3.2 Invariant Based Synchronization 2.2 Isolation Requirements 3 Ideas and Approaches 3.1 Context -the Data Model of Contracts 2 Critical Points 2.1 Requirements for the Data Model Contracts -a Low-level Mechanism for Building General-purpose Workflow Management-systems

The isolation concept used in ConTracts is a powerful means to express the isolation needs of workflow processes. It was built to allow processes to release access restrictions on shared objects as soon as possible without violating their correctness. Therefore a new correctness criterion has been introduced which allows parallelism far beyond serializability. Furthermore, the design allows us to handle conflicts explicitly instead of reacting to a conflict by simply aborting the current transaction. Although the prototypical implementation of a ConTract system (APRICOTS) [Schw93] has been under development for more than three years now, not all features have been completely implemented yet. We are currently working on the efficient implementation of the concurrency control concepts. Additionally, we are looking at the problem on how existing transactional environments like e.g. Encina from Transarc can be enhanced to support transactional workflows like ConTracts directly. Likewise, we are looking at database systems and other " resource managers " to find out on how they can be leveraged to join a ConTract environment. state of concurrently accessed objects which in result guarantees the executability of the Compensation Step. The constraints defined after Steps are called Invariants. In detail, the mechanism works as follows: • The programmer assigns predicates with Steps in the Script which represent the Invariants • At execution time the run-time system checks at the end of the execution of a Step if the predicates are valid. If they are valid, the constraint is established and the transaction which protects the Step is allowed to commit. • If the constraint cannot be established the ConTract-Manager is informed. That means the ConTract -Manger gets information about an isolation conflict and is able to initiate actions to resolve the conflict. As introduced in the original model, Invariants can also be used to support the executability of a ConTract. For example, a Script can contain two Steps: checking a flight and the reservation of a flight. Instead of grouping those two Steps under one transaction and restricting the access on the flight database more than needed, a predicate can be established after the first Step which expresses the pre-reservation of a certain amount of flight seats. This ensures the executability of the second step without restricting the accessibility of other seats. On the other hand, this not a real Invariant of the whole ConTract because the constraint can be removed after the second Step has …