Книга: Distributed operating systems

10.1.4. Cells

10.1.4. Cells

Users, machines, and other resources in a DCE system are grouped together to form cells. Naming, security, administration, and other aspects of DCE are based upon these cells. Cell boundaries usually mirror organizational units, for example, a small company or a department of a large company might be one cell.

When determining how to group machines into cells, four factors should be taken into consideration:

1. Purpose.

2. Security.

3. Overhead.

4. Administration.

The first factor, purpose, means that the machines in a cell (and their users) should all be working toward a common goal or on a common long-term project (probably measured in years). The users should know each other and have more contact with each other than with people outside the cell. Departments in companies are often structured this way. Cells can also be organized around a common service offered, such as online banking, with all the automated teller machines and the central computer belonging to a single cell.

The second factor, security, has to do with the fact that DCE works better when the users in a cell trust each other more than they trust people outside the cell. The reason for this is that cell boundaries act like firewalls — getting at internal resources is straightforward, but accessing anything in another cell requires the two cells to perform an arms' length negotiation to make sure that they trust one another.

The third factor, overhead, is important because some DCE functions are optimized for intracell operation (e.g., security). Geography can play a role here because putting distant users in the same cell means that they will often have to go over a wide-area network to communicate. If the wide-area network is slow and unreliable, extra overhead will be incurred to deal with these problems and compensate for them where possible.

Finally, every cell needs an administrator. If all the people and machines in a cell belong to the same department or project, there should be no problem appointing an administrator. However, if they belong to two widely separated departments, each with its own czar, it may be harder to agree upon one person to administer the cell and make cell-wide decisions.

Subject to these constraints, it is desirable to have as few cells as possible to minimize the number of operations that cross cell boundaries. Also, if an intruder ever breaks into a cell and steals the security data base, new passwords will have to be established with every other cell. The more of them there are, the more work is involved.

To make the idea of cells clearer, let us consider two examples. The first is a large manufacturer of electrical equipment whose products range from aircraft engines to toasters. The second is a publisher whose books cover everything from art to zoos. Since the electrical manufacturer's products are so diverse, it may organize its cells around products, as shown in Fig. 10-2(a), with different cells for the toaster group and the jet engine group. Each cell would contain the design, manufacturing, and marketing people, on the grounds that people marketing jet engines need more contact with people manufacturing jet engines than they do with people marketing toasters.


Fig. 10-2. (a) Two cells organized by product. (b) Three cells organized by function.

In contrast, the publisher would probably organize the cells as in Fig. 10-2(b) because manufacturing (i.e., printing and binding) a book on art is pretty much like manufacturing a book on zoos, so the differences between the departments are probably more significant than the differences between the products.

On the other hand, if the publisher has separate, autonomous divisions for childrens' books, trade books, and textbooks, all of which were originally separate companies with their own management structures and corporate cultures, arranging the cells by division rather than function may be best.

The point of these examples is that the determination of cell boundaries tends to be driven by business considerations, not technical properties of DCE.

Cell size can vary considerably, but all cells must contain a time server, a directory server, and a security server. It is possible for the time server, directory server, and security server all to run on the same machine. In addition, most cells will either contain one or more application clients (for doing work) or one or more application servers (for delivering a service). In a large cell, there might be multiple instances of the time, directory, and security servers, as well as hundreds of applications client and servers.

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