Ambient-Oriented Programming

Transcription

1 Ambient-Oriented Programming Designing an Object-Oriented Language for Ambient Intelligence Dr. Éric Tanter University of Chile DCC/CWR Presented at the JCC (c) E. Tanter, All Rights Reserved 1

2 Contents What is Ambient Intelligence (AmI)? OO Language Design for AmI AmbientTalk - A First OO Language for AmI Future Directions 2

3 Credits Programming Technology Lab - Vrije Universiteit Brussel Prof. Dr. Theo D Hondt Wolfgang de Meuter Move Considered Harmful -- A Language Design Approach to Distribution and Mobility in Open Networks PhD Thesis, 2004 Jessie Dedecker, Tom Van Cutsem, Stijn Mostinckx PhD students (VUB), AmbientTalk development Sebastián González PhD student (UCL) ORION: Open Reflective Infrastructure for Open Networks Project DCC/CWR - VUB

4 Ambient Intelligence A Peculiar Context for Application Development [Dedecker+05b] 4

5 Hardware Environment Advent of mobile networks wireless technologies (wifi, bluetooth) nomadic and ad hoc (aka. open networks) Mobile devices limited connection range and computational resources Ambient Intelligence European Council s IST Advisory Group [ISTAG03] aka. ubiquitous computing [Weiser91], pervasive computing what is fundamentally different? 5

6 Autonomicity & Concurrency no central server natural concurrency Communication Autonomous Concurrent Devices Interconnected via ad hoc wireless networks 6

7 Connection Volatility cannot assume stable connection but tasks need to be done Failure Volatile Wireless Connections exceptions are no more exceptional! 7

8 Resource Discovery resources depend on location Discovery Ambient Resources I provide a printing service I require a printing service Devices (dis)appear in the 'Ambient' Spontaneous Service Discovery 8

9 Programming for AmI Communication Failure Discovery A robust AmI application must address these characteristics Current alternatives Middleware Distributed programming languages 9

10 Middleware RPC-based middleware Alice [Haarh+99], Dolmen [Reynolds+99] CORBA on lightweight devices no support for long disconnections Publish-subscribe middleware [Cugola+02] good for context-aware communication forces callback programming Tuple-space based middleware [Murphy+01] uncoupling in both time and space not object-oriented 10

11 Distributed Languages Languages for LAN Emerald [Jul+88], Obliq [Cardelli95], etc. communication mechanisms assume reliable connection Languages for open networks Janus [Kahn+90], Salsa [Varela+98], E [Miller] pure asynchronous communication no support for resource discovery 11

12 Position Statement [Dedecker+05] Currently insufficient support for programming nomadic / ad hoc distributed systems low-level APIs, middleware, etc. can be done, but hard no dedicated abstractions for AmI specificities Need of dedicated languages: AmOP abstracting without hiding 12

13 Object-Oriented Languages for AmI 13

14 Design Dimensions object model class-based vs. prototype-based static typing vs. dynamic typing communication model concurrency model communication primitives failure handling acquaintance management 14

15 Why Classes Do Not Fit and neither static typing... [Gonzales+04] 15

16 OOP without classes?? object = encapsulated state + message passing objects are good encapsulation and polymorphism are fundamental for highly dynamic environments 16

17 OOP without classes??? Classification of OOP languages [Wegner87] object-based (objects + messages) class-based (classes) object-oriented (classes + inheritance) prototype-based (objects + delegation/sharing)... 17

18 Fundamental Problem [Gonzales+04] Classes a static, implicit sharing mechanism of state and behaviour [Treaty of Orlando] Open Networks Distribution transparency is impossible to achieve in practice, and precisely because of that impossibility, it is dangerous to provide the illusion of transparency. [Guerraoui&Fayad99] 18

19 State Sharing class attributes (eg. Java statics) semantics = value shared by all instances not true under distribution! Solution: add yet more keywords to Java... public static-replicated synchronized Vector v; protected static-centralized String s; or fix a choice and make it implicit in the language. It s no solution: a new node may appear with the same class, but with a different value... 19

