1. Copyright © Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy. All rights reserved. Applied Architectures Software Architecture Lecture 9

2. Software Architecture: Foundations, Theory, and Practice Objectives Illustrate how principles have been used to solve challenging problems u Usually means combining elements Highlight some critical issues u I.e., ignore them at your peril Show how architecture can be used to explain and analyze common commercial systems 2

3. Software Architecture: Foundations, Theory, and Practice Outline Distributed and networked architectures u Limitations of the distributed systems “viewpoint” u REST u Commercial Internet-scale applications Decentralized applications u Peer-to-peer u Web services Some interesting domains u Robotics u Wireless sensors u Flight simulators 3

4. Software Architecture: Foundations, Theory, and Practice Limitations of the Distributed Systems Viewpoint The network is reliable Latency is zero Bandwidth is infinite The network is secure Topology doesn’t change There is one administrator Transport cost is zero The network is homogeneous -- Deutsch & Gosling 4

5. Software Architecture: Foundations, Theory, and Practice Architecture in Action: WWW (From lecture #1) This is the Web 5 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

6. Software Architecture: Foundations, Theory, and Practice Architecture in Action: WWW (cont’d) So is this 6 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

7. Software Architecture: Foundations, Theory, and Practice Architecture in Action: WWW And this 7 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

8. Software Architecture: Foundations, Theory, and Practice WWW’s Architecture The application is distributed (actually, decentralized) hypermedia Architecture of the Web is wholly separate from the code There is no single piece of code that implements the architecture. There are multiple pieces of code that implement the various components of the architecture. u E.g., different Web browsers Stylistic constraints of the Web’s architectural style are not apparent in the code u The effects of the constraints are evident in the Web One of the world’s most successful applications is only understood adequately from an architectural vantage point. 8

9. Software Architecture: Foundations, Theory, and Practice REST Principles [RP1] The key abstraction of information is a resource, named by an URL. Any information that can be named can be a resource. [RP2] The representation of a resource is a sequence of bytes, plus representation metadata to describe those bytes. The particular form of the representation can be negotiated between REST components. [RP3] All interactions are context-free: each interaction contains all of the information necessary to understand the request, independent of any requests that may have preceded it. 9

10. Software Architecture: Foundations, Theory, and Practice REST Principles (cont’d) [RP4] Components perform only a small set of well- defined methods on a resource producing a representation to capture the current or intended state of that resource and transfer that representation between components. These methods are global to the specific architectural instantiation of REST; for instance, all resources exposed via HTTP are expected to support each operation identically. 10

11. Software Architecture: Foundations, Theory, and Practice REST Principles (cont’d) [RP5] Idempotent operations and representation metadata are encouraged in support of caching and representation reuse. [RP6] The presence of intermediaries is promoted. Filtering or redirection intermediaries may also use both the metadata and the representations within requests or responses to augment, restrict, or modify requests and responses in a manner that is transparent to both the user agent and the origin server. 11

12. Software Architecture: Foundations, Theory, and Practice REST — Data Element Decisions Resource u Key information abstraction Resource ID Representation u Data plus metadata Representation metadata Resource metadata Control data u e.g., specifies action as result of message 12

13. Software Architecture: Foundations, Theory, and Practice An Instance of REST 13

14. Software Architecture: Foundations, Theory, and Practice Derivation of REST Key choices in this derivation include: Layered Separation (a theme in the middle portion of diagram) is used to increase efficiencies, enable independent evolution of elements of the system, and provide robustness; Replication (left side of the diagram) is used to address latency and contention by allowing the reuse of information; Limited commonality (right side) addresses the competing needs for universally understood operations with extensibility. The derivation is driven by the application(!) 14

15. Software Architecture: Foundations, Theory, and Practice Derivation of REST (cont’d) 15 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

16. Software Architecture: Foundations, Theory, and Practice REST: Final Thoughts REpresentational State Transfer Style of modern web architecture u Web architecture one of many in style Web diverges from style on occasion u e.g., Cookies, frames u Doesn’t explain mashups 16

17. Software Architecture: Foundations, Theory, and Practice Commercial Internet-Scale Applications Akamai u Caching to the max Google u Google distributed file system (GFS) u MapReduce Data selection and reduction All parallelization done automatically 17

18. Software Architecture: Foundations, Theory, and Practice Architectural Lessons from Google Abstraction layers abound: GFS hides details of data distribution and failure, for instance; MapReduce hides the intricacies of parallelizing operations; By designing, from the outset, for living with failure of processing, storage, and network elements, a highly robust system can be created; Scale is everything: Google’s business demands that everything be built with scaling issues in mind; 18

19. Software Architecture: Foundations, Theory, and Practice Architectural Lessons from Google (cont’d) By specializing the design to the problem domain, rather than taking the generic “industry standard” approach, high performance and very cost-effective solutions can be developed; By developing a general approach (MapReduce) to the data extraction/reduction problem, a highly reusable service was created. 19

20. Software Architecture: Foundations, Theory, and Practice Decentralized Architectures Networked applications where there are multiple authorities In other words u Computation is distributed u Parts of the network may behave differently, and vary over time It’s just like collaboration in the real world 20

21. Software Architecture: Foundations, Theory, and Practice Grid Protocol Architecture 21 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission. Coordinated resource sharing in a distributed environment u E.g., Folding-at-home GLOBUS u A commonly used infrastructure “Standard architecture” u Fabric manages low-level resources u Connectivity: communication and authentication u Resource: sharing of a single r. u Collective: coordinating r. usage

22. Software Architecture: Foundations, Theory, and Practice Grid GLOBUS (Recovered) 22 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

