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WebSphere Scalability - Feedback

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I had asked Simon a question about typical application server configurations in a production environment
He responded (Thanks Simon):
After Simon's suggestion, I read the first 100 pages of the IBM WebSphere V4.0 Advanced Edition Scalability Redbook.
I highly recommend it for anyone with similar questions. In the spirit of the JavaRanch of giving back, and not just asking questions, here is my feedback, FWIW.
One of my initial concerns about seperating web servers and application servers was network latency. Simply, why not just keep everything on one physical server. Turns out there are several reason including competing for resources, and as Simon stated, serving up static content. However, the one that really won me over was the fact that you would tune a web server PC differently than a servlet or ejb server. Same for a database, for that matter. One example the Redbook gives concerning a database is that in some situations, you often tune the OS (Solaris) for a particular database. Another obvious reason to seperate web servers and database into different physical servers from application servers is simple ratios (i.e. probably web server to application server is typically something like 1:2 or 1:X). So seperate web servers and dbms servers are no-brainers (we are assuming an application that needs to be able to scale. For a department (small) application, a single server may work fine, and be easier to maintain). Based on the Redbook, seperating the servlet container and ejb container on different phyical servers is NOT a no-brainer. My take from the Redbook is that you may get some gains depending on your application by splitting up servlets and ejb's but usually not enough to make it worth it. I guess I would start out with the goal of not splitting them up unless a problem arose forcing it. (Anyone with different opinions?).
Another important concept mentioned in the Redbook is vertical cloning of application servers (assume I'm talking about combined servlet and ejb from this point forward) vs horizontal cloning. A WebSphere application server instance runs in a single JVM process. According to the Redbook, due to the nature of JVM and concurrency issues, it is unlikely that a single JVM on a server will be able to take full advantage of the server's processing capability. By vertically cloning instances of the same application server on a PC, you are able to scale performance. The only way to find out how many makes sense is by trial and error. Keep adding clones on the same box until you get diminishing returns. (I would be interested in other feedback about other reasons to limit vertical cloning. Maybe you only want to limit to two vertical clones per box?).
Obviously vertical cloning does not solve the problem of failover. Horizontal cloning (across phyical servers does this). So my take is to tune a single server with vertical cloning, and then use that as the model for horizontal clones. There is a good discussion in the Redbook about Server Groups (templates) and cloning, and WebSphere tools and techniques to help you with this.
That's is a very brief overview of my reading in the Redbook. I have been looking for feedback on these issues for sometime, and recommend this reading for others that are interested.
Oh yeah... great place to pick up good interview buzzwords.
vertical cloning, horizontal cloning, topology, IP spraying (Dispatcher), WAS plugin, Demilitarized zone, ....
[ April 06, 2002: Message edited by: Mike Jones ]
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