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From Jules Gosnell <ju...@mortbay.com>
Subject Re: Effectiveness of WADI's Design and Implementation Comforted
Date Thu, 18 Oct 2007 22:37:32 GMT
ikarzali wrote:
> 
> 
> jgenender wrote:
>>
>>> Excerpt of the conclusion:
>>>
>>> "
>>> The effectiveness of the design and implementation of WADI's distributed
>>> session lookup engine and replication engine is further comforted by the
>>> observed average response times and scalability characteristics.
>>> For the considered scenarios, WADI performs better than Terracotta,
>>> which is not really surprising as...
>>>
>> If I may comment here...Without fine-grained clustering capabilities, I
>> have a hard time believing that WADI can outperform Terracotta.
>> Especially with large objects...WADI would push over the entire object
>> each time, where Terracotta would only ship the changed members.  If you
>> are going to publish the numbers you did, you probably should explain
>> what is getting pushed across.

Jeff,

from Gianny's article:

"In order to efficiently cluster the few online solutions having a 
rather big session size, WADI offers an alternative replication 
mechanism, which only replicates field updates or method executions 
against replicas."

So, WADI offers a delta-based replication route as well - but this was 
not the one under test.

>>
> 
> Interesting test of Terracotta.  I wouldn't trust any test that pegs the CPU
> at 100%.  May I suggest the following potential changes:
> 
> 1. Running Jetty, Grinder, and Terracotta on a single laptop should change. 
> Run Terracotta on its own server.  It will run faster even though it won't
> be over loopback.
> 
> 2. Run sticky and see what happens.  See, the test is not testing the same
> thing with WADI and Terracotta.  With WADI, the clustering implementation is
> configured to keep data on  a finite number of nodes.  With Terracotta, you
> have a consistent clustered view of sessions.  Since you are round-robin,
> with Terracotta every node is holding a reference to every session and as
> the sessions change, all Jetty nodes are updated with the change. 

Ari,

I know nothing about Terracotta, so would you mind spending a little 
more time comparing and contrasting architectures...

I agree that the best case scenario is fully functional stickiness - but 
  then you would simply be testing session replication - see Gianny's 
"Session Replication Test" explanation and results.

Every now and then, stickiness is not enough - for instance when you 
take down or lose a node (unless Terracotta has some form of integrated 
load-balancer which negotiates a session's new location). WADI is 
designed to work with the LCD - a loadbalancer which will just dump the 
subsequent request for a dead node anywhere in the cluster and then restick.

Thus the worst case scenario, often an interesting thing to examine, is 
not temporary, as above, but prolonged lack of stickiness in the load 
balancer.

WADI will maintain 1+numReplicas copies of the session, no matter how 
many different nodes may have to shelter the session during its lifetime.

Are you saying that each Terracotta node that shelters a session becomes 
part of its replication group (i.e. has every change to that session 
synchronously replicated to it) ? Can you expand ? Can you not place a 
limit on group size ? If I churn nodes regularly, am I not going to end 
up with a lot of unecessary replication ?

> So, round
> robin WITH WADI replication off is actually pretty much cheating because TC
> has the sessions in all nodes and WADI has them in one.

"cheating" is a bit harsh...

>  Run sticky sessions
> in your load balancer.  Then Terracotta will have the session in one node
> just like WADI.  _Then_ you will have apples-to-apples and maybe find TC
> latency to be lower and throughput higher.

As I said - see the "apples-to-apples" straight replication test results 
in the article.

> 
> I would be happy to help explain more but this use of WADI and Terracotta
> seem like you are getting opposite behaviors out of the products (full n-way
> replication with no SPoF under Terracotta versus zero replication under
> WADI) 

!! - I would hope that Terracotta doesn't always do one-to-all 
replication and should point out that my reading of "One again, the cost 
of keeping one session replica, i.e. the cost of keeping a copy of a 
session on an another node than the one currently owning the session, 
remains constant while the number of nodes in the cluster increases for 
WADI." (see the Session Migration & Replication Test" observations), 
indicates that WADI is running in a 1+1 configuration, that means it IS 
replicating - each session has 1 (configurable) backup session off-node. 
and therefore no SPoF either.

and a different test will more accurately reflect the relative
> performance of the systems.

This is an interesting thread - but I think that there is a lot of 
misunderstanding/miscommunication occurring that it would be useful to 
clear up :-)


Jules


> 
> --Ari
> 


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