23. Software Architecture: Foundations, Theory, and Practice Peer-to-Peer Architectures Decentralized resource sharing and discovery u Napster u Gnutella P2P that works: Skype u And BitTorrent 23

24. Software Architecture: Foundations, Theory, and Practice Peer-to-Peer Style State and behavior are distributed among peers which can act as either clients or servers. Peers: independent components, having their own state and control thread. Connectors: Network protocols, often custom. Data Elements: Network messages Topology: Network (may have redundant connections between peers); can vary arbitrarily and dynamically Supports decentralized computing with flow of control and resources distributed among peers. Highly robust in the face of failure of any given node. Scalable in terms of access to resources and computing power. But caution on the protocol! 24

25. Software Architecture: Foundations, Theory, and Practice Peer-to-Peer LL 25 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

26. Software Architecture: Foundations, Theory, and Practice Napster (“I am the Napster!”) 26 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission. --Lyle, “The Italian Job” (2003)

27. Software Architecture: Foundations, Theory, and Practice Gnutella (original) 27 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2008 John Wiley & Sons, Inc. Reprinted with permission.

28. Software Architecture: Foundations, Theory, and Practice Skype 28 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

29. Software Architecture: Foundations, Theory, and Practice Insights from Skype A mixed client-server and peer-to-peer architecture addresses the discovery problem. Replication and distribution of the directories, in the form of supernodes, addresses the scalability problem and robustness problem encountered in Napster. Promotion of ordinary peers to supernodes based upon network and processing capabilities addresses another aspect of system performance: “not just any peer” is relied upon for important services. A proprietary protocol employing encryption provides privacy for calls that are relayed through supernode intermediaries. Restriction of participants to clients issued by Skype, and making those clients highly resistant to inspection or modification, prevents malicious clients from entering the network. 29

30. Software Architecture: Foundations, Theory, and Practice Web Services 30 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

31. Software Architecture: Foundations, Theory, and Practice Web Services (cont’d) 31 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

32. Software Architecture: Foundations, Theory, and Practice Mobile Robotics Manned or partially manned vehicles Uses u Space exploration u Hazardous waste disposal u Underwater exploration Issues u Interface with external sensors & actuators u Real-time response to stimuli u Response to obstacles u Sensor input fidelity u Power failures u Mechanical limitations u Unpredictable events 32

33. Software Architecture: Foundations, Theory, and Practice Robotics: Sense-Plan-Act 33 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

34. Software Architecture: Foundations, Theory, and Practice Robotics Subsumption Architecture: Abandon Complete World Model 34 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission. Inhibit (delay input for time t) Suppress (override input for time t)

35. Software Architecture: Foundations, Theory, and Practice Robotics: Three-Layer 35 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

36. Software Architecture: Foundations, Theory, and Practice Wireless Sensor Networks 36 Software Architecture: Foundations, Theory, and Practice; Richard N. Taylor, Nenad Medvidovic, and Eric M. Dashofy; © 2010 John Wiley & Sons, Inc. Reprinted with permission.

37. Software Architecture: Foundations, Theory, and Practice Flight Simulators: Key Characteristics Real-time performance constraints u Extremely high fidelity demands Requires distributed computing Must execute at high fixed frame rates  Often called harmonic frequencies e.g. often 60Hz, 30Hz; some higher (100Hz) u Coordination across simulator components All portions of simulator run at integral multiple of base rate  e.g. if base rate = 60Hz, then 30Hz, 15Hz, 12Hz, etc. Every task must complete on time  Delays can cause simulator sickness 37

38. Software Architecture: Foundations, Theory, and Practice Flight Simulators: Key Characteristics (cont’d) Continuous evolution Very large & complex u Millions of SLOC u Exponential growth over lifetime of simulator Distributed development u Portions of work sub­contracted to specialists u Long communication paths increase integration complexity Expensive verification & validation u Direct by-product of above characteristics Mapping of Simulation SW to HW components unclear u Efficiency muddies abstraction u Needed because of historical HW limitations 38

39. Software Architecture: Foundations, Theory, and Practice Functional Model 39

40. Software Architecture: Foundations, Theory, and Practice How Is the Complexity Managed? New style u “Structural Modeling” u Based on Object–oriented design to model air vehicle’s  Sub-systems  Components Real-time scheduling to control execution order Goals of Style u Maintainability u Integrability u Scalability 40

41. Software Architecture: Foundations, Theory, and Practice How Is the Complexity Managed? (cont’d) Style principles u Pre-defined partitioning of functionality among SW elements u Restricted data- & control-flow Data-flow through export areas only  Decoupling objects u Small number of element types Results in replicated subsystems 41

42. Software Architecture: Foundations, Theory, and Practice How Is the Complexity Managed? (cont’d) Style principles (continued) u Objects fully encapsulate their own internal state u No side-effects of computations u Narrow set of system-wide coordination strategies Employs pre-defined mechanisms for data & control passing Enable component communication & synchronization 42

43. Software Architecture: Foundations, Theory, and Practice F-Sim In The Structural Modeling Style Five basic computational elements in two classes u Executive (or infrastructure) Periodic sequencer Event handler Synchronizer u Application Components Subsystems Each of the five has u Small API u Encapsulated state u Severely limited external data upon which it relies 43

44. Software Architecture: Foundations, Theory, and Practice Level–0 Architecture 44

45. Software Architecture: Foundations, Theory, and Practice Level–1 Architecture 45

46. Software Architecture: Foundations, Theory, and Practice Takeaways A great architecture is the ticket to runaway success A great architecture reflects deep understanding of the problem domain A great architecture probably combines aspects of several simpler architectures Develop a new architectural style with great care and caution. Most likely you don’t need a new style. 46