=Paper=
{{Paper
|id=None
|storemode=property
|title=Design, Modelling and Analysis of a Workflow Reconfiguration
|pdfUrl=https://ceur-ws.org/Vol-723/paper2.pdf
|volume=Vol-723
|dblpUrl=https://dblp.org/rec/conf/apn/MazzaraADB11
}}
==Design, Modelling and Analysis of a Workflow Reconfiguration==
https://ceur-ws.org/Vol-723/paper2.pdf
Design, Modelling and Analysis of
a Workflow Reconfiguration
Manuel Mazzara1 , Faisal Abouzaid2 ,
Nicola Dragoni3 , and Anirban Bhattacharyya1
1
Newcastle University, Newcastle upon Tyne, UK
{Manuel.Mazzara, Anirban.Bhattacharyya}@ncl.ac.uk
2
École Polytechnique de Montréal, Canada
m.abouzaid@polymtl.ca
3
Technical University of Denmark (DTU), Copenhagen
ndra@imm.dtu.dk
Abstract. This paper describes a case study involving the reconfigu-
ration of an office workflow. We state the requirements on a system
implementing the workflow and its reconfiguration, and describe the sys-
tem’s design in BPMN. We then use an asynchronous π-calculus and
W ebπ∞ to model the design and to verify whether or not it will meet
the requirements. In the process, we evaluate the formalisms for their
suitability for the modelling and analysis of dynamic reconfiguration of
dependable systems.
1 Introduction
Competition drives technological development, and the development of depend-
able systems is no exception. Thus, modern dependable systems are required to
be more flexible, available and dependable than their predecessors, and dynamic
reconfiguration is one way of achieving these requirements.
A significant amount of research has been performed on hardware reconfig-
uration (see [5] and [9]), but little has been done for reconfiguration of services,
especially regarding computational models, formalisms and methods appropri-
ate to the service domain. Furthermore, much of the current research assumes
that reconfiguration can be instantaneous, or that the environment can wait dur-
ing reconfiguration for a service to become available (see [14] and [13]). These
assumptions are unrealistic in the service domain. For example, instantaneous
mode change in a distributed system is generally not possible, because the system
usually has no well-defined global state at a specific instant (due to significant
communication delays). Also, waiting for the reconfiguration to complete is not
acceptable if (as a result) the environment becomes dangerously unstable or the
service provider loses revenue by the environment aborting the service request.
These observations lead to the conclusion that further research is required
on dynamic reconfiguration of dependable services, and especially on its formal
foundations, modelling and verification. In a preliminary paper [16], we exam-
ined a number of well-known formalisms for their suitability for reconfigurable
The invited speaker is Manuel Mazzara.
M. Mazzara et al.: Workflow Reconfiguration 11
dependable systems. In this paper, we focus on one of the formalisms (W ebπ∞ )
and compare it to a π-calculus in order to perform a deeper analysis than was
possible in [16]. We use a more complex case study involving the reconfiguration
of an office workflow for order processing, define the requirements on a system
implementing the workflow and its reconfiguration, and describe the design of a
system in BPMN (see section 2). We then use an asynchronous π-calculus with
summation (in section 3) and W ebπ∞ [18] (in section 4) to model the design and
to verify whether or not the design will meet the reconfiguration requirements.
We chose process algebras because they are designed to model interaction be-
tween concurrent activities. An asynchronous π-calculus was selected because
π-calculi are designed to model link reconfiguration, and asynchrony is suitable
for modelling communication in distributed systems. W ebπ∞ was selected be-
cause it is designed to model composition of web services.
Thus, the contribution of this paper is to identify strengths and weaknesses
of an asynchronous π-calculus with summation and W ebπ∞ for modelling dy-
namic reconfiguration and verifying requirements (discussed in section 5). This
evaluation may be useful to system designers intending to use formalisms to
design dynamically reconfigurable systems, and also to researchers intending to
design better formalisms for the design of dynamically reconfigurable systems.
