moving computations to client M A A R T E N FIRST NAME LAST NAME E-MAIL Server Client Check form Process form MAARTEN MVS VAN-STEEN.NET @ VAN STEEN FIRST NAME LAST NAME E-MAIL Server Client Check form Process form MAARTEN MVS@VAN-STEEN.NET VAN STEEN MAARTEN VAN STEEN MVS@VAN-STEEN.NET Scaling techniques 23 / 56
- 30. Introduction: Design goals Being scalable Techniques for scaling Partition data and computations across multiple machines Move computations to clients (Java applets) Decentralized naming services (DNS) Decentralized information systems (WWW) Scaling techniques 24 / 56
- 31. Introduction: Design goals Being scalable Techniques for scaling Replication and caching: Make copies of data available at different machines Replicated file servers and databases Mirrored Web sites Web caches (in browsers and proxies) File caching (at server and client) Scaling techniques 25 / 56
- 32. Introduction: Design goals Being scalable Scaling: The problem with replication Applying replication is easy, except for one thing Scaling techniques 26 / 56
- 33. Introduction: Design goals Being scalable Scaling: The problem with replication Applying replication is easy, except for one thing Having multiple copies (cached or replicated), leads to inconsistencies: modifying one copy makes that copy different from the rest. Scaling techniques 26 / 56
- 34. Introduction: Design goals Being scalable Scaling: The problem with replication Applying replication is easy, except for one thing Having multiple copies (cached or replicated), leads to inconsistencies: modifying one copy makes that copy different from the rest. Always keeping copies consistent and in a general way requires global synchronization on each modification. Scaling techniques 26 / 56
- 35. Introduction: Design goals Being scalable Scaling: The problem with replication Applying replication is easy, except for one thing Having multiple copies (cached or replicated), leads to inconsistencies: modifying one copy makes that copy different from the rest. Always keeping copies consistent and in a general way requires global synchronization on each modification. Global synchronization precludes large-scale solutions. Scaling techniques 26 / 56
- 36. Introduction: Design goals Being scalable Scaling: The problem with replication Applying replication is easy, except for one thing Having multiple copies (cached or replicated), leads to inconsistencies: modifying one copy makes that copy different from the rest. Always keeping copies consistent and in a general way requires global synchronization on each modification. Global synchronization precludes large-scale solutions. Observation If we can tolerate inconsistencies, we may reduce the need for global synchronization, but tolerating inconsistencies is application dependent. Scaling techniques 26 / 56
- 37. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. 27 / 56
- 38. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. False (and often hidden) assumptions 27 / 56
- 39. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. False (and often hidden) assumptions The network is reliable 27 / 56
- 40. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. False (and often hidden) assumptions The network is reliable The network is secure 27 / 56
- 41. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. False (and often hidden) assumptions The network is reliable The network is secure The network is homogeneous 27 / 56
- 42. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. False (and often hidden) assumptions The network is reliable The network is secure The network is homogeneous The topology does not change 27 / 56
- 43. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. False (and often hidden) assumptions The network is reliable The network is secure The network is homogeneous The topology does not change Latency is zero 27 / 56
- 44. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. False (and often hidden) assumptions The network is reliable The network is secure The network is homogeneous The topology does not change Latency is zero Bandwidth is infinite 27 / 56
- 45. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. False (and often hidden) assumptions The network is reliable The network is secure The network is homogeneous The topology does not change Latency is zero Bandwidth is infinite Transport cost is zero 27 / 56
- 46. Introduction: Design goals Pitfalls Developing distributed systems: Pitfalls Observation Many distributed systems are needlessly complex caused by mistakes that required patching later on. Many false assumptions are often made. False (and often hidden) assumptions The network is reliable The network is secure The network is homogeneous The topology does not change Latency is zero Bandwidth is infinite Transport cost is zero There is one administrator 27 / 56
- 47. Introduction: Types of distributed systems Three types of distributed systems High performance distributed computing systems Distributed information systems Distributed systems for pervasive computing 28 / 56
- 48. Introduction: Types of distributed systems High performance distributed computing Parallel computing Observation High-performance distributed computing started with parallel computing Multiprocessor and multicore versus multicomputer Shared memory Processor P P P P M M M Interconnect Private memory Memory P P P P M M M M Interconnect 29 / 56