20 Code Sharing Exactly same issue with methods A node containing the same class as another node may appear in the network, but with a different method implementation... Methods are part of a class s state, just the same as class attributes. If a method changes, the class is no longer the same, even if semantically it remains compatible. 20

21 Transitive Closures instance-of link + subclass-of link when an object is transmitted, its class and all superclasses must be transmitted too (an object is not a self-contained entity!) static typing transmit argument-type classes, returntype class and exception-type classes and their transitive closures 21

22 Conclusion on Classes Classes are dangerous under distribution Classes (and static typing) are a barrier to adaptability sharing is important, but should be managed explicitly prototype-based languages! 22

23 Prototypes eg. Self [Ungar+87] Self-contained objects state + code created ex-nihilo or by cloning self Extension mechanism true delegation to parent self Sharing mechanism parent sharing managed explicitly message forwarding delegation eg. traits [Ungar+91] 23

24 Communication Model [Dedecker+05b] 24

25 OO Concurrency Model Thread-based Actor model [Agha86] 25

26 Communication Primitives Blocking primitives are evil deadlocks & diminished availability of resources Communication Non-blocking Asynchronous Communication Primitives send transmit receive 26

27 Failure Recovery Inconsistent states cannot be avoided need for reversibility Failure Reified Communication Traces rollback strategies delivery guarantees yes Agenda Actor agenda slot available? Agenda Actor reservation confirmed? Agenda Actor reserve slot Agenda Actor 27

28 Acquaintance Management No third party for communication (P2P) no explicit reference: distributed naming Discovery Reified Environmental Context Provided service patterns actors specify services required from the ambient provided to the ambient Required service patterns description of service interface 28

29 Design Conclusions Classless language, dynamically-typed Actor model Non-blocking communication primitives Reified communication trace Reified ambient context C? C++? Java? C#? 29

30 AmbientTalk A First AmOP Language [Dedecker+05a] 30

31 AmbientTalk Dynamically typed language Prototype-based object model Actor-based concurrency model VM runs on Java J2SE and J2ME platforms VMs communicate via sockets over WLAN Plug-and-play: no additional setup Tested and deployed on QTek9090 PocketPC smartphones Actors AmbientTalk VM jat 31

32 Communication Communication Mailboxes reify actor-actor interaction «outbox» «sentbox» «inbox» «rcvbox» 32

33 Discovery Discovery Mailboxes reify actor-ambient interaction «required» «provided» «joinedbox» «disjoinedbox» 33

34 Metaobjects First-class Mailboxes interaction between mailboxes goverened by a metaobject protocol ontransmit onsend onjoin ondisjoin onprocess onreceipt Meta-level events trapped using mailbox observers 34

35 Actors AmbientTalk VM jat PrintFileBehavior :: object({ printer : void; file : void; new(afile,dpi) :: copy({ printer := AmbientRef("Printer"); file := afile }); printit() :: { when(printer#print(file), { display("doc Printed! ) }) } }) PrintFileActor(file,dpi) :: { actor(printfilebehavior.new(file,dpi)) } { p: PrintFileActor(aFile,300); p#printit() } 35

36 Higher-level Constructs [Dedecker+05a] AmI is a moving target AmbientTalk is a testbed for AmOP experimentation Some higher-level constructs Non-blocking futures Application-specific message routing Group communication... 36

37 Language-level Security [DeMeuter+05] Security must be addressed at all levels comm. channels, OS, language, application, user Extreme encapsulation principle objects subject to message passing alone no omnipotent operators But powerful object model cloning, extension, parent sharing, reflection eg. mirror methods [Tanter05] 37

38 Mobility [Vallejos05] Controversial feature Security-wise challenging A killer application? follow-me applications? 38

39 Open Infrastructure [ORION] Reflective Virtual Machine for AmI Fine-tune tradeoffs/strategies at the VM level garbage collection failure recovery strategy partial reversible computation partial object matching for resource discovery 39

40 Context Awareness Context-aware object references [VanCutsem+05] Context-oriented programming [Costanza+05] Context-aware aspects [Tanter+05] 40

