A data-centric concurrency control mechanism for three tier systems

Concurrency control (CC) algorithms targeted at web systems need to be different from that of traditional transactional processing systems. In web systems, transactions could be generated in a burst mode, leading to scalability problems at the server. Transactions may also suffer from network delays due to the unpredictable response time over the web. This paper proposes a CC mechanism for web-based three tier systems. In this mechanism, initial validation of the transactions is performed at the application server. The transactions that pass the initial validation are sent to the database server (DB server) for final validation. The serializability criterion is achieved by associating a data-counter with each data-item. This reduces the load on the DB server. Also, in the proposed model, the middle tier contains multiple number of application servers. A part of the database is dynamically replicated in these servers. The modifications made on the data-items are known immediately to clients and the data-items at the DB server are locked only during the final validation phase and write phase. Consequently, the model is also suitable for transactions that suffer from unpredictable delays between read and write operations. The model is scalable as it can support large number of application servers hi the middle tier. Performance studies have been carried out to depict the efficiency of the proposed model over the existing models. The proposed model is simple to implement and it performs extremely well compared to existing models when the transactions are generated in a burst mode.