Answer: Domino for the AS/400 has been available for about two and a half years. During this period the AS/400 has become the second most popular Domino server platform. Many of the traditional strengths of the AS/400 are relevant and important when looking at a system to do Domino serving.
One of the key ones would indeed be scaleability. The NotesBench Release 5 benchmark run on the new Model 840 24-way processor gave a figure of 75,000 mail and calendaring users supported with an average response time of 276 milliseconds. If you compare this with the Model 270 AS/400 with an entry price that starts at under £7,000, then it is clear that there really is a dramatic level of scaleability across the AS/400 range.
There are other aspects of the AS/400 though that make it such an attractive option when doing Domino serving. Perhaps the most important is reliability. The most obvious reason for this is the integrated nature of OS/400 and also because the hardware and software are developed together in Rochester.
Another aspect of reliability is that the AS/400 architecture provides for application isolation which prevents errors in one application from affecting other applications or the operating system. The main aspects of this include the large portion of code within OS/400 that is error recovery code; the attributes that are associated with pointers that stop applications misusing them; and the fact that the system ensures that only the appropriate operations can be executed against a particular object type, such as a program.
OS/400 work management ensures that appropriate resources such as main storage and processing power are allocated correctly to applications in order to provide acceptable performance.
Domino has two concepts which aid reliability. One is partitioned servers - this means multiple Domino servers running in a single physical machine. This allows isolation of one workload from another, while giving the benefits of administering a single box. On AS/400 Domino uses a different subsystem for each partitioned server. The resources for Domino servers, such as main storage and processor priority, are managed like other applications running on the AS/400. Again, since processing power is priority based, this ensures workload balancing in an effective manner.
Also, on the AS/400 when Domino is started, the first program that starts is a thin managing program. This then launches the first of programs that would normally be the Domino server on other platforms - the main server task. This is a multithreaded process. It handles Notes client database access, as well as other processes such as the mail router, the process to run agents, the HTTP server process, and several others.
These other processes are all descendant processes of the managing program. If any of these fails for any reason the managing program is notified, and it then takes the Domino server down and brings it back up. In many cases this will clear up the problem . This automatic restart capability is only possible because of the AS/400's ability to protect the operating system, and the monitoring program from the effects of unexpected error conditions in the Domino server.
The other concept is Domino clustering. This is a function where business critical databases use event-driven replication of a database across Domino servers for failover and workload balancing. The Domino cluster manager maintains information about the availability of databases and server workload, and Notes clients use this to switch to another server in a cluster, when appropriate. Again, because of the application isolation supported by the AS/400, clustering partitioned servers together in a single system is much more worthwhile than would be the case on other systems where failure in one Domino server could well cause the others to fail.
There are several benefits to clustering partitioned servers within a single AS/400. Firstly the clustered partitioned servers can communicate with each other for event-driven replication much more efficiently, without having to go out to the network - and the network is freed up for other traffic. Secondly, load balancing will operate much more efficiently than it will across physically separate systems. Thirdly, all partitioned servers will have access to non-Domino data resident on the AS/400. Lastly, the deployment and management of a single system will clearly be easier.
Domino integrates with many AS/400 functions. These include a variety of ways to access the DB2 universal database, the AS/400 anymail framework, security, the system distribution directory, the Integrated File System, the Java Virtual Machine and AS/400 work management.
In summary, the Model 840 with its 12-way and 24-way processors offers immense power at the top end of the AS/400 range. Domino benefits from the power and scaleability of the new 8xx models. Domino also benefits from other AS/400 characteristics, such as the integration and reliability, and takes full advantage of these to provide important capabilities, such as clustered partitioned servers and automatic server restart.