Databases Reference
In-Depth Information
While this example is an
intra-instance
SOD constraint, more complex
constraints specified over multiple workflow instances, called
inter-instance
constraints may be necessary. Broadly speaking, SOD constraints can be cat-
egorized as follows.
Intra-instance
constraints [18, 10, 9] are specified on a workflow schema
and therefore apply to a single instance. While some of these constraints
can be enforced at the time of workflow schema specification, others can
only be enforced at run-time. Based on this criteria, these can be catego-
rized as follows.
-
•
Static constraints:
These constraints can be evaluated without execut-
ing the workflow. Examples of such constraints include: (i) At least
three roles must be involved in executing the workflow. (ii) The same
role must execute tasks
T
1
and
T
2
.
-
Dynamic constraints:
These constraints can be evaluated only during
the execution of a workflow, because they express restrictions based on
the execution history of an instance of the workflow. If John belongs
to role
R
1
and has performed task
T
1
, then he cannot perform
T
2
.
The constraint mentioned above in the context of claim processing is
a dynamic constraint.
-
Hybrid constraints:
These are constraints whose satisfiability can be
partially verified without executing the workflow. An example of such
a constraint would be, task
T
2
must be executed by a role dominating
the role, which executes task
T
3
.
•
Inter-instance
constraints are specified on instances rather than on the
workflow schema. These can either be specified on
multiple instances of the same workflow that can only be enforced at run-
time, or can be specified on the history of
all
the workflow instances and
therefore are not necessarily limited to one workflow. Although the moti-
vation to recognize such constraints is to limit fraud, note that they can
also be used for the purpose of workload and resource distribution.
Several researchers have proposed constraint specification languages for
describing these types of constraints [11, 27, 12, 21, 16, 28]. Given a set of
constraints, we now need to ensure that a workflow can be executed. Specif-
ically, one needs to ensure that a workflow specification with constraints is
satisfiable, that the conditions and constraints are actually satisfied during
execution. Finally, one must ensure that given a set of task assignments, the
workflow can complete - that is there are enough users available to complete
the workflow tasks given the constraints.
In [10], Bertino et al. present a language to express different types of intra-
instance authorization constraints as clauses in a logic program, and propose
solutions to verify the consistency of the constraint specification and to assign
users and roles to tasks of the workflow in a such a way that no constraints
are violated.
