The article collection

1. The article collection PRIS F7 Fredrik Kilander

2. Content • “On agent-based software engineering” Nick Jennings, 1999 • “An agent-based approach for building complex software systems” Nick Jennings 2001 • “A note on distributed computing” Waldo, Wyant, Wollrath, Kendall, 1994 • “The seven deadly sins of distributed systems” Steve Muir, 2004

3. On agent-based software engineering • introduction • essential concepts of agent-based computing • the appeals of the agent-based approach • why agent-based techniques will succeed • the drawbacks • modelling on the social level • the wider implications for AI and CS

4. In traditional systems development... • interactions between components are too rigidly defined • organisational structure is not captured well

5. Agents, on the other hand... • are well defined (they have boundaries and interfaces) • are embedded in an environment • are designed to fulfill a purpose • are autonomous (need no external activation) • are reactive and proactive • are already organised in hierarchies

6. Agents are also... • flexible in their interactions • adaptable to changes in the environment • able to work in teams for a common goal

7. Jennings argues that... (1/3) • ”agent-oriented decomposition is an effective way of partitioning the problem space” Decomposition → distribution and decentralization Decentralization → less control complexity

8. Jennings argues that... (2/3) • ”the key abstractions of the agent-oriented mindset are a natural means of modeling complex systems” Subsystems → agent hierarchies Subsystem components → social interactions

9. Jennings argues that... (3/3) • ”the agent-oriented philosophy for dealing with organisational relationships is appropriate for complex systems” Agent-based systems flexibly form and reshape structures Individual agents or groups can be developed separately and incrementally added to the system

10. However... • interaction patterns are unpredictable • emergent system behaviour is unpredictable

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12. A note on distributed computingWaldo et al • Distributed computing is different from local computing • The programmer should be aware of this • The language should not hide it

13. A (false) vision of unified computing • Object-oriented design conquers all • Remote objects appear to be local • Failures depend on the implementation, not on design

14. Hard problems in distributed computing • Latency • Memory access (no pointers => copy) • Partial failure • Concurrency

15. Interfaces and design • Design remote interfaces to expect breakdowns • Accept that the programmer must be informed about breakdowns • E.g. java.rmi.RemoteException

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17. The seven deadly sinsof distributed systems • Experiences of real systems (PlanetLab Node Manager) • 400+ Internet nodes at 175 locations.

18. 1 Networks are unreliable • Large heterogenous networks are fundamentally unreliable • Packet loss • Packet duplication • Packet reordering • Highly variable latency

19. 2 DNS names make poor node identifiers • Human errors • Network reorganisation • Infrastructure failures • Network asymmetry (e.g. NAT) • Non-static addresses, multihoming

20. 3 Local clocks are unreliable • System timing is poor • High load • Bugs • NTP not always available NTP: Network Time Protocol

21. 4 Inconsistent node configuration is the norm • Version skew • Delays • Inaccessible nodes • Update times • No global synchronization

22. 5 Improbable events are frequent • Lots of nodes • 24/7 • Rare bugs WILL manifest themselves • Application programmers must not cut corners! (“Oh, this error will never happen…”)

23. 6 Over-utilization is the norm • All systems are used • No idle time • Resource expiration times must be extended

24. 7 Limited system transparency hampers debugging • Incomplete access to remote nodes • Virtualization hides details

25. Conclusions for distributed systems • Local assumptions fail • Applications must cover all failure modes • Resource management is harder • Local operations are affected too

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