A cloud service is an execution model azure offers to execute an application. Similar to Web Sites execution model, the cloud service is also a PaaS offering of Microsoft.
Typical cloud applications contain web roles and worker roles and both these roles are VMs. All these VMs of a particular application can reside in a container called cloud service.
Windows Azure supports two types of roles, web role and a worker role. A web role instance is a Virtual Machine created by the Fabric controller, which contains windows OS and IIS.
A worker role instance is also a VM created by the Fabric controller and contains a copy of the Windows OS. A web role (VM) can host the web application within IIS and is a front end of the application and takes the requests from the user.
Why do we create multiple instances of web roles?
Multiple instances of each web role and worker roles is for scalability and availability.
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