Cloud Computing 之 Service models

Boss最近接了华为的一个云计算相关的项目，于是自己的研究生生涯也就这样开始了。

其实自己真是不能称得上是一个合格的本科生，很多专业相关的名词真是仅仅停留在听过的层面上，根本没有好好去理解。

云计算这个词算是很火的了，但真没有好好去思考什么是云计算。

下面这张图算是很好解释了什么叫做云计算（自己的理解哈）：把服务放在云端，用户共享云端的资源，所有拥有权限的用户甚至可以同时地使用这些资源。云服务内部的管理和维护对用户而言是透明的，用户无需了解云计算服务内部的调度处理，这样一来方便了用户在实际中的操作，一切都在云端完成。

Cloud computing metaphor: For a user, the network elements representing the provider-rendered services are invisible, as if obscured by a cloud.

上图中也说明了云服务一般是3种模式的：Application、Platform以及Infrastructure. 

那么就详细介绍吧。

Infrastructure as a service (IaaS)[edit]

See also: Category:Cloud infrastructure and Software-defined application delivery

In the most basic cloud-service model & according to the IETF (Internet Engineering Task Force), providers of IaaS offer computers – physical or (more often) virtual machines – and other resources. (A hypervisor, such as Xen, Oracle VirtualBox, KVM, VMware ESX/ESXi, or Hyper-V runs the virtual machines as guests. Pools of hypervisors within the cloud operational support-system can support large numbers of virtual machines and the ability to scale services up and down according to customers' varying requirements.) IaaS clouds often offer additional resources such as a virtual-machine disk image library, raw block storage, and file or object storage, firewalls, load balancers, IP addresses, virtual local area networks (VLANs), and software bundles.^[50] IaaS-cloud providers supply these resources on-demand from their large pools installed in data centers. Forwide-area connectivity, customers can use either the Internet or carrier clouds (dedicated virtual private networks).

To deploy their applications, cloud users install operating-system images and their application software on the cloud infrastructure. In this model, the cloud user patches and maintains the operating systems and the application software. Cloud providers typically bill IaaS services on a utility computing basis: cost reflects the amount of resources allocated and consumed.^[51][52][53]

Iaas模式提供的主要是实实在在的物理层面的设施，例如物理机、存储、负载均衡器等等。这些是比较具象的东西，比如说你需要搭建一个集群，自己物理机不够，就算是可以创建不少的虚拟机，但是对于大集群而言根本就无法满足，此时的云服务就提供了虚拟机和物理机来为用户解决这样的问题。Iaas模式是比较好理解的，感觉就像是把服务器移到了云端，然后是共享的、不要需要用户亲自管理维护，其它方面感觉和远程机房里摆着的服务器没太大的区别。

Platform as a service (PaaS)[edit]

Main article: Platform as a service

See also: Category:Cloud platforms

In the PaaS models, cloud providers deliver a computing platform, typically including operating system, programming language execution environment, database, and web server. Application developers can develop and run their software solutions on a cloud platform without the cost and complexity of buying and managing the underlying hardware and software layers. With some PaaS offers like Microsoft Azure and Google App Engine, the underlying computer and storage resources scale automatically to match application demand so that the cloud user does not have to allocate resources manually. The latter has also been proposed by an architecture aiming to facilitate real-time in cloud environments.^[54]

Paas模式就好像是一个应用，有客户端和服务器，客户端在本地，服务器在云端，然后云端提供这样的一个平台，你可以将自己的应用在本地启动，应用部署在云上，应用的后台数据处理都是在云端完成。其实平台也可以理解为接口，Paas模式的服务就是提供给用户调用云服务的接口。实在不好理解就拿 Web Server 来举例吧，将原有的 Tomcat 服务器换成是云端的服务器，很好理解啦。

Paas模式和Iaas模式的区别大概在于Iaas模式提供的服务是偏硬件、实际设施的，Paas模式更像是软件但又不这么软件的服务。打个比方，Iaas模式是实实在在的物理机，有处理器、有存储，那么Paas模式就是操作系统。

Software as a service (SaaS)[edit]

Main article: Software as a service

In the business model using software as a service (SaaS), users are provided access to application software and databases. Cloud providers manage the infrastructure and platforms that run the applications. SaaS is sometimes referred to as "on-demand software" and is usually priced on a pay-per-use basis. SaaS providers generally price applications using a subscription fee.

In the SaaS model, cloud providers install and operate application software in the cloud and cloud users access the software from cloud clients. Cloud users do not manage the cloud infrastructure and platform where the application runs. This eliminates the need to install and run the application on the cloud user's own computers, which simplifies maintenance and support. Cloud applications are different from other applications in their scalability—which can be achieved by cloning tasks onto multiple virtual machines at run-time to meet changing work demand.^[55] Load balancers distribute the work over the set of virtual machines. This process is transparent to the cloud user, who sees only a single access point. To accommodate a large number of cloud users, cloud applications can be multitenant, that is, any machine serves more than one cloud user organization.

The pricing model for SaaS applications is typically a monthly or yearly flat fee per user,^[56] so price is scalable and adjustable if users are added or removed at any point.^[57]

Proponents claim SaaS allows a business the potential to reduce IT operational costs by outsourcing hardware and software maintenance and support to the cloud provider. This enables the business to reallocate IT operations costs away from hardware/software spending and personnel expenses, towards meeting other goals. In addition, with applications hosted centrally, updates can be released without the need for users to install new software. One drawback of SaaS is that the users' data are stored on the cloud provider's server. As a result, there could be unauthorized access to the data. For this reason, users are increasingly adopting intelligent third-party key management systems to help secure their data.

自己的理解吧：软件和支持软件的后台都是在云端完成的，连软件本身的安装和维护过程都省去了。

可能是刚接触这方面的知识，要是理解不正确的话今后再一点点修改。