0.3 What is a distributed system?  (Page 3/4)

Special challenges for distributed computing

We are now moving from having the number of computers working on a problem being small enough to fit in a building or two to having tens of thousands of computers working on single problems. This brings many special challenges that mark distributed computing as being a vastly more complex enterprise than what has gone before.

The first of these special problems is security. The first aspect of security is the security of the computers themselves, since few people feel like giving some wannabe digital mobster a free pass to misuse their computers. The second aspect is the security of the data being processed, much of which may be highly confidential or a trade secret (e.g., individual patients’ medical data, or the designs for products under development). The third aspect of security is the security of the messages used to control the other computers, which are often important in themselves and could be used to conduct a wide range of other mischief if intercepted.

The second special problem of distributed computing is due to the use of systems owned by others, either other people or other organizations. The issues here are to do with the fact that people ultimately retain control over their own systems; they do not like to cede it to others. This behaviour could be considered just a matter of human nature, but it does mean that it is extremely difficult to trust others in this space. The key worries relate to either the computer owner lying about what actions were taken on their systems (for pride, for financial gain, for spite, or just out of straight ignorance) or the distributed computer user using the system for purposes other than those that the computer owner wants to permit.

Interoperability presents the third challenge for distributed computing. Because the systems that people use to provide large-scale computing capabilities have grown over many years, the ways in which they are accessed are quite diverse. This does mean that a lot of effort has been put into finding out access methods that balance the need for efficiency with those of security and flexibility, but it also means that frequently it is extremely difficult to make these systems work together as one larger system. Past attempts by hardware and software vendors to lock people in to specific solutions have not been helpful here either; researchers and practitioners want to solve a far more diverse collection of challenges than the vendors have imagined there to be. After all, the number of things that people wish to do is limited only by the human imagination.

These challenges can be surmounted though, even if the final form of the solutions is not yet clear. We know that the demands of security can be met through a combination of encryption, digital signatures, firewalls, and placing careful constraints on what can be done by any program. The second challenge is being met through the use of techniques from digital commerce like formal contracts, service level agreements, and appropriate audit and provenance trails. The third, which will become ever more important as the size of problems people wish to tackle expands, is primarily dealt with through standardization of both the access mechanisms (whether for computation or for data) and the formal understanding of the systems being accessed by common models, lexicons and ontologies.