Design applications, e.g., CAD or media production, often require multiple users to work cooperatively on shared data, e.g., XML documents. Using explicit transactions in such environments is difficult, because designers usually do not want to consider transactions or ACID. However, applying transactions in order to control visibility of changes or specify recovery units, is reasonable, but determining transaction boundaries must be transparent for the designer. For this reason we propose a novel approach for the automatic determination of transaction boundaries which considers the degree of cooperation designers want to achieve. Furthermore, we present an optimistic synchronization model based on the traditional backward oriented concurrency control (BOCC) algorithm, in order to synchronize the determined transactions in multi-user environments. It exploits the semantics of tree operations on XML data and enforces a correctness criterion weaker than serializability. As our evaluation shows, when multiple users work cooperatively on shared data, this model significantly reduces the number of transaction aborts in comparison to the traditional BOCC approach.
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