Multiuser web applications often need to manage concurrent (and potentially conflicting) operations on shared data. The parallel processing of web requests and the disconnected client-side view on that shared data raise ample opportunities for users to make conflicting changes. When such conflicts arise, we may or we may not need to inform the user of the conflict. To this end, database transaction mechanisms are a performant and proven technology for detecting and managing conflicting changes. When each user's web session maintains its isolated view on the database and operations are wrapped in transactions, we are able to prevent, detect and handle all such conflicts. At the same time, however, such applications easily become dependent on a particular database's technical abilities. Even when only considering the object-oriented databases GOODS, Magma and Gemstone, the differences in how database views and transactions work are vast. For example, Magma supports nested transactions while GOODS does not. An even more important difference is that when using Magma and GOODS, we can maintain a separate database view per web session and rely on the transaction mechanism to handle all conflicts. In Gemstone's GLASS integration of Seaside, however, incoming requests always share the same database view. This means only parallel execution conflicts are trapped by the transaction mechanism. With DALi, a database abstraction layer, we build Seaside-based web applications that are database independent and rely on database transactions to manage inter-user application-level conflicts as well as parallel execution conflicts directly. DALi is currently implemented for Pharo Smalltalk (with GOODS, Magma or image-based backends) and GemStone GLASS.
Johan is founder of Inceptive.be, the Smalltalk startup who is the technological partner in the development of Yesplan, an innovative next-generation event-planning system built with Seaside. He currently devotes all his time as the software architect of 2Rivers, the joint venture created around Yesplan by Inceptive and the cultural centre 'Vooruit'. Johan has more than 12 years of experience in Smalltalk, which he started to use as a PhD researcher for the development of research tools that focus on advanced software engineering techniques. He obtained a PhD in computer science from the Vrije Universiteit Brussel in 2005 and is a (co-)author of open-source projects in Smalltalk such as Javaconnect, Soul and IntensiVE. Since 2009, he is a full-time software development professional.