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From "gianny DAMOUR" <>
Subject Re: Web State Replication... (long)
Date Fri, 31 Oct 2003 01:27:19 GMT
Jules Gosnell wrote:
>>I see. I understand why you need to tell your lb about migrations and, if 
>>I understand it correctly, this is mainly an optimization. However, you 
>>are not compeled to keep the lb aware of migrations:
>?? - If you don't, it will continue to deliver requests to the node that 
>the session used to be at, not the node that it is currently at. This is 
>bad news. You will either have to forward/redirect (clumsy), migrate the 
>session back to this node (wasted cycles/bandwidth/time) or continue 
>without the session (failure).
No: it tries the primary. The primary is not available. It then tries 
another available server. This server can be the secondary or not. One does 
not care about the migration process because this available server will 
request to the secondary the session attached to the incoming request. At 
the end of the day, the lb is aware only of the available servers.

>>I assume that in the cookies or in the URL will be embedded the 
>>identifiers of the replica. If the primary node fails, the lb can try any 
>>available servers. This server is part of the domain and by using the 
>>cookies or the URL of the incoming request, it can request to the replica 
>>the session attached to the incoming request.
>If you had a loadbalancer that did this sort of thing, then there is no 
>reason why you shouldn't encode a 'failover-list' into the session id, or 
>extra cookie/url-param etc.... - I believe that this is a strategy use by 
>WebLogic, but they have a custom Apache module... You might be able to 
>extend e.g. mod_backhand to do this, or perhaps more major surgery on 
>another module might achieve it...
Indeed, WebLogic works like this in the case of a soft lb. In the case of an 
hardware load-balancing, WebLogic implements what I have just described: 
send to an available server and then this server is in charge of requesting 
the session to the secondary.

>Hope that helps,
Thanks for that. I start to understand the design.


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