2 Office Workflow: Requirements and Design
This case study describes dynamic reconfiguration of an office workflow for order
processing that is commonly found in large and medium-sized organizations
[7]. These workflows typically handle large numbers of orders. Furthermore, the
organizational environment of a workflow can change in structure, procedures,
policies and legal obligations in a manner unforseen by the original designers
of the workflow. Therefore, it is necessary to support the unplanned change of
these workflows. Furthermore, the state of an order in the old configuration may
not correspond to any state of the order in the new configuration. These factors,
taken in combination, imply that instantaneous reconfiguration of a workflow is
not always possible; neither is it practical to delay or abort large numbers of
orders because the workflow is being reconfigured. The only other possibility is
to allow overlapping modes for the workflow during its reconfiguration.
2.1 Requirements
A given organization handles its orders from existing customers using a number
of activities arranged according to the following procedure:
1. Order Receipt: an order for a product is received from a customer. The order
includes customer identity and product identity information.
2. Evaluation: the product identity is used to perform an inventory check on the
availability of the product. The customer identity is used to perform a credit check
on the customer using an external service. If both the checks are positive, the order
is accepted for processing; otherwise the order is rejected.
12 PNSE’11 – Petri Nets and Software Engineering
3. Rejection: if the order is rejected, a notification of rejection is sent to the customer
and the workflow terminates.
4. If the order is to be processed, the following two activities are performed concur-
rently:
(a) Billing: the customer is billed for the total cost of the goods ordered plus
shipping costs.
(b) Shipping: the goods are shipped to the customer.
5. Archiving: the order is archived for future reference.
6. Confirmation: a notification of successful completion of the order is sent to the
customer.
In addition, for any given order, Order Receipt must precede Evaluation,
which must precede Rejection or Billing and Shipping.
After some time, managers notice that lack of synchronisation between the
Billing and Shipping activities is causing delays between the receipt of bills and
the receipt of goods that are unacceptable to customers. Therefore, the managers
decide to change the order processing procedure, so that Billing is performed
before Shipping (instead of performing the two activities concurrently). During
the transition interval from one procedure to the other, the following require-
ments must be met:
1. The result of the Evaluation activity for any given order should not be
affected by the change in procedure.
2. All accepted orders must be billed and shipped exactly once, then archived,
then confirmed.
3. All orders accepted after the change in procedure must be processed accord-
ing to the new procedure.
2.2 Design
We designed the system implementing the office workflow using the Business
Process Modeling Notation (BPMN) [4]. We chose BPMN because it is a widely
used graphical tool for designing business processes. In fact, BPMN is a standard
for business process modelling, and is maintained by the Object Management
Group (see http://www.omg.org/).
The system is designed as a collection of eight pools: Office Workflow, Or-
der Generator, Credit Check, Inventory Check, Reconf. Region, Bill&Ship1,
Bill&Ship2 and Archive. The different pools represent different functional en-
tities, and each pool can be implemented as a separate concurrent task (see
Figure 1). Office Workflow coordinates the entire workflow: it receives a request
from a customer, and makes a synchronous call to Order Generator to create an
order. It then calls Credit Check (with the order) to check the creditworthiness
of the customer, and tests the returned value using an Exclusive Data-Based
Gateway. If the test is positive, Office Workflow calls Inventory Check (with the
order) to check the availability of the ordered item, and tests the returned value.
If either of the two tests is negative, the customer is notified of the rejected order
M. Mazzara et al.: Workflow Reconfiguration 13
Fig. 1. Office workflow - BPMN diagram of the reconfiguration
and the workflow terminates. If both tests are positive, Office Workflow calls Re-
conf. Region, which acts as a switch between configuration 1 and configuration
2 of the workflow, and thereby handles the reconfiguration of the workflow.
Reconf. Region calls Bill&Ship1 by default: it makes an asynchronous call
to the Main pool within Bill&Ship1, which uses a Parallel Gateway to call Bill
and Ship concurrently and merge their respective results, and then returns these
14 PNSE’11 – Petri Nets and Software Engineering
results to Office Workflow. The Office Workflow then calls Archive to store the
order, then notifies the customer of the successful completion of the order, and
then terminates the workflow. However, if Reconf. Region receives a change
configuration message, it calls the Main pool within Bill&Ship2 instead, which
makes sequential a call to Bill and then to Ship, and then returns the results to
Office Workflow.
