1,794 research outputs found
When Are Two Workflows the Same?
In the area of workflow management, one is confronted with a large number of competing languages and the relations between them (e.g. relative expressiveness) are usually not clear. Moreover, even within the same language it is generally possible to express the same workflow in different ways, a feature known as variability. This paper aims at providing some of the formal groundwork for studying relative expressiveness and variability by defining notions of equivalence capturing different views on how workflow systems operate. Firstly, a notion of observational equivalence in the absence of silent steps is defined and related to classical bisimulation. Secondly, a number of equivalence notions in the presence of silent steps are defined. A distinction is made between the case where silent steps are visible (but not controllable) by the environment and the case where silent steps are not visible, i.e., there is an alternation between system events and environment interactions. It is shown that these notions of equivalence are different and do not coincide with classical notions of bisimulation with silent steps (e.g. weak and branching)
Behavioural equivalences for timed systems
Timed transition systems are behavioural models that include an explicit
treatment of time flow and are used to formalise the semantics of several
foundational process calculi and automata. Despite their relevance, a general
mathematical characterisation of timed transition systems and their behavioural
theory is still missing. We introduce the first uniform framework for timed
behavioural models that encompasses known behavioural equivalences such as
timed bisimulations, timed language equivalences as well as their weak and
time-abstract counterparts. All these notions of equivalences are naturally
organised by their discriminating power in a spectrum. We prove that this
result does not depend on the type of the systems under scrutiny: it holds for
any generalisation of timed transition system. We instantiate our framework to
timed transition systems and their quantitative extensions such as timed
probabilistic systems
Generalized Strong Preservation by Abstract Interpretation
Standard abstract model checking relies on abstract Kripke structures which
approximate concrete models by gluing together indistinguishable states, namely
by a partition of the concrete state space. Strong preservation for a
specification language L encodes the equivalence of concrete and abstract model
checking of formulas in L. We show how abstract interpretation can be used to
design abstract models that are more general than abstract Kripke structures.
Accordingly, strong preservation is generalized to abstract
interpretation-based models and precisely related to the concept of
completeness in abstract interpretation. The problem of minimally refining an
abstract model in order to make it strongly preserving for some language L can
be formulated as a minimal domain refinement in abstract interpretation in
order to get completeness w.r.t. the logical/temporal operators of L. It turns
out that this refined strongly preserving abstract model always exists and can
be characterized as a greatest fixed point. As a consequence, some well-known
behavioural equivalences, like bisimulation, simulation and stuttering, and
their corresponding partition refinement algorithms can be elegantly
characterized in abstract interpretation as completeness properties and
refinements
Process algebra for performance evaluation
This paper surveys the theoretical developments in the field of stochastic process algebras, process algebras where action occurrences may be subject to a delay that is determined by a random variable. A huge class of resource-sharing systems – like large-scale computers, client–server architectures, networks – can accurately be described using such stochastic specification formalisms. The main emphasis of this paper is the treatment of operational semantics, notions of equivalence, and (sound and complete) axiomatisations of these equivalences for different types of Markovian process algebras, where delays are governed by exponential distributions. Starting from a simple actionless algebra for describing time-homogeneous continuous-time Markov chains, we consider the integration of actions and random delays both as a single entity (like in known Markovian process algebras like TIPP, PEPA and EMPA) and as separate entities (like in the timed process algebras timed CSP and TCCS). In total we consider four related calculi and investigate their relationship to existing Markovian process algebras. We also briefly indicate how one can profit from the separation of time and actions when incorporating more general, non-Markovian distributions
Axiomatizing Flat Iteration
Flat iteration is a variation on the original binary version of the Kleene
star operation P*Q, obtained by restricting the first argument to be a sum of
atomic actions. It generalizes prefix iteration, in which the first argument is
a single action. Complete finite equational axiomatizations are given for five
notions of bisimulation congruence over basic CCS with flat iteration, viz.
strong congruence, branching congruence, eta-congruence, delay congruence and
weak congruence. Such axiomatizations were already known for prefix iteration
and are known not to exist for general iteration. The use of flat iteration has
two main advantages over prefix iteration: 1.The current axiomatizations
generalize to full CCS, whereas the prefix iteration approach does not allow an
elimination theorem for an asynchronous parallel composition operator. 2.The
greater expressiveness of flat iteration allows for much shorter completeness
proofs.
In the setting of prefix iteration, the most convenient way to obtain the
completeness theorems for eta-, delay, and weak congruence was by reduction to
the completeness theorem for branching congruence. In the case of weak
congruence this turned out to be much simpler than the only direct proof found.
In the setting of flat iteration on the other hand, the completeness theorems
for delay and weak (but not eta-) congruence can equally well be obtained by
reduction to the one for strong congruence, without using branching congruence
as an intermediate step. Moreover, the completeness results for prefix
iteration can be retrieved from those for flat iteration, thus obtaining a
second indirect approach for proving completeness for delay and weak congruence
in the setting of prefix iteration.Comment: 15 pages. LaTeX 2.09. Filename: flat.tex.gz. On A4 paper print with:
dvips -t a4 -O -2.15cm,-2.22cm -x 1225 flat. For US letter with: dvips -t
letter -O -0.73in,-1.27in -x 1225 flat. More info at
http://theory.stanford.edu/~rvg/abstracts.html#3
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