41 References (1) [Agha86] G. Agha, Actors: a model of concurrent computation in distributed systems, MIT Press [Cardelli95] L. Cardelli, Obliq: a language with distributed scope, Proceedings of POPL [Costanza+05] P. Costanza, R. Hirschfeld, Language Constructs for Context-Oriented Programming, Proceedings of DLS [Cugola+02] G. Cugola, H.-A. Jacobsen, Using Publish-Subscribe Middleware for Mobile Systems, SIGMOBILE Mob. Comp. Commun. 6(4), [Dedecker+05a] J. Dedecker, S. Mostinckx, T. Van Cutsem, W. De Meuter, T. D Hondt, AmbientTalk: A Small Reflective Kernel for Programming Mobile Network Applications, VUB TR [Dedecker+05b] J. Dedecker, T. Van Cutsem, S. Mostinckx, W. De Meuter, T. D Hondt, Ambient-Oriented Programming, OOPSLA Onward! [DeMeuter+05] W. De Meuter, É. Tanter, S. Mostinckx, T. Van Cutsem, J. Dedecker, Flexible Object Encapsulation for Ambient-Oriented Programming, Proceedings of DLS [Guerraoui&Fayad99] R. Guerraoui, M. E. Fayad, OO Distributed Programming is Not Distributed OO Programming, CACM 42(4), [Gonzales+04] S. Gonzalez, W. De Meuter, K. Mens, T. D Hondt, The Unavoidable Failure of Class-Based Languages n the Processor Cloud Era, ECOOP Workshop on Object-Oriented Language Engineering for the Post-Java Era, [Haarh+99] M. Haarh, R. Cunningham, V. Cahill, Supporting CORBA Applications in a Mobile Environment, Proceedings MobiCom [ISTAG03] ISTAG, Ambient Intelligence: From Vision to Reality, [Jul+88] E. Jul, H. Levy, N. Hutchinson, A. Black, Fine-grained Mobility in the Emerald System, ACM TCS 6 (1),

42 References (2) [Kahn+90] K.M. Kahn, V.A. Saraswat, Actors as a special case of concurrent constraint programming, Proceedings ECOOP/OOPSLA [Miller] M. Miller, E: Open Source Distributed Capabilities, [Murphy+01] A.L. Murphy, G.P. Picco, G.-C. Roman, Lime: A Middleware for Physical and Logical Mobility, Proceedings DCS [ORION] Open Reflective Infrastructure for Open Networks, draft proposal for a bilateral project between the VUB Prog Lab and the DCC U. Chile, [Reynolds+99] P. Reynolds, R. Brangeon, DOLMEN - Service Machine Development for Open Long-Term Mobile and Fixed Network Environment [Tanter05] É. Tanter, Mirror Methods - Reconciling Reflection and Extreme Encapsulation, Proc. OT4AmI [Tanter+05] É.Tanter, K. Gybels, M. Denker, A. Bergel, Context-Aware Aspects, DCC U.Chile TR [Treaty of Orlando] H. Lieberman, L. Stein, D. Ungar, Treaty of Orlando, OOPSLA Add [Ungar+87] D. Ungar, R. Smith, Self: the Power of Simplicity, Proc. OOPSLA [Ungar+91] D. Ungar, C. Chambers, B. Chang, U. Hölzle, Organizing Programs without Classes, Lisp Symb. Comp. 4(3), [Vallejos05] J. Vallejos, Mobile Actors Supporting Reconfigurable Applications in Open P2P Networks, MSc Thesis, VUB, Belgium, [Varela+98] C.A. Varela, G. Agha, What after Java? From Objects to Actors, Proceedings ICWWW [VanCutsem+05] T. Van Cutsem, J. Dedecker, S. Mostinckx, W. De Meuter, Abstractions for Context-aware Object References, OOPSLA Workshop on Pervasive Computing, [Wegner87] P. Wegner, Dimensions of Object-based Language Design, Proceedings OOPSLA [Weiser91] M. Weiser, The Computer for the 21st Century, Scientific American 265(3)