Notice that for the sake of simplicity, we assume neither Bill nor Ship pro-
duces a negative result. Furthermore, the Bill and Ship pools are identical in both
configurations, which suggests their code is replicated (rather than shared) in
the two configurations. Finally, we assume the reconfiguration is planned rather
than unplanned.
3 Asynchronous π-Calculus
The asynchronous π-calculus ([10], [3]) is a subset of Milner’s π-calculus [20], and
it is known to be more suitable for distributed implementation. It is considered a
rich paradigm for asynchronous communication, although it is not as expressive
as Milner’s π-calculus in representing mixed-choice constructs, such as a.P +b.P 0
(see [22]).
We recall the (monadic) asynchronous π-calculus. Let N be a set of names
(e.g. a, b, c, ...) and V be a set of variables (e.g. x, y, z, ...). The set of the asyn-
chronous π-calculus processes is generated by the following grammar:
P ::= x̄z G P |P [a = b]P (νx)P A(x1 , ..., xn )
where guards G are defined as follows:
G ::= 0 x(y).P τ.P G+G
Intuitively, an output x̄z represents a message z tagged with a name x indi-
cating that it can be received (or consumed) by an input process x(y).P which
behaves as P {z/y} upon receiving z. Furthermore, x(y).P binds the name y in
P and the restriction (νx)P declares a name x private to P and thus binds x.
Outputs are non-blocking.
The parallel composition P |Q means P and Q running in parallel. G + G is
the non-deterministic choice that is restricted to τ and input prefixes.
[a = b]P behaves like P if a and b are identical.
A(y1 , ..., yn ) is an identifier (also call, or invocation) of arity n. It represents
the instantiation of a defined agent. We assume that every such identifier has
def
a unique, possibly recursive, definition A(x1 , ..., xn ) = P where the xi s are
pairwise distinct, and the intuition is that A(y1 , ..., yn ) behaves like P with each
yi replacing xi .
def
Furthermore, for each A(x1 , ..., xn ) = P we require: f n(P ) ⊆ {x1 , ..., xn },
where f n(P ) stands for the set of free names in P , and bn(P ) for the set of
bound names in P . The input prefix and the ν operator bind the names. For
example, in a process x(y).P , the name y is bound. In (νx)P , x is considered to
M. Mazzara et al.: Workflow Reconfiguration 15
be bound. Every other occurrences of a name like x in x(y).P and x, y in x̄hyi.P
are free.
Due to lack of space we omit to give details on structural congruence and
operational semantics for the asynchronous π-calculus. They can be found in [1]
for the version of the calculus we use in this paper.
The Model in Asynchronous π-Calculus The model in asynchronous π-
calculus needs to keep the synchronization between actions in sequence coherent
with the workflow definition. So sequence is implemented by using parallel com-
position with prefix and postfix on the same channel. Channel names are not
restricted since the full system is not described here and has to be put in paral-
lel with the detailed implementation of the environment process described (that
will be omitted here).
