【正文】 k of servers or even individual PCs interconnected in a grid. These puters run in parallel, bining the resources of each to generate superputinglike power. What, exactly, is the “ cloud” ? Put simply, the cloud is a collection of puters and servers that are publicly accessible via the Inter. This hardware is typically owned and operated by a third party on a consolidated basis in one or more data center locations. The machines can run any bination of operating systems。 he doesn’t even have to remember where the data is. All that matters is that the data is in the cloud, and thus immediately available to that user and to other authorized users. From Collaboration to the Cloud: A Short History of Cloud Computing Cloud puting has as its antecedents both client/server puting and peertopeer distributed puting. It’ s all a matter of how centralized storage facilitates collaboration and how multiple puters work together to increase puting power. Client/Server Computing: Centralized Applications and Storage In the antediluvian days of puting (pre1980 or so), everything operated on the client/server model. All the software applications, all the data, and all the control resided on huge mainframe puters, otherwise known as servers. If a user wanted to access specific data or run a program, he had to connect to the mainframe, gain appropriate access, and then do his business while essentially “ renting” the program or data from the server. Users connected to the server via a puter terminal, sometimes called a workstation or client. This puter was sometimes called a dumb terminal because it didn’t have a lot (if any!) memory, storage space, or processing power. It was merely a device that connected the user to and enabled him to use the mainframe puter. Users accessed the mainframe only when granted permission, and the information technology (IT) staff weren’t in the habit of handing out access casually. Even on a mainframe puter, processing power is limited— and the IT staff were the guardians of that power. Access was not immediate, nor could two users access the same data at the same time. Beyond that, users pretty much had to take whatever the IT staff gave them—with no variations. Want to customize a report to show only a subset of the normal information? Can’ t do it. Want to create a new report to look at some new data? You can’ t do it, although the IT staff can— but on their schedule, which might be weeks from now. The fact is, when multiple people are sharing a single puter, even if that puter is a huge mainframe, you have to wait your turn. Need to rerun a financial report? No problem— if you don’ t mind waiting until this afternoon, or tomorrow morning. There isn’t always immediate access in a client/server environment, and seldom is there immediate gratification. So the client/server model, while providing similar centralized storage, differed from cloud puting in that it did not have a usercentric focus。 they’ re stored on a collection of servers accessed via the Inter. Anyone with permission can not only access the documents, but can also edit and collaborate on those documents in real time. Unlike traditional puting, this cloud puting model isn’ t PCcentric, it’ s documentcentric. Which PC you use to access a document simply isn’ t important. But that’ s a simplification. Let’ s look in more detail at what cloud puting is— and, just as important, what it isn’ t. What Cloud Computing Isn’ t First, cloud puting isn’t work puting. With work puting, applications/documents are hosted on a single pany’ s server and accessed over the pany’ s work. Cloud puting is a lot bigger than that. It enpasses multiple panies, multiple servers, and multiple works. Plus, unlike work puting, cloud services and storage are accessible from anywhere in the world over an Inter connection。 with work puting, access is over the pany’ s work only. Cloud puting also isn’ t traditional outsourcing, where a pany farms out (subcontracts) its puting services to an outside firm. While an outsourcing firm might host a pany’ s data