Subway - Public Cloud Computing
Bob is living in a very busy city. In order for him to go to work, either he could take the subway train or drive his own car. Bob is a recent college graduate. He neither has a lot of money nor a good credit score. Therefore, buying a car is somewhat out of his options. Besides, it is really time consuming to drive in the city to commute. So, Bob opts to take the subway to work. This is exactly how cloud computing works. Instead of Bob buying his own vehicle, he is using the public transportation to commute. In cloud computing, companies, without having to invest on computer hardware, rent hardware or software in the cloud.
When Bob takes the subway, he shares the ride with many other commuters. Similarly, many organizations share the computing infrastructure made available through cloud computing.
Bob pays for his subway ride everyday. He pays only when he takes it and only for the distance he travels. For example, Alice, who is also a subway commuter, has to pay more than Bob as she travels longer distance than Bob. Likewise, with cloud computing, companies pay only for the resources they use. If a company uses more resources than another, they have to pay more for the cloud provider.
Subway trains are available 24x7. Bob can catch a train virtually any time he wants. Similarly, cloud is available 24x7.
Trains have more compartments during peak hours than the normal hours. So, Bob does not have to worry about getting on-board anytime he wants to take a train. Subway accommodates all passengers catering for varying demands through out the day. The cloud is also like that. The resources in the cloud can expand or shirk based on the demand. We call it elasticity. Elasticity is a very useful property that lets businesses to timely respond to their customers for varying amounts of demand through out the year.
If Bob used his own vehicle, he needs to regularly maintain his car on his own and repair any damages. Since Bob is using the public transport, he does not have to worry about any of that. Similarly, companies can have the peace of mind that they don't have to worry about maintaining computer hardware and having a dedicated staff to look after them.
When Bob boards in to the train, he gets a seat to sit and travel peacefully. Usually, everyone respects each other and does not invade others' space. Bob can browse his smart phone or read a book without getting unnecessary peaking from others. Similarly, even though the cloud is public, companies get their own bounded space to do their business so that their data is not mixed with other companies.
Usually subways have one or more train operators and ticket security officers. They make sure that Bob and other passengers travel safely as well as catch bad people who misuse it. Similarly, the cloud provides a several security measure to make sure that your business is safe and secure.
In summary, a public cloud is a shared resource, paid-per-usage, elastic, 24x7 available, transparent, virtually isolated, and secure.