- 49. Introduction: Types of distributed systems High performance distributed computing Distributed shared memory systems Observation Multiprocessors are relatively easy to program in comparison to multicomputers, yet have problems when increasing the number of processors (or cores). Solution: Try to implement a shared-memory model on top of a multicomputer. Example through virtual-memory techniques Map all main-memory pages (from different processors) into one single virtual address space. If process at processor A addresses a page P located at processor B, the OS at A traps and fetches P from B, just as it would if P had been located on local disk. Problem Performance of distributed shared memory could never compete with that of multiprocessors, and failed to meet the expectations of programmers. It has been widely abandoned by now. 30 / 56
- 50. Introduction: Types of distributed systems High performance distributed computing Cluster computing Essentially a group of high-end systems connected through a LAN Homogeneous: same OS, near-identical hardware Single managing node Local OS Local OS Local OS Local OS Standard network Component of parallel application Component of parallel application Component of parallel application Parallel libs Management application High-speed network Remote access network Master node Compute node Compute node Compute node Cluster computing 31 / 56
- 51. Introduction: Types of distributed systems High performance distributed computing Grid computing The next step: lots of nodes from everywhere Heterogeneous Dispersed across several organizations Can easily span a wide-area network Note To allow for collaborations, grids generally use virtual organizations. In essence, this is a grouping of users (or better: their IDs) that will allow for authorization on resource allocation. Grid computing 32 / 56
- 52. Introduction: Types of distributed systems High performance distributed computing Architecture for grid computing Applications Collective layer Resource layer Fabric layer Connectivity layer The layers Fabric: Provides interfaces to local resources (for querying state and capabilities, locking, etc.) Connectivity: Communication/transaction protocols, e.g., for moving data between resources. Also various authentication protocols. Resource: Manages a single resource, such as creating processes or reading data. Collective: Handles access to multiple resources: discovery, scheduling, replication. Application: Contains actual grid applications in a single organization. Grid computing 33 / 56
- 53. Introduction: Types of distributed systems High performance distributed computing Cloud computing Application Infrastructure Computation (VM) torage (block ) , s , file Hardware Platforms Software framework (Java/Python/.Net) Storage ( ) databases Infrastructure aa Svc Platform aa Svc Software aa Svc MS Azure Google App engine Amazon S3 Amazon EC2 Datacenters CPU, memory, disk, bandwidth Web services, multimedia, business apps Google docs Gmail YouTube, Flickr Cloud computing 34 / 56
- 54. Introduction: Types of distributed systems High performance distributed computing Cloud computing Make a distinction between four layers Hardware: Processors, routers, power and cooling systems. Customers normally never get to see these. Infrastructure: Deploys virtualization techniques. Evolves around allocating and managing virtual storage devices and virtual servers. Platform: Provides higher-level abstractions for storage and such. Example: Amazon S3 storage system offers an API for (locally created) files to be organized and stored in so-called buckets. Application: Actual applications, such as office suites (text processors, spreadsheet applications, presentation applications). Comparable to the suite of apps shipped with OSes. Cloud computing 35 / 56
- 55. Introduction: Types of distributed systems High performance distributed computing Is cloud computing cost-effective? Observation An important reason for the success of cloud computing is that it allows organizations to outsource their IT infrastructure: hardware and software. Essential question: is outsourcing also cheaper? Approach Consider enterprise applications, modeled as a collection of components, each component Ci requiring Ni servers. Application now becomes a directed graph, with a vertex representing a component, and an arc h − → i,ji representing data flowing from Ci to Cj . Two associated weights per arc: Ti,j is the number of transactions per time unit that causes a data flow from Ci to Cj . Si,j is the total amount of data associated with Ti,j . Cloud computing 36 / 56
- 56. Introduction: Types of distributed systems High performance distributed computing Is cloud computing cost-effective? Migration plan Figure out for each component Ci , how many ni of its Ni servers should migrate, such that the monetary benefits reduced by additional costs for Internet communication, are maximal. Requirements migration plan 1 Policy constraints are met. 2 Additional latencies do not violate specific delay constraints. 3 All transactions continue to operate correctly; requests or data are not lost during a transaction. Cloud computing 37 / 56
- 57. Introduction: Types of distributed systems High performance distributed computing Computing benefits Monetary savings Bc: benefits of migrating a compute-intensive component Mc: total number of migrated compute-intensive components Bs: benefits of migrating a storage-intensive component Ms: total number of migrated storage-intensive components Obviously, total benefits are: Bc ·Mc +Bs ·Ms Cloud computing 38 / 56
