[mod_cluster-dev] clustered mod_cluster?

[Marek Goldmann]

On Oct 21, 2009, at 11:39 AM, Bela Ban wrote:

> This doesn't work currently, as the apache mod-cluster instances  
> don't exchange information about their states, e.g. which sticky  
> session is routed to which worker.
>
> No wait, I guess I'm wrong: they don't need to !
>
> Say we have Apaches A1 and A2, and workers W1, W2 and W3.
>
> If a client request hits A2 and A2 creates a new session on W3, then  
> the session id is going to be SESSION.W3. When the same client now  
> hits A1, A1 should actually also route the request to W3 ! This of  
> course assumes that all workers have registered with all Apache  
> daemons.

Yes, I'm assuming that.

> So IMO we don't even need to exchange routing information between  
> the Apache daemons... can someone confirm this ?

Good catch, I'll try this today and will let you know!

> This is with sticky session. If you turn this off, you can pick any  
> Apache to route your request, and the route will be different every  
> time. In this case, you'd have to use synchronous replication to  
> prevent potentially stale data between 2 requests for the same  
> session going to different workers.

Yes, we're talking about sticky sessions only, because those who don't  
want to have sticky sessions, don't want sessions at all, I think. In  
other words if I care about sessions, I want to have sticky sessions  
too, otherwise I'm not interested in sticky sessions. Am I wrong?

--Marek

> With Infinispan and DIST, stale data shouldn't be an issue as the  
> routing is always based on the content (keys to be accesssed) rather  
> than a client's identity (session-id), and L1 caches should make  
> things fast, so data doesn't have to be fetched by one worker from  
> another one.
>
>
>
> Marek Goldmann wrote:
>> Hi all,
>>
>> One of JBoss Cloud users asked me about possible clustering of our
>> front-end appliances. This in real is the question: can we have
>> multiple mod_cluster instances that share state between them?
>>
>> If this is possible how I can configure it? If this is not possible
>> now, is it maybe in roadmap?
>>
>> This could greatly enhance HA of web clusters. How? For example we
>> have an Elastic Load Balancer in Amazon. It doesn't support sticky
>> sessions and it just forwards requests to selected instances. If we
>> could for example forward it to two our clustered front-end  
>> appliances
>> we could achieve great failover.
>
> -- 
> Bela Ban
> Lead JGroups / Clustering Team
> JBoss