Cloud computing is the leading technology for delivery of reliable, secure, fault-tolerant, sustainable, and scalable computational services.For assurance of such characteristics in cloud systems under development, it is required timely, repeatable, and controllable methodologies for evaluation of new cloud applications and policies before actual development of cloud products. Because utilization of real testbeds limits the experiments to the scale of the testbed and makes the reproduction of results an extremely difficult undertaking, simulation may be used. CloudSim goal is to provide a generalized and extensible simulation framework that enables modeling, simulation, and experimentation of emerging Cloud computing infrastructures and application services, allowing its users to focus on specific system design issues that they want to investigate, without getting concerned about the low level details related to Cloud-based infrastructures and services.

CloudSim is an open source software under the GPL license developed in the Cloud Computing and Distributed Systems (CLOUDS) Laboratory, at the Computer Science and Software Engineering Department of the University of Melbourne. CloudSim simulation framework enables users to
1. test application in controlled and repeatable environment
2. find the system bottlenecks without the need of real clouds
3. try different configurations for developing adaptive provisioning techniques

CloudSim features include :

•  modeling and simulating large scale data centers
• modeling and simulating virtualized server hosts
•  modeling and simulating energy-aware computational resources
•  modeling and simulating federated clouds
•  inserting simulation elements dynamically
• user-defined policies for allocation of hosts to virtual machines