Cloud computing is an attracting technology in the field of computer science. In researchers' report, it says that the cloud will bring changes to the IT industry. The cloud is changing the people life by providing users with new types of services. Users get service from a cloud without paying attention to the details. NIST gave a definition of cloud computing as a model for enabling ubiquitous, convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction.