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From Jules Gosnell <ju...@coredevelopers.net>
Subject Re: WADI clustering
Date Wed, 18 Jan 2006 12:18:09 GMT
Oh Rajith - you've got me thinking :-(

I'm not happy with the last answer - lets try again....

lets agree some points :

1) since changes made to sessions are made in app-space, apps are not 
written with the expectation that a change collision may occur and the 
container would not be able to avoid such a collision, it must never happen.

2) in order for a change-collision to occur multiple concurrent 
requests/invocations must hit multiple copies of the session

In the web world, clients commonly throw multiple concurrent requests at 
clusters, however, if we could assure total affinity, these would always 
arrive at the same copy, avoiding the chance of a collision. There are 
various situations within the web tier that may cause the breakdown of 
affinity. Different loadbalancers handle these situations with varying 
degrees of correctness. I have decided that it is safer to assume that, 
whilst affinity is a substantial optimisation, it cannot be relied on 100%.

So, in the web tier, it is possible for concurrent requests for the same 
session to arrive at different session copies. So we need a pessimistic 
distributed locking strategy to ensure that collisions do not occur.

In the EJB world, we have more control over the load-balancer, because 
it is effectively built into the proxy  that we supplied, and we could 
enforce the serial nature of invocations at this point. So it might be 
possible to move forward on the assumption that we don't need 
pessimistic locking (provided that no-one ever passes a session handle 
to another client).

I'm going to give this a little more thought...

I think the outcome will be that I can avoid some locking in the EJB 
world, but need to send the same messages anyway... but we'll see.

Thanks for getting me to revisit this,


Jules



Jules Gosnell wrote:

> Rajith Attapattu wrote:
>
>> More question if you don't mind.
>>  
>> > 2.) Assuming sombody wants to do session replication (All
>> > Active) instead of (one Active and "n" backups) is there provision
>> > within the WADI api to plug in this stratergy?
>>
>> >I'm giving this some thought in terms of SFSB support, I'm not aware of
>> >similar constraints in the EJB world...
>>
>> >I guess we could relax this constraint in the web world, but I am not
>> >sure that I think that this is a good idea. Can you see a way to do 
>> this
>> >and maintain spec compliance and performance ?
>> Is WADI designed primarily for Web?? (bcos u talked about being 
>> servlet spec compliant) and u also mention about SFSB support.
>
>
> WADI was initially designed for the web - because I saw the issues 
> surrounding HttpSession distribution, particularly the requirement for 
> a single 'active' session as unresolved in any open source offering 
> and I thought it was about time that there was a truly compliant 
> solution.
>
> It soon became clear that many of the problems faced by sessions in 
> the web-tier were also faced by sessions in the EJB-tier...
>
>> Can we abstract the Replication problem to a more higher level and 
>> have the two (or more if there is) stratergies as impls of the 
>> replication API that installs as a pluggin by the user.
>
>
> Well, we could, but you would have to convince me that SFSBs would 
> benefit from a 'multiple-active-sessions' approach... I haven't given 
> it much thought, but I don't see any advantage - bear with me :
>
> - in the EJB world, we own the client side proxy. We can impose strict 
> affinity. An invocation arriving at a node that is not carrying the 
> primary session copy will be an exceptional occurance.
>
> - If you go with the 'single-active-session' model, and an invocation 
> does land on a secondary, you then pay an exceptional cost - 
> deserialisation and promotion (messaging) from secondary to primary. 
> This is OK, since you are in an exceptional situation.
>
> - If you go with the 'multiple-active-sessions' approach you have two 
> choices regarding deserialisation of replication messages.
>
> 1) you can deserialise them as they arrive - bad idea, because 
> deserialisation is extremely expensive and most of the time these 
> copies will never be used.
> 2) you can deserialise them lazily - only bother to do the work, 
> if/when an invocation arises.
>
> Regardless of which you choose (and I hope you would choose 2), you 
> are now in a situation where two copies of a session may diverge from 
> each other. Lets say you make a change to one, then you make a change 
> to the other, but the replication message from the first session 
> arrives at the second session after your second change and wipes it 
> out, and the replication message from your second change then 
> overwrites the first session with what is now a different value than 
> that carried by the second.... you can detect these issues by 
> versioning, but the best way to protect against them occuring (see 
> reasons for needing a pessimistic algorithm below) is by having some 
> form of distributed locking. In effect, the guy with the lock is the 
> primary and the guy without it the secondary.
>
> OK, so now we have a working 'multiple-active-sessions' model - but 
> hold on, it is doing lazy deserialisation and distributed locking - it 
> looks very like the 'single-active-session' model....
>
> Does that help ?
>
>>  
>> We can abstract things like a ReplicationManager that 
>> handles/controls no of replicas etc.. and a ReplicatedSession which 
>> decides wether it's active or passive (backup) based on the 
>> parameters passed to the ReplicatedSessionFactory at create time from 
>> the ReplicationManager.
>
>
> sure - and all of these things are already pluggable in WADI.
>
>>  
>> The ReplicationManager impl could be the stratergy that decides 
>> wether it maintains n of active replicas or 1 active and n backups or 
>> any other stratergy.
>
>
> Yes it could - but I think that this is still being driven by your 
> attachment to the multiple-active-sessions model and I do not see that 
> as viable.
>
>>  
>> Also the ReplicatedSession could impl stratergies like in 
>> MemoryReplication or PassiveReplication (based on active or passive) 
>> or anything else. And PassiveReplication can be extended to file 
>> based, database backed (not recomended) or anything else.
>>  
>
>
> WADI's replication strategy is already pluggable. We have a basic DB 
> replication scheme and are working on the in-vm scheme. Other schemes 
> could easily be added.
>
>> If we open up the API and let the user choose the stratergy they want 
>> then we are delaying our concerns to the user level and allow them to 
>> make the decesion.
>> I am sure we cannot address every situation, and the user is the best 
>> to judge about there env.
>>  
>> But we can always provide some sensible stratergies and 
>> recomendations and use cases around them to make an informed decesion.
>>  
>> Then We can leave the decesion to the user about 
>> spec-complient/performance.
>>  
>> What do u think??
>
>
> Unless you can demonstrate a clear win for a strategy that is 
