When you type http://www.pcmech.com in your browser’s address bar, how does the page appear on your screen?
Let us start with a familiar analogy – the telephone. Calling is the easiest way to talk to a friend. To place a call, you must have a telephone connection and a phone number. How do you get a telephone connection and a number? You call your local authority, the telephone company, and ask to be connected to the telephone network. The telephone company verifies your name and street address, and allots you a unique number on the telephone network – your telephone number.
Once you have a connection and a phone number, you can call your friend. Naturally, he must have a telephone connection too. Even then, you can’t dial his name; you must know his telephone number. In other words, you must be able to cross-reference (or, in geek-speak, resolve) his number if you know his name and street address.
The ideal situation is that you know his number off the top of your head so you can do the resolution in your head. But you possibly can’t remember everybody’s telephone numbers. A viable solution is to write the names and phone numbers of people you are likely to call, in a little notebook. You could then open your notebook and dial the number against your friend’s name.
If your notebook does not have an entry for your friend, you have to look it up in an authoritative cross-reference. The authority for telephone numbers is your telephone company. It provides such a cross-reference – the telephone directory.
Your telephone directory does not have entries for all the people in the world who are on the telephone network; it would get incredibly thick. To remain of manageable size, your directory only includes people on the telephone network who live in your local area. If your friend happens to live in your local area, you could open the local telephone directory, find your friend’s name, and dial the number listed against it.
What if your friend lives in another town? You will not find his name and address in your local telephone directory; he would be listed in his town’s local telephone directory. You have to look for his telephone number in that directory. The problem, however, is that you would have to have a directory from his town handy. In general, to call any person at random, you would have to stock a directory from every town.
If you are like me, you already have plenty of junk in your house. You probably will not find the idea of storing hundreds of directories in your living room particularly appealing. Would it not be great if you could just ask someone? For 50 cents or so, you can. You can call a directory enquiry service like 411.
Unless your friend is a celebrity, the 411 operator in your town is unlikely to be acquainted with him. Therefore you must provide her your friend’s full name and address – say
John Doe
23 Washington Avenue,
Jacksonville, FL
A modern-day operator simply searches for the phone number at that address on her computer and gives it to you. But in pre-historic days, the operator either had to store all the telephone directories herself or call someone else who had a copy of the relevant directory. That someone, in your friend’s case, was an operator in the telephone exchange in Jacksonville, FL. All your local operator had to do was to know the Jacksonville exchange’s number. She then called Jacksonville exchange and provided the operator there with the information you provided her. The Jacksonville operator looked the number up in his local directory and gave it to your local operator. She, in turn, passed it on to you.
If you are a meticulous person, you would write the number down in your little notebook for future use. On the other hand, if you are like most people, you would write it down on the first piece of paper you lay your hands on and promptly lose it. If you wanted to call your friend again, you would have to call the operator in your town again and she would have to repeat the whole exercise with the operator in Jacksonville. After a couple of such round trips, she would get tired of you and write the number down in her little notebook so that the next time you called, she would have it handy.
Storing of information for quick retrieval is called “cache”ing. Notice that information can be cached at various stages. You could cache the number in your notebook; your local operator could cache it in her notebook; the operator in Jacksonville could cache it in his notebook.
This system works well because no one has to store hundreds of telephone directories. The only information each person in the chain must have is
1. A copy of the local telephone directory
2. Phone numbers of one or more authoritative sources that would be able to find a number not present in her copy of the local telephone directory.
In this example, your authoritative source is your local operator. Her authoritative source is the operator in Jacksonville who has access to the local phonebook in Jacksonville. If your friend lived in another country, the chain would simply be longer and involve more operators
In addition, each person would do well to have a little notebook (cache) to write down often referenced numbers. The cache is not essential for the system to function; it merely makes retrievals faster.
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