36 research outputs found
Soft Concurrent Constraint Programming
Soft constraints extend classical constraints to represent multiple
consistency levels, and thus provide a way to express preferences, fuzziness,
and uncertainty. While there are many soft constraint solving formalisms, even
distributed ones, by now there seems to be no concurrent programming framework
where soft constraints can be handled. In this paper we show how the classical
concurrent constraint (cc) programming framework can work with soft
constraints, and we also propose an extension of cc languages which can use
soft constraints to prune and direct the search for a solution. We believe that
this new programming paradigm, called soft cc (scc), can be also very useful in
many web-related scenarios. In fact, the language level allows web agents to
express their interaction and negotiation protocols, and also to post their
requests in terms of preferences, and the underlying soft constraint solver can
find an agreement among the agents even if their requests are incompatible.Comment: 25 pages, 4 figures, submitted to the ACM Transactions on
Computational Logic (TOCL), zipped file
Timed Soft Concurrent Constraint Programs: An Interleaved and a Parallel Approach
We propose a timed and soft extension of Concurrent Constraint Programming.
The time extension is based on the hypothesis of bounded asynchrony: the
computation takes a bounded period of time and is measured by a discrete global
clock. Action prefixing is then considered as the syntactic marker which
distinguishes a time instant from the next one. Supported by soft constraints
instead of crisp ones, tell and ask agents are now equipped with a preference
(or consistency) threshold which is used to determine their success or
suspension. In the paper we provide a language to describe the agents behavior,
together with its operational and denotational semantics, for which we also
prove the compositionality and correctness properties. After presenting a
semantics using maximal parallelism of actions, we also describe a version for
their interleaving on a single processor (with maximal parallelism for time
elapsing). Coordinating agents that need to take decisions both on preference
values and time events may benefit from this language. To appear in Theory and
Practice of Logic Programming (TPLP)
Dynamic Programming on Nominal Graphs
Many optimization problems can be naturally represented as (hyper) graphs,
where vertices correspond to variables and edges to tasks, whose cost depends
on the values of the adjacent variables. Capitalizing on the structure of the
graph, suitable dynamic programming strategies can select certain orders of
evaluation of the variables which guarantee to reach both an optimal solution
and a minimal size of the tables computed in the optimization process. In this
paper we introduce a simple algebraic specification with parallel composition
and restriction whose terms up to structural axioms are the graphs mentioned
above. In addition, free (unrestricted) vertices are labelled with variables,
and the specification includes operations of name permutation with finite
support. We show a correspondence between the well-known tree decompositions of
graphs and our terms. If an axiom of scope extension is dropped, several
(hierarchical) terms actually correspond to the same graph. A suitable
graphical structure can be found, corresponding to every hierarchical term.
Evaluating such a graphical structure in some target algebra yields a dynamic
programming strategy. If the target algebra satisfies the scope extension
axiom, then the result does not depend on the particular structure, but only on
the original graph. We apply our approach to the parking optimization problem
developed in the ASCENS e-mobility case study, in collaboration with
Volkswagen. Dynamic programming evaluations are particularly interesting for
autonomic systems, where actual behavior often consists of propagating local
knowledge to obtain global knowledge and getting it back for local decisions.Comment: In Proceedings GaM 2015, arXiv:1504.0244
A Calculus for Orchestration of Web Services
Service-oriented computing, an emerging paradigm for distributed computing based on the use of services, is calling for the development of tools and techniques to build safe and trustworthy systems, and to analyse their behaviour. Therefore, many researchers have proposed to use process calculi, a cornerstone of current foundational research on specification and analysis of concurrent, reactive, and distributed systems. In this paper, we follow this approach and introduce CWS, a process calculus expressly designed for specifying and combining service-oriented applications, while modelling their dynamic behaviour. We show that CWS can model all the phases of the life cycle of service-oriented applications, such as publication, discovery, negotiation, orchestration, deployment, reconfiguration and execution. We illustrate the specification style that CWS supports by means of a large case study from the automotive domain and a number of more specific examples drawn from it
