Cloud computing

1.定義 by NIST(2011)
	A model for enabling convenient, on-demand network access to a shared pool of configurable computing resources. It can be rapidly provisioned and released with minimal management effort or service provider interaction. It promotes availability. It consists of five essential characteristics, three service models, and four deployment models.
2.基本特徵
	有五個基本特徵
2.1 On-demand self-service
	Users are able to provision, monitor and manage computing resources as needed without the help of human administrators
2.2 Broad network access
	Computing services are delivered over standard networks and heterogeneous devices
2.3 Resource pooling
	IT resources are shared across multiple applications and tenants in a non-dedicated manner
2.4 Rapid elasticity
	IT resources are able to scale out and in quickly and on an as needed basis
2.5 Measured service
	IT resource utilization is tracked for each application and tenant, typically for public cloud billing or private cloud chargeback
3. 三個service model
3.1 IaaS
	The capability provided to the consumer is to use the provider’s applications running on a cloud infrastructure.(Ex. Google docs)
3.2 PaaS
	The capability provided to the consumer is to deploy onto the cloud infrastructure consumer-created or -acquired applications created using programming languages and tools supported by the provider. (Ex. Google App, Amazon S3, Windows Azure)
3.3 SaaS
	The capability provided to the consumer is to provision processing, storage, networks, and other fundamental computing resources where the consumer is able to deploy and run arbitrary software, which can include operating systems and applications. (Ex. Amazon EC2)
4. 四個佈署模式
4.1 Private cloud
	The cloud infrastructure is operated solely for an organization.
4.2 Community cloud
	The cloud infrastructure is shared by several organizations and supports a specific community that has shared concerns.
4.3 Public cloud
	For use by multiple organizations (tenants) on a shared basis and hosted and managed by a third party service provider .
4.4 Hybrid cloud
	The cloud infrastructure is a composition of two or more clouds.
5. The General Statements For Cloud Computing
5.1 Network dependency
	If the network is not reliable, the cloud will not be reliable from the subscriber's point of view.
5.2 Subscribers still need IT skills
	subscribers will still access the cloud from onsite subscriber-managed client systems that must be maintained, secure, etc.
5.3 Workload locations are dynamically assigned and are thus hidden from clients
	providers must be able to migrate subscriber workloads between machines without inconveniencing the clients.
5.4 Risks from multi-tenancy
	The workloads of different clients may reside concurrently on the same system and local network.
5.5 Data import/export, and performance limitations
	subscribers access a cloud over a network, on-demand bulk data import or export may exceed the network's ability to carry the data in a timely manner.
6. The Key-Technologies of Cloud Computing
	Virtualization. Utility Computing. Parallel & Distributed Computing(Grid). Fault-Tolerant Computing. Automatic Systems Computing. Client-server computing. Web technique.