Storage and Retrieval in the Physical World

We can own a book or a hammer without giving it a name or a permanent place of residence in our houses. A book can be identified by characteristics other than a name—a color or a shape, for example. However, after we accumulate a large number of items that we need to find and use, it helps to be a bit more organized.

Everything in its place: Storage and retrieval by location

It is important that there be a proper place for our books and hammers, because that is how we find them when we need them. We can't just whistle and expect them to find us; we must know where they are and then go there and fetch them. In the physical world, the actual location of a thing is the means to finding it. Remembering where we put something—its address—is vital both to finding it, and putting it away so it can be found again. When we want to find a spoon, for example, we go to the place where we keep our spoons. We don't find the spoon by referring to any inherent characteristic of the spoon itself. Similarly, when we look for a book, we either go to where we left the book, or we guess that it is stored with other books. We don't find the book by association. That is, we don't find the book by referring to its contents.

In this model, which works just fine in your home, the storage system is the same as the retrieval system: Both are based on remembering locations. They are coupled storage and retrieval systems.

Indexed retrieval

This system of everything in its proper place sounds pretty good, but it has a flaw: It is limited in scale by human memory. Although it works for the books, hammers, and spoons in your house, it doesn't work at all for the volumes stored, for example, in the Library of Congress.

In the world of books and paper on library shelves, we make use of another tool to help us find things: the Dewey Decimal system (named after its inventor, American philosopher and educator John Dewey). The idea was brilliant: Give every book title a unique number based on its subject matter and title and shelve the books in this numerical order. If you know the number, you can easily find the book, and other books related to it by subject would be near by—perfect for research. The only remaining issue was how to discover the number for a given book. Certainly nobody could be expected to remember every number.

The solution was an index, a collection of records that allows you to find the location of an item by looking up an attribute of the item, such as its name. Traditional library card catalogs provided lookup by three attributes: author, subject, and title. When the book is entered into the library system and assigned a number, three index cards are created for the book, including all particulars and the Dewey Decimal number. Each card is headed by the author's name, the subject, or the title. These cards are then placed in their respective indices in alphabetical order. When you want to find a book, you look it up in one of the indices and find its number. You then find the row of shelves that contains books with numbers in the same range as your target by examining signs. You search those particular shelves, narrowing your view by the lexical order of the numbers until you find the one you want.

You physically retrieve the book by participating in the system of storage, but you logically find the book you want by participating in a system of retrieval. The shelves and numbers are the storage system. The card indices are the retrieval system. You identify the desired book with one and fetch it with the other. In a typical university or professional library, customers are not allowed into the stacks. As a customer, you identify the book you want by using only the retrieval system. The librarian then fetches the book for you by participating only in the storage system. The unique serial number is the bridge between these two interdependent systems. In the physical world, both the retrieval system and the storage system may be very labor intensive. Particularly in older, non-computerized libraries, they are both inflexible. Adding a fourth index based on acquisition date, for example, would be prohibitively difficult for the library.