The entire model is expressed in asynchronous π-calculus as follows:
Entire Model
Let params =
{customer, item, Archive, ArchiveReply, Bill, BillReply, BillShip, Conf irm,
CreditCheck, CreditOk, CreditReject, InventoryCheck,
InventoryOk, InventoryReject, OrderGenerator,
OrderGeneratorReply, OrderReceipt, Reject, Ship, ShipReply, reco, recn}
We can define the W orkf low process as follows:
W orkf low(params) ,
(ν order) (OrderReceipt(customer, item).OrderGenerator customer, item
| OrderGeneratorReply(order).CreditCheck customer
| (creditOk().InventoryCheck item + CreditReject().Reject order)
| (InventoryOk().BillShip + InventoryReject().Reject order)
| reco ().BillShip().(Bill customer, item, order | Ship customer, item, order)
| BillReply(order).ShipReply(order).Archive order
+recn ().BillShip().(Bill customer, item, order
| BillReply(order).Ship customer, item, order) | ShipReply(order).Archive order
| ArchiveReply(order).Conf irm order) | W orkf low(params)
In the model, the old region is identified as follows:
reco ().BillShip().(Bill customer, item, order | Ship customer, item, order)
| BillReply(order).ShipReply(order).Archive order
And the new region is:
recn ().BillShip().(Bill customer, item, order
| BillReply(order).Ship customer, item, order) | ShipReply(order).Archive order
In the asynchronous π-calculus, two outputs cannot be in sequence. In order
to impose ordering between Bill and Ship, in the new region, it is necessary to
put a guard on Ship, which requires enlarging the boundary of the old region to
16 PNSE’11 – Petri Nets and Software Engineering
include the processes in the environment of the workflow that synchronize with
Bill and Ship. We did not model these processes because they are outside the
system being designed, but the limitations of the asynchronous π-calculus imply
that we must be able to access the logic of external services for which we know
only the interfaces. For a more detailed description of this problem, please see
[12].
The entire model represents a specific instance of the workflow that spawn
concurrently another instance with fresh customer and item which here are as-
sumed to be fresh names but in reality will be user entered (but it is not relevant
to our purposes). We have to assume the existence of a “higher level” process
(at the level of the BPEL engine) that activates the entire workflow and bounds
the names that are free in the above π-calculus process. In this model channels
creditOK, creditReject, InventoryOK and InventoryReject are used to receive
the result of the credit check and inventory check, respectively. The old/new re-
gion is externally triggered using specific channels reco and recn chosen according
to the value x received on channel region:
(ν x)W orkf low(param) | region(x).([x = new]recn | [x = old]reco )
In section 4 we show a more efficient solution using Webπ∞ .
Analysis in π-logic Logics have long been used to reason about complex
systems, because they provide abstract specifications that can be used to describe
system properties of concurrent and distributed systems. Verification frameworks
can support checking of functional properties of such systems by abstracting
away from the computational contexts in which they are operating.
In the context of π-calculi, one can use the π-logic with the HAL Toolkit
model-checker [8]. The π-logic has been introduced in [8] to specify the behav-
ior of systems in a formal and unambiguous manner by expressing temporal
properties of π-processes.
Syntax of the π-logic The logic integrates modalities defined by Milner ([21])
with EF φ and EF {χ}φ modalities on possible future. The π-logic syntax is:
φ ::= true | ∼ φ | φ ∧ φ0 | EX{µ}φ | EF φ | EF {χ}φ
W
where µ is a π-calculus action and χ could be µ, ∼ µ, or i∈I µi and where I is
a finite set.
Semantics of π-formulae is given below:
• P |= true for any process P ;
• P |=∼ φ iff P 6|= φ;
• P |= φ ∧ φ0 iff P |= φ and P |= φ0 ;
µ
• P |= EX{µ}φ iff there exists P 0 such as P −→ P 0 and P 0 |= φ (strong
next);
M. Mazzara et al.: Workflow Reconfiguration 17
• P |= EF φ iff there exists P0 , ..., Pn and µ1 , ..., µn , with n ≥ 0, such as
µ1 µn
P = P0 −→ P1 ... −→ Pn and Pn |= φ. The meaning of EF φ is that φ
must be true sometimes in a possible future.
• P |= EF {χ}φ if and only if there exists P0 , ..., Pn and ν1 , ..., νn , with n ≥ 0,
ν1 νn
such that P = P0 −→ P1 ... −→ Pn and Pn |= φ with:
• χ = µ for all 1 ≤ j ≤ n, νj = µ or νj = τ ;
• χ =∼Wµ for all 1 ≤ j ≤ n, νj 6= µ or νj = τ ;
• χ = i∈I µi : for all 1 ≤ j ≤ n, νj = µi for some i ∈ I or νj = τ .
The meaning of EF {χ}φ is that the truth of φ must be preceded by the
occurrence of a sequence of actions χ.
