Seminarie Computernetwerken 2012-2013: Lecture I, 26-02-2013

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Information about Seminarie Computernetwerken 2012-2013: Lecture I, 26-02-2013

Published on March 1, 2014

Author: VincenzoDeFlorio



Seminarie Computernetwerken is a course given at Universiteit Antwerpen, Belgium
A series of seminars focusing on various themes changing from year to year.
This year's themes are: resilience, behaviour, evolvability; in systems, networks, and organizations
In what follows we describe:
themes of the course
view to the seminars
rules of the game

Seminarie Computernetwerken 2012-2013 Lecture I, 26-02-2013 Vincenzo De Florio PATS / University of Antwerp & iMinds

Seminarie Computernetwerken • A series of seminars focusing on a set of themes • This year: resilience, behaviour, evolvability; in systems, networks, and organizations • In what follows: 1. themes of the course 2. view to the seminars 3. rules of the game 26 February 2013 2001WETSCN-01 2

Themes • Resilience, evolvability, behaviour: interrelated properties • Behavior: the characteristics of the way systems respond to changes • Evolvability: the ability to rapidly adapt to novel environments • Resilience: identity robustness w.r.t. changes • An ancillary property: Dependability 26 February 2013 2001WETSCN-01 3

Rationale • This lecture is to introduce the themes of the course • Next ones: seminars describing systems and algorithms and their adaptive / resilient behaviors • Aim: learn how to assess those properties in existing systems/algorithms. 26 February 2013 2001WETSCN-01 4

Behaviour • “Any change of an entity with respect to its surroundings” (Rosenblueth et al., 1943) • Behavioral method: Entities are classified according to peculiar characteristics of their behaviors (behavioral classes) • Passive, active, purposeful, teleological, predictive behaviors • Individual and social dimension 26 February 2013 2001WETSCN-01 5

Behaviour  passive • Entity changes its state only by receiving energy from an external source. • A kicked ball does not produce the energy that sets it in motion—it simply receives that energy  passive behaviour. 26 February 2013 2001WETSCN-01 6

Behaviour  active • Active behavior occurs when an entity “is the source of the output energy involved in a given particular reaction”. 26 February 2013 2001WETSCN-01 7

Behaviour  purposeful active • Active change meant to attain a goal—for instance survival or economical profit • Output energy is exerted so as to move from a certain state into another one • Opposed to purposeless (that is, random) active behavior • Purposeful active behavior pertains e.g. to servo-mechanisms, cyber-physical systems, and legal persons. 26 February 2013 2001WETSCN-01 8

Behaviour  teleological • Purposeful behavior that is “controlled by the margin of error at which the [entity] stands at a given time with reference to a relatively specific goal” • Requires two capabilities: perceiving the relationship between one's actions and one's goal, and 2. adjusting dynamically one's behavior so as to maximize the chances to reach one's goal. 1. 26 February 2013 2001WETSCN-01 9

Behaviour  predictive (individual) • Simple individual extrapolative (i.e., predictive) teleological behavior • Ability to formulate one’s action in function of an extrapolated future state along a single or a few dimensions • Individual: action is chosen in isolation, i.e., without considering the choices of the entities co-existing in the same environment  E.g. speculation in compilers. 26 February 2013 2001WETSCN-01 10

Behaviour  predictive (social) • Simple social predictive behavior 3. Ability to operate “quorum sensing”: choices take into account the possible future states of the neighboring entities • E.g. Bacillus subtilis: When subjected to a stressful environment such bacteria adopt quorum sensing and choose between cooperative and selfish strategies • E.g. Pelotons. 26 February 2013 2001WETSCN-01 11

Behaviour  complex multivariate predictive • Multivariate prediction • Computing the future state also requires 4. the ability to perform 4.1 multiple extrapolations 4.2 along different dimensions, e.g. a temporal and a spatial axis, 4.3 on an individual or a social scale 26 February 2013 2001WETSCN-01 12

Behaviour  future-responsive collective strategies • Collective and proactive forms of organizational adaptation to the environment the ability to build “collectively constructed and controlled social environments” on top of the physical environments (Astley & Fombrun, 1983)  Social “overlay networks”, e.g. business ecosystems, cyber-physical societies, serviceoriented communities, knowledge ecosystems, mutual-assistance communities…  “The subject of human ecology”. 5. 26 February 2013 2001WETSCN-01 13

