The next generation of cloud computing must be reliable to fulfill dynamically changing end-user requirements. Key factor to handle this issue is the elasticity. Elasticity represents the ability to dynamically reconfigure cloud resources to adapt to these varying resource requirements. With the dynamic nature of cloud resources and rapid change of user requirements, elasticity has to be effectively supported. This involves considering possible obstacles that can hamper the successful execution of elasticity actions. More specifically, obstacles are situations that could evolve into failures, if they are not efficiently and promptly (i.e.,~no preventive measures taken) tackled. In this paper, we propose a transactional approach to effectively support reliable cloud resource elasticity by handling obstacles. Our proposition is built on an existing cloud resource description model but with emphasis on transactional properties during the execution of elasticity actions. We rely on a proof of concept implementation to demonstrate the applicability and the feasibility of our approach