Some useful dual operators are defined as usual:
f alse, φ ∨ φ, AX{µ}φ (∼ EX{µ} ∼ φ), < µ > φ (weak next), [µ]φ (Dual of
weak next), AGφ (AG{χ}) (always).
Properties of the dynamic reconfiguration model
We need to verify that during the reconfiguration interval the requirements
given in section 2.1 hold. For this purpose, we need to express the requirements
formally, if possible, using the π-logic.
The result of the Evaluation activity for any given order should not be
affected by the change in procedure. The following formula means what-
ever the chosen path (old or new region), an order will be billed, shipped and
archived or refused:
AG{EF {OrderReceipt()}true}
`
AG{ EF {Bill customer, item, order}true ∧ EF {Ship customer, item, order}true∧
´
EF {Archive order}true ∨ EF {Reject }true}
All accepted orders must be billed and shipped exactly once, then
archived, then confirmed. The following formula means that after an order
is billed and shipped, it is archived and confirmed, and cannot be billed nor
shipped again:
AG{EF {BillShip()}true}
AG{EF {Bill customer, item, order}true ∧ EF {Ship customer, item, order}true∧
EF {Archive order}true} ∧ EF {Conf irm order}true}
AG{{Bill customer, item, order}f alse ∧ {Ship customer, item, order}f alse}
All orders accepted after the change in procedure must be processed
according to the new procedure We can express in the π-logic the following
requirement: “after a reception on the channel recn , no other reception on chan-
nel rec0 will be accepted”. This meets the desired requirement since it is obvious
18 PNSE’11 – Petri Nets and Software Engineering
from the model that, if a signal is received on channel recn , the order will be
processed according to the new procedure.
AG{{recn ()}true AG{rec0 ()}f alse}
However, since the choice between the old procedure and the new one is non-
deterministic, this formula will not be true, although it is an essential require-
ment for the model. This result illustrates the difficulty of the asynchronous
π-calculus to model the dynamic reconfiguration properly. A first attempt to
answer this problem is presented in the next section.
4 Webπ∞
Webπ∞ is a conservative extension of the π-calculus developed for modelling and
analysis of Web services and Service Oriented Architectures. The basic theory
has been developed in [18] and [15], whilst its applicability has been shown in
other work: [12] gives a BPEL semantics in term of Webπ∞ , [6] clarifies some
aspects of the Recovery Framework of BPEL, and [17] exploits a web transaction
case study (a toy example has also been discussed in [16]).
Syntax and Semantics The syntax of webπ∞ processes relies on a countable
set of names, ranged over by x, y, z, u, · · · . Tuples of names are written u
e. We
intend i ∈ I with I a finite non-empty set of indexes.
X
e |
P ::= 0 | x u xi (uei ).Pi | (x)P | P |P | !x(e
u).P | h|P ; P |ix
i∈I
It is worth noting that the syntax of webπ∞ simply augments the asyn-
chronous π-calculus with a workunit process. A workunit h|P ; Q|ix behaves as
the body P until an abort x is received, and then it behaves as the event handler
Q.
We give the semantics of webπ∞ in two steps, following the approach of
Milner [19], separating the laws that govern the static relations between processes
from the laws that rule their interactions. The static relations between processes
are governed by the structural congruence ≡, the least congruence satisfying the
Abelian monoid laws for parallel and summation (associativity, commutativity
and 0 as identity) and closed with respect to α-renaming and the axioms shown
in table 1.
