In general, transactions in cooperative environments need to support the interactive processing of activities of long, uncertain duration, in which the competition for resources is replaced by the need to cooperate. The emphasis is on the interoperation, i.e., the semantically correct exchange of information, among concurrent activities of cooperating, possibly geographically distributed or mobile, users. In such environments, failure atomicity atomicity may be too strict, and isolation among concurrent users contradicts the need of cooperation. Hence, we need to replace these criteria by new ones which are more suitable for cooperative applications. This chapter describes the TransCoop cooperative transaction model CoAct, and its underlying paradigm of cooperative work. The CoAct model supports alternating periods of individual and joint work, and allows to exchange results consistently among cooperating users. In order to achieve this, we take a behavior-oriented view rather than a state-oriented view: the consistency of common work results is determined, based on the semantics of the operations performed to obtain these results. We formally describe the CoAct model and introduce a new correctness criteria for concurrently processed transactions, i.e., legality of histories. The exchange of information among cooperating users is based on the history merging paradigm that keeps histories legal despite operations are exchanged among users.

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