Designing a Nonmonotonic Soft Concurrent Constraint Language for SLA Management
We present an extension of the Soft Concurrent Constraint language to allow the nonmonotonic evolution of the constraint store. To accomplish this, we introduce some new operations: the retract(c) reduces the current store by c, the updateX(c) transactionally relaxes all the constraints of the store that deal with the variables in X set, and then adds a constraint c (usually with support = X); the nask(c) tests if c is not entailed by the store.We present this framework as a possible solution to the management of resources (e.g. web services and network resource allocation) that need a given Quality of Service (QoS). The QoS requirements of all the parties should converge, through a negotiation process, on a formal agreement defined as the Service Level Agreement, which specifies the contract that must be enforced. The main advantage is to have a preference (or cost) measure directly embedded in the language, and to have a highly flexible and parametric abstraction
A Semiring-based framework for fair resources allocation
This paper studies the probable roles of interplanetary and geomagnetic parameters
in the generation of ‘intense’ and ‘very intense’ magnetic storms as well as the
correlation between magnetic field B and flow speed V , southward turning of Bz
(Bs) and Bs duration BT . 18 storm events were observed and for analysis were
divided into two sections. This include 8 ‘intense’ magnetic storm (–250 nT ≤ peak
Dst < −100 nT) events and 10 ‘Very intense’ storms (peak Dst < −250 nT); both
spanning January 1976 until May 2005. From our analysis, it was observed that the
interplanetary magnetic field Bz plays a prominent role alongside Dst in the generation
of intense storms. So also is the interplanetary electric field associated with
high-speed streams and the solar wind density Np in the ring current intensification.
The result further shows that over 67% of the storm events under investigation are
generated from magnetic clouds which are characterized by a low beta plasma, high
IMF magnitude and large scale coherent field rotations often including large and
steady north-south components.
As regards the geoeffectiveness of the flow speed V , the Bs and Bs interval (BT )
with the magnetic field B, it was observed that generally for all the selected storm
events, the flow speed is the most correlated, showing a correlation coefficient of
50.9% with B, and hence the most geoeffective. However, the statistical significance
of its correlation with B is not so directly implying a higher substorm occurrence
during the faster solar wind, but that it is one of the causes of substorm occurrence
at a value faster than 400 km/s. Furthermore, the result shows that ‘very intense’
storms whose main feature is a plasma flow speed greater than 550 km/s has a
negligible or very low correlation between the flow speed and the magnetic field
B; whereas, ‘intense’ storms have a 58.7% correlation between the two parameters.
Lastly, it could be argued that all ‘very intense’ storms are likely to have a plasma
flow speed greater than 550 km/s within the storm interval, but not all flow speed
greater than 550 km/s are ‘very intense’ storms
Service discovery and negotiation with COWS
To provide formal foundations to current (web) services technologies, we put forward using COWS, a process calculus for specifying, combining and analysing services, as a uniform formalism for modelling all the relevant phases of the life cycle of service-oriented applications, such as publication, discovery, negotiation, deployment and execution. In this paper, we show that constraints and operations on them can be smoothly incorporated in COWS, and propose a disciplined way to model multisets of constraints and to manipulate them through appropriate interaction protocols. Therefore, we demonstrate that also QoS requirement specifications and SLA achievements, and the phases of dynamic service discovery and negotiation can be comfortably modelled in COWS. We illustrate our approach through a scenario for a service-based web hosting provider
Analysis of Integrity Policies using Soft Constraints
An integrity policy defines the situations when modification of information is authorized and is enforced by the security mechanisms of the system. However, in a complex application system it is possible that an integrity policy may have been incorrectly specified and, as a result, a user may be authorized to modify information that can lead to an unexpected system compromise. In this paper we outline a scalable and quantitative technique that uses constraint solving to model and analyze the effectiveness of application system integrity policie