Evolvability • The ability of systems and populations to rapidly adapting to novel environments [J. Clune, J.-B. Mouret, H. Lipson, The evolutionary origins of modularity, Proc. R. Soc. B 2013 280] • Evolvability is associated to several other traits: diversity, modularity, self-similarity, selforganization… [D. C. Stark, “Heterarchy: Distributing Authorithy and Organizing Diversity”. In “The Biology of Business: Decoding the Natural Laws of Enterprise”, Jossey-Bass, 1999. p. 153–179.] 26 February 2013 2001WETSCN-01 14

Evolvability in networks • Measured also through modularity • “Networks are modular if they contain highly connected clusters of nodes that are sparsely connected to nodes in other clusters. • Intuitively, modular systems seem more adaptable: it is easier to rewire a modular network with functional subunits than an entangled, monolithic network” . [Clune et al., 2013] 26 February 2013 2001WETSCN-01 15

Resilience • Identity robustness throughout change and evolutions: A system’s ability to retain its intended functions and properties in spite of behaviors, endogenous conditions, and environmental changes • Two major methods  Entelechy (active behavior resilience)  Elasticity (passive b. r.) 26 February 2013 2001WETSCN-01 16

Resilience through entelechy • Aristotelian idea of entelechy : the ability of “being-at-work-staying-the-same”  Continue working  Without going astray. 26 February 2013 2001WETSCN-01 17

Being-at-work… • An entity (e.g. a physical person, an organization, or a cyber-physical system) is resilient when: 1.The entity is able to exert active behavior (purposeful or otherwise): it continuously adjusts its functions to compensate for changes (adaptivity) 26 February 2013 2001WETSCN-01 18

…staying-the-same 2. While 1., the entity is able to retain its “identity”: its peculiar and distinctive functional and non-functional features  in the face of the above mentioned conditions, actions, and changes,  and despite the entity’s active behavior (the adjustments carried out by the entity).  Features include timeliness, jitter, scalability, quality-of-service attributes… 26 February 2013 2001WETSCN-01 19

Resilience through elasticity • “The ability of a body that has been subjected to an external force to recover its size and shape, following deformation” (McGraw-Hill, 2003) • In this case the system does not exert any purposeful behavior; it just makes use of its internal characteristics and resources so as to mask the action of external forces → Redundancy-based. 26 February 2013 2001WETSCN-01 20

Software resilience • Depending on the enacted behaviors, software resilience may be obtained through software elasticity, state recovery, software adaptation strategies, and collective resilience strategies • Several of our seminars focus on software adaptivity and software resilience of systems, communication algorithms, and organizations 26 February 2013 2001WETSCN-01 21

Software elasticity • …corresponds to simple protection mechanisms, e.g., error correcting codes, redundant data mechanisms, fault masking strategies based on voting • Redundant provisions are accommodated at design time to compensate for certain classes of events—to some predefined extent  E.g. NVP; adaptively redundant data structures, adaptive voting, etc. (see next lectures) 26 February 2013 2001WETSCN-01 22

State recovery software resilience • Software mechanisms that reach resilience by recovering trustworthy system states when the system is affected by errors  Purposeful teleological behaviors • Two major forms:  backward recovery (turns system back to a previously saved “safe state”: checkpoint & rollback, recovery blocks…)  forward recovery (synthesizes a new valid state: e.g., recovery languages + formal methods) 26 February 2013 2001WETSCN-01 23

Software adaptation-based resilience • Based on several complex features perception and introspection to reveal conditions and situations threatening the intended behaviors & identity; 2. diagnosis, e.g. ability to compare current and past situations; unravel trends; identify causes; 3. planning reactive (resp. proactive) strategies to compensate for current (resp. future) erroneous behaviors / assumption failures / identity losses; 4. strategy enactment through parametric and structural adaptation. 1. 26 February 2013 2001WETSCN-01 24

Software adaptation-based resilience • …corresponds to complex teleological extrapolative behaviors • Calls for formal methods to guarantee persistence of identity  More information: V. De Florio, “On the Role of Perception and Apperception in Ubiquitous and Pervasive Environments”. PDF Available . 26 February 2013 2001WETSCN-01 25