The scope laws are standard while novelties regard workunit and floating
laws. The law h|0 ; Q|ix ≡ 0 defines committed workunit, namely workunit with
0 as body. These ones, being committed, are equivalent to 0 and, therefore, can-
not fail anymore. The law h|h|P ; Q|iy | R ; R0 |ix ≡ h|P ; Q|iy | h|R ; R0 |ix moves
workunit outside parents, thus flattening the nesting. Notwithstanding this flat-
tening, parent workunits may still affect the children by means of names. The
law h|z u
e | P ; Q|ix ≡ z ue | h|P ; Q|ix floats messages outside workunit bound-
aries. By this law, messages are particles that independently move towards their
M. Mazzara et al.: Workflow Reconfiguration 19
Scope laws (u)0 ≡ 0, (u)(v)P ≡ (v)(u)P
P | (u)Q ≡ (u)(P | Q) , if u 6∈ fn(P )
h|(z)P ; Q|ix ≡ (z)h|P ; Q|ix , if z 6∈ {x} ∪ fn(Q)
Workunit laws h|0 ; Q|ix ≡ 0
h|h|P ; Q|iy | R ; R0 |ix ≡ h|P ; Q|iy | h|R ; R0 |ix
h|(z)P ; Q|ix ≡ (z)h|P ; Q|ix , if z 6∈ {x} ∪ fn(Q)
Floating law h|z u
e | P ; Q|ix ≡ z u e | h|P ; Q|ix
Table 1. webπ∞ Structural Congruence
inputs. The intended semantics is the following: if a process emits a message,
this message traverses the surrounding workunit boundaries until it reaches the
corresponding input. In case an outer workunit fails, recoveries for this message
may be detailed inside the handler processes.
The dynamic behavior of processes is instead defined by the reduction relation
→ which is the least relation satisfying the axioms and rules shown in table 2
and closed with respect to ≡, (x)_ , _ | _, and h| _ ; Q|iz . In the table we use
def
the shortcut: h|P ; Q|i = (z)h|P ; Q|iz where z 6∈ fn(P ) ∪ fn(Q)
COM xi ve | i∈I xi (uei ).Pi → Pi ve/uei
P ˘ ¯
REP u).P → P ve/e
˘ ¯
x ve | !x(e u | !x(eu).P
FAIL
Q P Q
x | h| i∈I s∈S xis (uf is ).Pis | ej ).Pj ; Q|ix → h|Q ; 0|i
j∈J !xj (u
where J 6= ∅ ∨ (I 6= ∅ ∧ S 6= ∅)
Table 2. webπ∞ Reduction Semantics
Rules (com) and (rep) are standard in process calculi and model input-output
interaction and lazy replication. Rule (fail) models workunit failures: when a
unit abort (a message on a unit name) is emitted, the corresponding body is
terminated and the handler activated. On the contrary, aborts are not possible
if the transaction is already terminated (namely every thread in the body has
completed its own work), for this reason we close the workunit restricting its
name.
The model in Webπ∞ For the modelling purposes of this work, the idea
of workunit and event handler turn out to be particularly useful. Webπ∞ uses
the mechanism of workunit to bound the identified regions, and event raising
is exploited to operate the non immediate change (reconfiguration). The model
can be expressed as follows (as a shortcut we will use here process invocation):
W orkf low(customer, item) ,
20 PNSE’11 – Petri Nets and Software Engineering
(ν order) OrderReceipt(customer, item).OrderGenerator customer, item
| OrderGeneratorReply(order).CreditCheck customer
| (CreditCheckReplyt (order).InventoryCheck item
+CreditCheckReplyf (order).Reject order)
| (InventoryCheckReplyt (order).BillShip
+InventoryCheckReplyf (order).Reject order)
| h|BillShip().(Bill customer, item, order | Ship customer, item, order
| (ν customer)(ν item) W orkf low(customer, item))
; (ν customer)(ν item) W orkf lown (customer, item)|irec
| BillReply(order).ShipReply(order).Archive order
| ArchiveReply(order).Conf irm order
Webπ∞ shows here a subtle feature which is important for modelling recon-
figurable systems. Since the floating laws of structural congruence allow the asyn-
chronous outputs in a workunit to freely escape, once the region to reconfigure
has been entered and the BillShip has been triggered, Bill customer, item, order
and Ship customer, item, order will not be killed by any incoming rec signal.
