# Difference between revisions of "D32 Scalability"

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==== Qualitative Reasoning ==== | ==== Qualitative Reasoning ==== | ||

+ | * Ideally, we would like to have a new value for the '''convergence''' attribute of Event-B events. However, this is not currently supported by the Rodin platform. Instead, a new '''probabilistic''' attribute is defined for events, with the value is either ''standard'' or ''probabilistic''. | ||

+ | |||

+ | * Since standard refinement does not maintain probabilistic convergent property, we put a restriction on the development method for ''almost-certain termination systems'' as follows. | ||

+ | *# Establish the model of the systems with various ''anticipated'' events | ||

+ | *# | ||

=== Available Documentation === | === Available Documentation === |

## Revision as of 12:56, 1 December 2010

## Contents

### Overview

Regarding scalability of the Rodin platform, the following contributions has been made:

- (Alexei) Flow plug-in

- (Alexei) Group Refinement

- (Alexei) Modes

- Emre Yilmaz and Thai Son Hoang (ETH Zurich) has developed a plug-in supporting an extension of Event-B with qualitative reasoning. The extension allows developer to declare an event converges probabilistically (with probability 1). This is in contrast with standard certain termination, where convergent events
*must decrease*the declared variant. Provided that the variant is bounded above, an event probabilistically converges if it*might decrease*the variant. The extension enables Event-B to model systems with*almost certain termination*properties, e.g. IEEE 1394 Firewire protocol^{[1]}or Rabin's Choice Coordination algorithm^{[2]}.

### Motivations

#### Flow plug-in

(Alexei)

#### Group Refinement

(Alexei)

#### Modes

(Alexei)

#### Qualitative Reasoning

Probability is used in many distributed systems for breaking the symmetry between different components/processes, e.g. IEEE 1394 Firewire protocol ^{[1]}, Rabin Choice Coordination algorithm ^{[2]}. For such systems, termination cannot be guaranteed for certain. Instead, a slightly weaker property is mostly appropriate: *termination with probability one*. As an example for this type of systems is to consider tossing a coin until it comes up tail. Provided that the coin is fair (in that sense that no face is ignored forever), eventually, the coin will eventually come up head.

Qualitative probabilistic reasoning has been integrated into Event-B ^{[3]}: a new kind of actions is added, namely *probabilistic actions* with an assumption that the probability for each possible alternative is bounded away from 0 or 1. Most of the time, probabilistic actions behave the same as (standard) non-deterministic actions (e.g. invariant preservation). The difference between probabilistic and non-deterministic actions is with convergence proof obligation: probabilistic actions are interpreted *angelically*, whereas non-deterministic actions are interpreted *demonically*. The result is a practical method for handling qualitative reasoning that generates proof obligations in the standard first-order logic of Event-B.

The plug-in allows developers to declare an event to be probabilistic convergent and generate appropriate proof obligations. Since the obligations are in standard first-order logic supported by the Rodin platform, we do not need to make any extension for the provers to handle the new proof obligations.

### Choices/Decisions

#### Flow plug-in

(Alexei)

#### Group Refinement

(Alexei)

#### Modes

(Alexei)

#### Qualitative Reasoning

- Ideally, we would like to have a new value for the
**convergence**attribute of Event-B events. However, this is not currently supported by the Rodin platform. Instead, a new**probabilistic**attribute is defined for events, with the value is either*standard*or*probabilistic*.

- Since standard refinement does not maintain probabilistic convergent property, we put a restriction on the development method for
*almost-certain termination systems*as follows.- Establish the model of the systems with various
*anticipated*events

- Establish the model of the systems with various

### Available Documentation

- Master thesis of Emre Yilmaz on developing tool support for qualitative reasoning in Event-B
^{[4]}.

- The development of Rabin's Choice Coordination Algorithm is available at the DEPLOY Repository
^{[5]}.

- A paper describing the development of Rabin Choice Coordination algorithm and tool support in the Proceedings of AVoCS'10
^{[6]}.

### Planning

#### Flow plug-in

(Alexei)

#### Group Refinement

(Alexei)

#### Modes

(Alexei)

#### Qualitative Reasoning

In DEPLOY's fourth year, we plan to implement the missing proof obligations. More importantly, we will investigate the interaction between refinement and almost certain termination. This allows us to prove convergence properties early in the development and make sure that refinement will maintain these convergent properties.

### References

- ↑
^{1.0}^{1.1}J.-R. Abrial, D. Cansell, D. Mery. A Mechanically Proved and Incremental Development of IEEE 1394 Tree Identify Protocol.*Formal Asp. of Comput.*14(3):215-227, 2003 - ↑
^{2.0}^{2.1}M. Rabin. The Choice Coordination Problem.*Acta Informatica, 17:121-134*, 1982. - ↑ S. Hallerstede, T.S. Hoang. Qualitative Probabilistic Modelling in Event-B.
*iFM 2007: Integrated Formal Methods*, Oxford, U.K. July 2007 - ↑ E. Yilmaz. Tool support for qualitative reasoning in Event-B. Master's thesis, Department of Computer Science, ETH Zurich, Switzerland, Aug. 2010
- ↑ E. Yilmaz. Rabin's Choice Coordination Development.
- ↑ E. Yilmaz and T.S. Hoang, Development of Rabin's Choice Coordination Algorithm in Event-B.
*Proceedings of AVoCS'10*, Dusseldorf, Germany, 2010