Defining Cloud Computing
Cloud computing refers to both the applications delivered as services over the Internet and the hardware and systems software in the data centers that provide those services. The services themselves have long been referred to as Software as a Service (SaaS). Some vendors use terms such as IaaS (Infrastructure as a Service) and PaaS (Platform as a Service) to describe their products, but we eschew these because accepted definitions for them still vary widely. The line between “low-level” infrastructure and a higher-level “platform” is not crisp. We believe the two are more alike than different, and we consider them together. Similarly, the related term “grid computing,” from the high-performance computing community, suggests protocols to offer shared computation and storage over long distances, but those protocols did not lead to a software environment that grew beyond its community.
通过网络提供软硬件服务可以广泛地定义云计算。云计算和网格计算有相似之处，他们都能提供高性能的远程计算力；但是网格计算相关协议的制定没能让其长远的发展（没有像云计算那样被人们所熟悉，其原因可能有商业或其他分歧）。从这可以初步了解其实云计算可以被归类为分布式计算。
The data center hardware and software is what we will call a cloud. When a cloud is made available in a pay-asyou-go manner to the general public, we call it a public cloud; the service being sold is utility computing. We use the term private cloud to refer to internal data centers of a business or other organization, not made available to the general public, when they are large enough to benefit from the advantages of cloud computing that we discuss here. Thus, cloud computing is the sum of SaaS and utility computing, but does not include small or mediumsized data centers, even if these rely on virtualization for management. People can be users or providers of SaaS, or users or providers of utility computing. We focus on SaaS providers (cloud users) and cloud providers, which have received less attention than SaaS users. Figure 1 makes provider-user relationships clear. In some cases, the same actor can play multiple roles. For instance, a cloud provider might also host its own customer-facing services on cloud infrastructure.
数据中心的软硬件被称作云（cloud）；当云通过网络以服务的形式灵活地（比如说按时收费）被售卖给大众时，就被称作共有云。相反的，不被售卖的被企业或私有组织享有的云称为私有云。云计算是大型云提供的软件服务及其相关计算服务的总称。
From a hardware provisioning and pricing point of view, three aspects are new in cloud computing.
- The appearance of infinite computing resources available on demand, quickly enough to follow load surges, thereby eliminating the need for cloud computing users to plan far ahead for provisioning.
- The elimination of an up-front commitment by cloud users, thereby allowing companies to start small and increase hardware resources only when there is an increase in their needs.b
- The ability to pay for use of computing resources on a short-term basis as needed (for example, processors by the hour and storage by the day) and release them as needed, thereby rewarding conservation by letting machines and storage go when they are no longer useful.
Types of cloud
通过网络提供的云服务可以从底层硬件到上层软件的角度来分。
Infrastructure as a Service (IaaS), 基础设施即服务
- 提供原生的计算机基础建设作为服务，一般通过平台（操作系统）虚拟化实现。
- 供应商管理网络，硬件存储，虚拟化和操作系统等，比如说 Amazon Web Service。
Platform as a Service (PaaS), 平台即服务
- 提供底层框架（应用开发环境）作为服务。
- 用户可以在框架之上开发、搭建应用，例如Window Azure
Software as a Service (SaaS),软件即服务
- 提供的软件作为服务
- Office 365
References
A View of Cloud Computing
https://s3.amazonaws.com/academia.edu.documents/34578652/a_view_of_cc.pdf?response-content-disposition=inline%3B%20filename%3DA_View_of_Cloud_Computing_Clearing_the_c.pdf&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIWOWYYGZ2Y53UL3A%2F20200116%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20200116T154217Z&X-Amz-Expires=3600&X-Amz-SignedHeaders=host&X-Amz-Signature=5ac29401ea51eaef12f19f063caac5b43de4d2e2a43e58c287d88719bacdd0f2
Data-intensive Systems slides of University of St Andrews