This means that, once the region has been entered by an order, that order will
go through without being interrupted by reconfiguration events and the old or-
der will be processed according to the old procedure, not the new one. Future
orders will find instead only the new procedure W orkf lown waiting for orders:
W orkf lown (customer, item) ,
(ν order) OrderReceipt(customer, item).OrderGenerator customer, item
| OrderGeneratorReply(order).CreditCheck customer
| (CreditCheckReplyt (order).InventoryCheck item +
CreditCheckReplyf (order).Reject order)
| (InventoryCheckReplyt (order).BillShip +
InventoryCheckReplyf (order).Reject order)
| BillShip().(Bill customer, item, order | BillReply(order).Ship customer, item, order)
| ShipReply(order).Archive order | ArchiveReply(order).Conf irm order
| (ν customer)(ν item) W orkf lown (customer, item)
As in the π-calculus model, we have to assume the existence of a top level
process activating the entire workflow and bounding all the names appearing
free in the above π-calculus process. The change in procedure will be activated
when the channel t is triggered.
(ν customer)(ν item)(ν rec) W orkf low(customer, item) | t().rec
This process is also responsible for triggering the reconfiguration.
Analysis in Webπ∞ Analysis in Webπ∞ is intended as equational reasoning.
At the moment, one severe weakness of Webπ∞ is its lack of tool support, i.e. au-
tomatic system verification. However, it is clearly possible to encode Webπ∞ into
the π-calculus, being the only technical complication the encoding of the worku-
nit and its asynchronous interrupt. Once the compilation into the π-calculus has
been done, we can proceed using HAL. From one side, Webπ∞ simplifies the
M. Mazzara et al.: Workflow Reconfiguration 21
modelling of dependable systems expressing with its workunit the recovery be-
havior. On the other side, it makes the verification more difficult. Luckily, there
is an optimal solution using Webπ∞ as modelling language and the π-calculus as
intermediate language, i.e. a verification bytecode. We can then offer a practical
modelling suite to the designer and still use the tool support for the π-calculus.
At the moment our research has not gone so far, so we will just discuss the three
requirements here. We will analyse the requirements in terms of equational rea-
soning (see [18] and [15]). The case study of this paper is interesting at showing
both the modelling power of Webπ∞ and the weaknesses of its reasoning system.
The result of the Evaluation activity for any given order should not
be affected by the change in procedure. The acceptability of an order
(Evaluation activity) is computed outside the region to be reconfigured, and
there is no interaction between Evaluation and the region. That means that the
Evaluation in the old procedure workf low is exactly the same as in the new pro-
cedure workf lown , i.e. the checks are performed in the same exact order. We can
formally express it, in term of equational reasoning, stating that the Evaluation
activity in the old procedure workf low is bisimilar to the Evaluation activity
in the new procedure workf lown which is trivially true.
All accepted orders must be billed and shipped exactly once, then
archived, then confirmed. The presence of a workunit does not affect how the
order itself is processed. The workflow of actions described by the requirement
can be formally expressed as follows:
(ν x)(ν y) (Bill customer, item, order | Ship customer, item, order
| BillReply(order).x | ShipReply(order).y | x().y().Archive order
| ArchiveReply(order).Conf irm order)
In plain words this process describes billing and shipping happening in any
order but both before archiving and confirming. The channels x and y are there
precisely to work as a joint for billing and shipping. If we want to express the
requirements in term of equational reasoning, we can require that both the old
and the new regions have to be bisimilar with the above process. However, this
is too strict since the above process allows a set of traces which is a superset
of both the set of traces of the old configuration and the new one. In this case
similarity could be considered instead of bisimilarity.
All orders accepted after the change in procedure must be processed
according to the new procedure To show this requirements has been imple-
mented in the model semantic reasoning is not necessary, structural congruence
is sufficient. The change in procedure is here modelled by triggering the rec chan-
nel and spawning the workunit handler. The handler then activates a new in-
stance of the workflow based on the new procedure scheme which has been called
workf lown . The floating laws of structural congruence of Webπ∞ (definition 1)
22 PNSE’11 – Petri Nets and Software Engineering
allow the asynchronous outputs in a workunit to freely escape the workunit itself.
Thus, once the region to reconfigure has been already entered and the BillShip
has been triggered, Bill customer, item, order and Ship customer, item, order
will not be killed by any incoming rec signal. Thus, once the region has been
entered by an order, that order will be not interrupted by reconfiguration events
so that old order will be processed according to the old procedure and not the
new one.