> non-compliant, I would be very hesitant to ship one.
>
> WADI is designed so that pretty much everything that you might want to 
> plug is pluggable. But the larger the piece that you want to plug in, 
> the more work you would have to do writing it and making sure that it 
> did not collide with any other fn-ality.
>
>>  
>> >If a request arrives at a secondary, primary and secondary swap roles
>> >and processing happens locally.
>> >If a request arrives on a node with no copy of the relevant session, it
>> >may be relocated to the primary, or the primary to it.
>>  
>> 1. Do u plan to have an abstraction around the above concerns as well??
>>     So we can have impls of different stratergies, So people can 
>> decide wether they want to relocate the primary or the request.
>
>
> yes - this decision is pluggable.
>
>>  
>>     In case of a relocation of either request or session I assume u 
>> have hidden the impls behind an interface/API sort of thing so ppl 
>> can do different impls of the same stratergies or impl their own 
>> stratergy.
>
>
> yes
>
>>  
>> 2. In the event of a primary and secondary swapping roles or having n 
>> of active replicas don't we need some sort of distributed locking 
>> mechanism.
>> I heard that in memory locking should be optimistic and storage 
>> backed replicas should be pessimistic locking.
>
>
> session locking has to be pessimistic, because changes are made by 
> app, not container code. So a collision of changes could not be 
> resolved by the container, so it cannot be allowed to happen.
>
> WADI contains a distributed locking mechanism within its Partitioning 
> system. When a copy is promoted, a message will pass from it 
> (containing its version number), to its partition (where it will take 
> a lock and find the location of the primary), on to the primary (where 
> it will compare version numbers), back to the secondary (with a 
> possible update, if its version is out of date) and finally back to 
> the partition (where the primary's new location will be stored and the 
> lock released). local locking will also occur around both secondary 
> and primary whilst they are involved in this interaction.
>
>>  
>> I hope I haven't got the too mixed up :)
>
>
> No, I don' think so, but I do think that you need to take a careful 
> look at exactly how you think a mutiple-active-sessions model might 
> work and whether this would, in fact, be any different from the model 
> that I am proposing.
>
>>  
>> Can u please touch on this problem as my knoweldge is limited on this 
>> area.
>
>
> is this enough detail ? :-)
>
>
> Jules
>
>>  
>> Regards,
>>  
>> Rajith.
>>
>>  
>> On 1/17/06, *Jules Gosnell* <jules@coredevelopers.net 
>> <mailto:jules@coredevelopers.net>> wrote:
>>
>>     Rajith Attapattu wrote:
>>
>>     >
>>     >  Hi,
>>     >
>>     > Some of these questions came up after reading the thread on totem.
>>     > However I started the new thread so that searching is easy and 
>> also
>>     > want distract the intense discussions on totem with out-of-topic
>>     > questions.
>>     >
>>     > Jules Gosnel wrote
>>     >
>>     > >This is not something that is really considered a significant
>>     saving in
>>     > >WADI (see my last posting's explanation of why you only want one
>>     > >'active' copy of a session). WADI will keep session backups
>>     serialised,
>>     > >to save resources being constantly expended deserialising session
>>     > >backups that may never be accessed. I guess actually, you could
>>     consider
>>     > >that WADI will do a lazy deserialisation in the case that you 
>> have
>>     > >outlined, as primary and secondary copies will actually swap
>>     roles with
>>     > >attendant serialisation/passivation and 
>> deserialisation/activation
>>     > >coordinated by messages.
>>     >
>>     > >If you are running a reasonable sized cluster ( e.g. 30 nodes -
>>     it's all
>>     > >relative) with a small number of backups configured ( e.g. 1),
>>     then, in
>>     > >the case of a session affinity brekdown (due to the leaving of a
>>     > >primary's node), you have a 1/30 chance that the request will
>>     hit the
>>     > >primary, a 1/30 that you will hit the secondary and a 28/30
>>     that you
>>     > >will miss :-) So, you are right :-)
>>     >
>>     > So just to figure out if I understand this correctly.
>>     >
>>     > 1.) WADI only has one active and one-two backups at most (I
>>     assume the
>>     > no of backups is configurable)
>>
>>     replication is under implementation at the moment. Any number of
>>     backups
>>     should be configurable, but the more you have the less performant 
>> you
>>     are. You trade off safety for speed.
>>
>>     >
>>     > 2.) WADI is built up on the assumption of session affinity. So the
>>     > probability of missing the primary and the secondary backup(s)  
>> goes
>>     > up as the cluster grows according to your example
>>
>>     WADI will work without session affinity, however, as you would 
>> expect,
>>     this will not perform as well as it might. If you switch on 
>> affinity,
>>     you will drastically cut down the amount of request/session 
>> relocation