- 58. Introduction: Types of distributed systems High performance distributed computing Internet costs Traffic to/from the cloud Trlocal,inet = ∑ Ci (Tuser,i Suser,i +Ti,user Si,user ) Tuser,i : transaction per time unit causing data flow from user to Ci Suser,i : amount of data associated with Tuser,i Cloud computing 39 / 56
- 59. Introduction: Types of distributed systems High performance distributed computing Rate of transactions after migration Some notations Ci,local : set of servers of Ci that continue locally. Ci,cloud : set of servers of Ci that are placed in the cloud. Assume traffic distribution is the same for local and cloud server Note that |Ci,cloud | = ni . Let fi = ni /Ni , and si a server of Ci . T∗ i,j = (1−fi )·(1−fj )·Ti,j when si ∈ Ci,local and sj ∈ Cj,local (1−fi )·fj ·Ti,j when si ∈ Ci,local and sj ∈ Cj,cloud fi ·(1−fj )·Ti,j when si ∈ Ci,cloud and sj ∈ Cj,local fi ·fj ·Ti,j when si ∈ Ci,cloud and sj ∈ Cj,cloud Cloud computing 40 / 56
- 60. Introduction: Types of distributed systems High performance distributed computing Overall Internet costs Notations costlocal,inet : per unit Internet costs to local part costcloud,inet : per unit Internet costs to cloud Costs and traffic before and after migration Tr∗ local,inet = ∑ Ci,local ,Cj,local (T∗ i,j S∗ i,j +T∗ j,i S∗ j,i )+ ∑ Cj,local (T∗ user,j S∗ user,j +T∗ j,user S∗ j,user ) Tr∗ cloud,inet = ∑ Ci,cloud ,Cj,cloud (T∗ i,j S∗ i,j +T∗ j,i S∗ j,i )+ ∑ Cj,cloud (T∗ user,j S∗ user,j +T∗ j,user S∗ j,user ) costs =costlocal,inet (Tr∗ local,inet −Trlocal,inet )+costcloud,inet Tr∗ cloud,inet Cloud computing 41 / 56
- 61. Introduction: Types of distributed systems Distributed information systems Integrating applications Situation Organizations confronted with many networked applications, but achieving interoperability was painful. Basic approach A networked application is one that runs on a server making its services available to remote clients. Simple integration: clients combine requests for (different) applications; send that off; collect responses, and present a coherent result to the user. Next step Allow direct application-to-application communication, leading to Enterprise Application Integration. 42 / 56
- 62. Introduction: Types of distributed systems Distributed information systems Example EAI: (nested) transactions Transaction Primitive Description BEGIN TRANSACTION Mark the start of a transaction END TRANSACTION Terminate the transaction and try to commit ABORT TRANSACTION Kill the transaction and restore the old values READ Read data from a file, a table, or otherwise WRITE Write data to a file, a table, or otherwise Issue: all-or-nothing Airline database Hotel database Subtransaction Subtransaction Nested transaction Two different (independent) databases Atomic: happens indivisibly (seemingly) Consistent: does not violate system invariants Isolated: not mutual interference Durable: commit means changes are permanent Distributed transaction processing 43 / 56
- 63. Introduction: Types of distributed systems Distributed information systems TPM: Transaction Processing Monitor TP monitor Server Server Server Client application Requests Reply Request Request Request Reply Reply Reply Transaction Observation In many cases, the data involved in a transaction is distributed across several servers. A TP Monitor is responsible for coordinating the execution of a transaction. Distributed transaction processing 44 / 56
- 64. Introduction: Types of distributed systems Distributed information systems Middleware and EAI Server-side application Server-side application Server-side application Client application Client application Communication middleware Middleware offers communication facilities for integration Remote Procedure Call (RPC): Requests are sent through local procedure call, packaged as message, processed, responded through message, and result returned as return from call. Message Oriented Middleware (MOM): Messages are sent to logical contact point (published), and forwarded to subscribed applications. Enterprise application integration 45 / 56
- 65. Introduction: Types of distributed systems Distributed information systems How to integrate applications File transfer: Technically simple, but not flexible: Figure out file format and layout Figure out file management Update propagation, and update notifications. Shared database: Much more flexible, but still requires common data scheme next to risk of bottleneck. Remote procedure call: Effective when execution of a series of actions is needed. Messaging: RPCs require caller and callee to be up and running at the same time. Messaging allows decoupling in time and space. Enterprise application integration 46 / 56
- 66. Introduction: Types of distributed systems Pervasive systems Distributed pervasive systems Observation Emerging next-generation of distributed systems in which nodes are small, mobile, and often embedded in a larger system, characterized by the fact that the system naturally blends into the user’s environment. Three (overlapping) subtypes 47 / 56
- 67. Introduction: Types of distributed systems Pervasive systems Distributed pervasive systems Observation Emerging next-generation of distributed systems in which nodes are small, mobile, and often embedded in a larger system, characterized by the fact that the system naturally blends into the user’s environment. Three (overlapping) subtypes Ubiquitous computing systems: pervasive and continuously present, i.e., there is a continuous interaction between system and user. 47 / 56