5 Discussion
In this section, we discuss three issues which arose during design and modelling:
how the modelling influenced our design, how the π-calculus and Webπ∞ com-
pare with respect to modelling, and correctness criteria for verification of the
workflow reconfiguration.
Modelling and Design Different formalisms have different biases on design
because of their different perspectives. In one of the alternative designs we con-
sidered, the Bill and Ship pools were outside the reconfiguration region, so that
their code was shared between the two configurations. Thus, the boundary of the
reconfiguration region was different. We chose the design in section 2.2 because
it is easier to model. It is the job of a formalist to model what the system design-
ers produce, and ask them to change the design if it cannot be modelled or is
unverifiable. Our experience with asynchronous π-calculi and Webπ∞ suggested
that extending the boundary of the reconfiguration region to include billing and
shipping was a practical choice. This is because in the asynchronous π-calculus
(and consequently in Webπ∞ ), two outputs cannot be in sequence. So, in order
to impose ordering between Bill and Ship, we had to enlarge the boundary
of the reconfiguration region to include the processes in the environment of the
workflow that synchronize with them. The negative side of this solution is that
we have been forced to include in the region parts of the system that were not
intended to be changed. Here the asynchronous π-calculus shows its weakness in
terms of reconfiguring processes dynamically.
Comparison of π-calculus and Webπ∞ This paper has shown the Webπ∞
workunit as being able to offer a more efficient solution to the problem of mod-
elling the case study. In particular, by means of the Webπ∞ floating laws, re-
configuration activities can be better handled. However, at the moment, one
weakness of Webπ∞ is its lack of tool support, whereas the π-calculus is sup-
ported by verification tools (e.g. TyPiCal [11] and HAL [8]). Therefore, Webπ∞
has to be intended as a a front end for modelling with the the π-calculus as the
verification bytecode. As mentioned above, neither the asynchronous π-calculus
nor Webπ∞ can have two outputs in sequence, and this leads to the specific
design choice.
M. Mazzara et al.: Workflow Reconfiguration 23
Correctness Criteria The standard notion of correctness used in process alge-
bras is congruence based on bisimulation. However, our requirements are not all
expressible as congruences between processes. The first and third requirements
can be expressed as congruences, and so bisimulation can be used in the reason-
ing. The second requirement cannot be expressed as a congruence because the
old and new configurations are not behaviourally congruent. So, we have used
reasoning based on simulation instead. Thus, we found that congruence as it has
been used in section 4 is not always applicable for verifying the correctness of
our models. Therefore, in section 3 we have investigated model checking.
The discussion leads us to the following:
1. It is easier to model workflow reconfiguration in Webπ∞ than in the asyn-
chronous π-calculus. However, modelling would be even easier in a syn-
chronous version of Webπ∞ .
2. Model checking is more widely applicable than equational reasoning based
on congruences for verifying workflow reconfiguration.
These two conclusions seem to have wider applicability than just reconfigu-
ration of workflows; but this needs to be verified.
Future Work We intend to proceed with a deeper analysis of alternative designs
for this case study, and evaluate other formalisms, such as VDM [2] and Petri
nets [23]. We are also working on a BPEL implementation of the system. We also
need larger industrial case studies to help us to design and evaluate formalisms
for the modelling and analysis of dynamic reconfiguration.
Acknowledgments
This work is partly funded by the EPSRC under the terms of a graduate studentship.
The paper has been improved by conversations with John Fitzgerald, Cliff Jones,
Alexander Romanovsky, Jeremy Bryans, Gudmund Grov, Mario Bravetti, Massimo
Strano, Michele Mazzucco, Paolo Missier and Mu Zhou. We also want to thank mem-
bers of the Reconfiguration Interest Group (in particular, Kamarul Abdul Basit, Carl
Gamble and Richard Payne), the Dependability Group (at Newcastle University) and
the EU FP7 DEPLOY Project (Industrial deployment of system engineering methods
providing high dependability and productivity).
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