>>     and most interactions should become local.
>>
>>     Switch off affinity, and of course, your chances of hitting a 
>> copy of
>>     the session will go down. There are a fixed number of sessions and
>>     you
>>     are increasing the number of nodes... If you are intending to use
>>     an lb
>>     without affinity, then you should really reconsider. The costs are
>>     tiny
>>     and the gains enormous. Affinity is a standard feature on any 
>> serious
>>     HTTP LB.
>>
>>     >
>>     > 3.) How does WADI handle a situation where there is no session
>>     affinity??
>>
>>     If a request lands on the primary, processing occurs locally.
>>     If a request arrives at a secondary, primary and secondary swap 
>> roles
>>     and processing happens locally.
>>     If a request arrives on a node with no copy of the relevant
>>     session, it
>>     may be relocated to the primary, or the primary to it.
>>
>>     >
>>     > 4.) Have you compared the overhead of maintaining session
>>     affinity vs
>>     > having R replicas (all-Active) to service the client.
>>
>>     I have worked on impls using both approaches and am satisfied 
>> that my
>>     most recent approach will be the most performant.
>>
>>     >
>>     > >If, however,  you did your deserialisation of replicants up
>>     front and
>>     > thus avoided further messages when a secondary was hit, by
>>     maintaining
>>     > >all copies 'active' (I think you would not be spec compliant 
>> if you
>>     > did this),
>>     >
>>     > 1.) What do u mean by spec here ?? Are u talking about the WADI
>>     spec?
>>
>>     There is no WADI spec :-) - I'm talking about the servlet spec -
>>     specifically :
>>
>>     SRV 7.7.2 - "Within an application marked as distributable, all
>>     requests
>>     that are part of a session must be handled by one Java Virtual
>>     Machine1
>>     ( JVM ) at a time." and "Containers must notify any session 
>> attributes
>>     implementing the HttpSessionActivationListener during migration of a
>>     session. They must notify listeners of passivation prior to
>>     serialization of a session, and of activation after
>>     deserialization of a
>>     session."
>>
>>     These two constraints make it, IMHO, much more difficult to try
>>     implementing any system that maintains multiple 'active', or
>>     'primary'
>>     copies of a session. The system needs to be absolutely clear 
>> where the
>>     single 'active' copy is at any one time, in order to remain 
>> compliant.
>>     To ensure that activation/passivation semantics work OK, only this
>>     session may be activated, whilst the other 'secondary' copies are
>>     passivated. By leaving the secondaries in serialised form, you save
>>     further cycles and arrive at WADI's current design.
>>
>>     >
>>     > 2.) Assuming sombody wants to do session replication (All
>>     > Active) instead of (one Active and "n" backups) is there provision
>>     > within the WADI api to plug in this stratergy?
>>
>>     I'm giving this some thought in terms of SFSB support, I'm not
>>     aware of
>>     similar constraints in the EJB world...
>>
>>     I guess we could relax this constraint in the web world, but I am 
>> not
>>     sure that I think that this is a good idea. Can you see a way to
>>     do this
>>     and maintain spec compliance and performance ?
>>
>>     >
>>     > If u remeber we talked about extention points within WADI.
>>     >
>>     > 1.) Is there a doc that describes WADI architecture
>>
>>     Not as yet, just a website with various resources hanging of it.
>>     WADI is
>>     still relatively young. The best source of architecture info is the
>>     conversations that we have been having.
>>
>>     >
>>     > 2.) Is there a doc that describes these extention points and how
>>     to do
>>     > it?? (Looking for a little more info than the API doc)
>>
>>     WADI is put together using Spring. You just check out the javadoc 
>> and
>>     plug the pojos together. A lot of what we have been talking about is
>>     architectural design and not implemented (although the
>>     primary/secondary
>>     stuff is all in and working).
>>
>>     regards,
>>
>>
>>     Jules
>>
>>     >
>>     > Thanks,
>>     >
>>     > Rajith.
>>     >
>>
>>
>>
>>     --
>>     "Open Source is a self-assembling organism. You dangle a piece of
>>     string into a super-saturated solution and a whole operating-system
>>     crystallises out around it."
>>
>>     /**********************************
>>     * Jules Gosnell
>>     * Partner
>>     * Core Developers Network (Europe)
>>     *
>>     *    www.coredevelopers.net <http://www.coredevelopers.net>
>>     *
>>     * Open Source Training & Support.
>>     **********************************/
>>
>>
>
>


-- 
"Open Source is a self-assembling organism. You dangle a piece of
string into a super-saturated solution and a whole operating-system
crystallises out around it."

/**********************************
 * Jules Gosnell
 * Partner
 * Core Developers Network (Europe)
 *
 *    www.coredevelopers.net
 *
 * Open Source Training & Support.
 **********************************/


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