- 68. Introduction: Types of distributed systems Pervasive systems Distributed pervasive systems Observation Emerging next-generation of distributed systems in which nodes are small, mobile, and often embedded in a larger system, characterized by the fact that the system naturally blends into the user’s environment. Three (overlapping) subtypes Ubiquitous computing systems: pervasive and continuously present, i.e., there is a continuous interaction between system and user. Mobile computing systems: pervasive, but emphasis is on the fact that devices are inherently mobile. 47 / 56
- 69. Introduction: Types of distributed systems Pervasive systems Distributed pervasive systems Observation Emerging next-generation of distributed systems in which nodes are small, mobile, and often embedded in a larger system, characterized by the fact that the system naturally blends into the user’s environment. Three (overlapping) subtypes Ubiquitous computing systems: pervasive and continuously present, i.e., there is a continuous interaction between system and user. Mobile computing systems: pervasive, but emphasis is on the fact that devices are inherently mobile. Sensor (and actuator) networks: pervasive, with emphasis on the actual (collaborative) sensing and actuation of the environment. 47 / 56
- 70. Introduction: Types of distributed systems Pervasive systems Ubiquitous systems Core elements 1 (Distribution) Devices are networked, distributed, and accessible in a transparent manner 2 (Interaction) Interaction between users and devices is highly unobtrusive 3 (Context awareness) The system is aware of a user’s context in order to optimize interaction 4 (Autonomy) Devices operate autonomously without human intervention, and are thus highly self-managed 5 (Intelligence) The system as a whole can handle a wide range of dynamic actions and interactions Ubiquitous computing systems 48 / 56
- 71. Introduction: Types of distributed systems Pervasive systems Mobile computing Distinctive features A myriad of different mobile devices (smartphones, tablets, GPS devices, remote controls, active badges. Mobile implies that a device’s location is expected to change over time ⇒ change of local services, reachability, etc. Keyword: discovery. Communication may become more difficult: no stable route, but also perhaps no guaranteed connectivity ⇒ disruption-tolerant networking. Mobile computing systems 49 / 56
- 72. Introduction: Types of distributed systems Pervasive systems Mobility patterns Issue What is the relationship between information dissemination and human mobility? Basic idea: an encounter allows for the exchange of information (pocket-switched networks). A successful strategy Alice’s world consists of friends and strangers. If Alice wants to get a message to Bob: hand it out to all her friends Friend passes message to Bob at first encounter Observation This strategy works because (apparently) there are relatively closed communities of friends. Mobile computing systems 50 / 56
- 73. Introduction: Types of distributed systems Pervasive systems Community detection Issue How to detect your community without having global knowledge? Gradually build your list 1 Node i maintains familiar set Fi and community set Ci , initially both empty. 2 Node i adds j to Ci when |Fj ∩Ci | |Fj | λ 3 Merge two communities when |Ci ∩Cj | γ|Ci ∪Cj | Experiments show that λ = γ = 0.6 is good. Mobile computing systems 51 / 56
- 74. Introduction: Types of distributed systems Pervasive systems How mobile are people? Experimental results Tracing 100,000 cell-phone users during six months leads to: 5 10 50 100 500 1000 1 10 -4 10 -6 10 -2 Displacement Probability Moreover: people tend to return to the same place after 24, 48, or 72 hours ⇒ we’re not that mobile. Mobile computing systems 52 / 56
- 75. Introduction: Types of distributed systems Pervasive systems Sensor networks Characteristics The nodes to which sensors are attached are: Many (10s-1000s) Simple (small memory/compute/communication capacity) Often battery-powered (or even battery-less) Sensor networks 53 / 56
- 76. Introduction: Types of distributed systems Pervasive systems Sensor networks as distributed databases Two extremes Operator's site Sensor network Sensor data is sent directly to operator Operator's site Sensor network Query Sensors send only answers Each sensor can process and store data Sensor networks 54 / 56
- 77. Introduction: Types of distributed systems Pervasive systems Duty-cycled networks Issue Many sensor networks need to operate on a strict energy budget: introduce duty cycles Definition A node is active during Tactive time units, and then suspended for Tsuspended units, to become active again. Duty cycle τ: τ = Tactive Tactive +Tsuspended Typical duty cycles are 10−30%, but can also be lower than 1%. Sensor networks 55 / 56
- 78. Introduction: Types of distributed systems Pervasive systems Keeping duty-cycled networks in sync Issue If duty cycles are low, sensor nodes may not wake up at the same time anymore and become permanently disconnected: they are active during different, nonoverlapping time slots. Solution Each node A adopts a cluster ID CA, being a number. Let a node send a join message during its suspended period. When A receives a join message from B and CA CB, it sends a join message to its neighbors (in cluster CA) before joining B. When CA CB it sends a join message to B during B’s active period. Note Once a join message reaches a whole cluster, merging two clusters is very fast. Merging means: re-adjust clocks. Sensor networks 56 / 56