Collective adaptation strategies • Strategies of social organizations (= “a set of roles tied together with channels of communication”) (Boulding, 1956)  Business ecosystems, knowledge ecosystems, cyber-physical societies, service-oriented communities, mutual-assistance communities…  Bio-inspired organizations • More information:  V. De Florio et al. Service-oriented Communities: Models and Concepts towards Fractal Social Organizations. PDF Available 26 February 2013 2001WETSCN-01 26

A major problem • The more complex the adaptation strategy, the more difficult it is to guarantee / prove that the system “stays-the-same” • But simple strategies often are not enough . 26 February 2013 2001WETSCN-01 27

Dependability • “The property of a system such that reliance can justifiably be placed on the service it delivers” • System identity with a focus on certain attributes 26 February 2013 2001WETSCN-01 28

Attributes of dependability 26 February 2013 2001WETSCN-01 29

Attributes of dependability • Availability  Readiness for usage  A(t) = probability that system is conform to specifications at time t • Reliability  Continuity of service  R(t) = probability that system is conform to specifications during [t0,t], provided that so it is at t0 26 February 2013 2001WETSCN-01 30

Attributes of dependability (2) • Safety  Non-occurrence of catastrophic consequences on environment  S(t) = probability that a system is either conform to specifications, or reaches a safe halt, at time t  Fail-safe systems  The focus of next seminar 26 February 2013 2001WETSCN-01 31

Attributes of dependability (3) • Maintainability  Aptitude to undergo repairs and adaptations without going astray  M(t) = probability that system is back to specifications at t if it failed at t0  “…recover its size and shape, following deformation…” 26 February 2013 2001WETSCN-01 32

Attributes of dependability (4) • Confidentiality  Non-occurrence of unauthorised disclosure of information • Integrity  Non-occurrence of improper alterations of information 26 February 2013 2001WETSCN-01 33

Related attributes • Testability  Ability to test features of a system  Related to maintainability • Security  Integrity + availability + confidentiality 26 February 2013 2001WETSCN-01 34

Means of dependability 26 February 2013 2001WETSCN-01 35

Avoidance/prevention and removal • Fault avoidance/prevention: design methodologies that try to make software provably fault-free • Fault removal: methods that aim to remove faults after system development. Done through testing. 26 February 2013 2001WETSCN-01 36

Fault tolerance • Starting point: “No amount of verification, validation and testing can eliminate all faults in an application and give complete confidence in the availability and data consistency of applications” (Randell) • Faults will occur, but we need to make sure that the system is elastic • FT: provisions for the system to operate correctly even in the presence of faults. 26 February 2013 2001WETSCN-01 37

Multiple-version Fault Tolerance • Idea: redundancy of software: independently designed versions of software  Randell (1975) : “All fault tolerance must be based on the provision of useful redundancy, both for error detection and error recovery. In software the redundancy required is not simple replication of programs but redundancy of design” • Assumption: random component failures. Correlated failures  sudden exhaustion of available redundancy  Ariane 5 flight 501: two crucial components were operating in parallel with identical hardware and software… 26 February 2013 2001WETSCN-01 38

MvFT: Recovery blocks #include <ftmacros.h> ... ENSURE(acceptance-test) { Alternate 1; } ELSEBY { Alternate 2; } ... ENSURE; 26 February 2013 2001WETSCN-01 39

MvFT: NVP #include <ftmacros.h> ... NVP VERSION{ block 1; SENDVOTE(v-pointer, v-size); } VERSION{ block 2; SENDVOTE(v-pointer, v-size); } … ENDVERSION(timeout, v-size); if (!agreeon(v-pointer)) error_handler(); ENDNVP; 26 February 2013 2001WETSCN-01 40

MvFT in general & in the context of this exam • MvFT  Implies N-fold design costs, N-fold maintenance costs;  The risk of correlated failures is not negligible;  How would you describe the behaviors of such systems?  What kind of behaviors?  What resilience strategy? 26 February 2013 2001WETSCN-01 41

MvFT’s behaviors and resilience  Behaviors are simple and predefined (system structure is fixed; no support for dynamic adaptability)  Resilience: simple software elasticity 26 February 2013 2001WETSCN-01 42

Next lectures • A variety of systems and algorithms will be presented • Their characteristics in terms of behaviours and resilience (B/R) will be highlighted 26 February 2013 2001WETSCN-01 43

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