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Is it possible to have a redundant cache server

Posted on 2012-03-14
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A cache server in Linux is typically used in front of a cluster. When a server in a cluster drop out, the cache server will determine which server in the cluster to use.
Question1:
Is it possible to use 2 cache servers in Linux (redundancy)


In Lotus Domino a cache server is called a ICM (internet cluster manager). It is a manager running on the Domino server.  Well it is not a cache, but is is used in the same manner as a Linux cluster front.
Question2:
Is it possible to use 2 ICMs in front of a Domino web server running in a Domino cluster.

It should not be possible, but the Domino Administrator manual suggest a solution. It seems pretty weered, and difficult to understand the thinking behind such a solution. Study the pictures below, and tell me how it is possible to have 2 ICMs?

From the Lotus Domino Administrator database:
Example of multiple ICMs-outside a clsuterExample of multiple ICMs inside a cluster
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Question by:tbruheim1967
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cohalex earned 500 total points
ID: 37720347
Related to question 2):

I can imagine that you would need multiple ICM servers for load balancing, in case one ICM server isn't enough to serve all of your incoming requests.
In this case, you could have an A record in your DNS system for each ICM server, resolving the same address to different ICM host names. If your DNS service is capable of round robin fetching, you have your basic load balancing done.
Round robin enabled DNS systems return all A records for a given address (the order of the entries being scrambled for each serving).

As to the high availability benefit described in the Admin manual, you would have to rely on the HTTP client to actually try the next A record in case it gets no reply from (most probably) the first one it tried ..
Are browsers actually doing that ? Seems so, check this thread:
http://webmasters.stackexchange.com/questions/10927/using-multiple-a-records-for-my-domain-do-web-browsers-ever-try-more-than-one
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Author Closing Comment

by:tbruheim1967
ID: 37720976
Awesome. Great answer. I did not have knowledge about the browser stuff you told me. Your information about DNS was also partially new stuff for me. Thank you very much.
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