| DCOM Explained by Rosemary Rock-Evans Digital Press ISBN: 1555582168 Pub Date: 09/01/98

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### In Summary

So overall are Microsoft’s Directory services available, secure, reliable, and high performance? As we saw, I believe the current Registry service has its failings. Availability should not be a problem, but security, reliability, and performance may be.

There is a fairly high administrative overhead involved in maintaining the Registry information, and as a consequence, errors may creep in and reliability suffer. Furthermore, although security checks can be applied to the Registry to prevent unauthorized access, the fact that these have to be set up on each node for each Registry is tedious and prone to error.

We also saw that performance may not be that good because where remote processes are to be accessed, the address has to be found by searching the Registries of other nodes. A further overhead is incurred because accesses are disk, not memory, based.

Given that the current Registry has these weaknesses, the Active Directory is a most important development and one that will be key to whether DCOM can provide enterprise level support or not.

Overall, the initial plans look promising. Active Directory has the potential to provide good performance and availability, as all the information needed for an application will be stored on the node—no network access is needed to obtain the data and contention because the shared Directory resource is lessened. At the time of writing, there were no plans for the data to be held in memory and loaded upon system startup from disk, so performance may suffer slightly here, but there may be good reasons for this approach.

We saw that if the configuration is an extremely large one with thousands of nodes, the memory needed to hold the entire configuration may actually be too great for smaller machines. On the whole, Microsoft is geared towards support for smaller machines, and as such a solution is needed to cover this problem. The alternative solution—that reliant on the single file solution approach (a set of clients is given a configuration file that shows where their nearest Directory service node or nodes is based)—may have been deemed unsatisfactory. In this case, a directory based solely on disk is an attractive alternative, even though it has performance implications.

Active Directory looks as though it will also be reliable. The administrator will only have to set up and create one master file; he or she will not have to set up numerous files on each node manually. If publish and subscribe is added to the services, reliability will further improve.

Security should also be easy to maintain as, in effect, no one will be able to look or change the data in the replicates as these are managed by the middleware itself. Restrictions on access can be applied to the master copy—and are more likely to be applied.

Altogether then, Active Directory looks to be a very promising development. Its failings?

One failing is that a question mark hangs over the Directory’s ability to handle load balancing. If each component/class is allowed only one address, then load balancing will not be possible as the component will only be capable of being stored once on the network (as it can only have one address). As the new Active Directory evolves it will be important to see how the final architecture copes with this problem.

Another more obvious failing is that its introduction is likely to cause considerable disruption, affecting as it does not only DCOM services such as MSMQ, SCM, and the Administration routines but many of Microsoft’s other products. It is worth remembering, for example, that at the moment, MSMQ has its own Directory and uses the Registry to store queues, so considerable adaptation may be needed to merge MSMQ’s Directory with Active Directory and devise a new way of supporting queues.

Similarly, all the developments I have described are currently plans, not a product, so we are all going to have to wait for it to appear. This is no simple development that Microsoft has undertaken, and time scales could also well slip.

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