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From Rajith Attapattu <rajit...@gmail.com>
Subject Re: WADI clustering
Date Mon, 23 Jan 2006 14:15:30 GMT
Jules,

More questions :)

If we assume the single-active-model (with n backups)

1. Do we still need a locking mechanism?? Bcos if we have only one live
session where updates can happen and the rest are serialized backup copies
there is no need for a locking mechanism. Is this correct?? or have I missed
something here?

2. Assuming a change happed within the live session (for ex an attribute was
added or modified)
  a) will it be immedialely broadcasted or are they batched (lets say two or
3 changes to gether) before being broadcasted

 b) Is it the delta that is sent or is it the whole session thats moved
across. (I assume it's the delta with some meta-info like session_id,
version, date_time..etc)


3. when these changes are recived by the other copies how do they use it?
   Is it stored seperately in a queue in serialized format and applied once
the session is de-srialized??

  I guess it will be an expensive operation if we keep serializing the
session, then apply the change and then de-serialize it.


4. How are these backup copies stored within WADI??
    a) Can they be stored in a central place like DB or a file

    b) Can they be in memory or a file within the node?


>ActiveCluster allows each node to maintain a small amount of published
>data about itself. It can write this data. All other nodes can read it.
>I use it, currently, to advertise human-readable-node-name, http-port,
>shutting-down?, etc... - I figure that if if e.g. the number of sessions
>that you are carrying fluctuates by more than 10% since the last
>published figure, you publish the new number etc. We want to throttle
>publishing a little because each publishing causes a 1->all message

1. Ok, where is this information about each node is stored??
   If a nide publish it's chnages I assume it will go to each node and
essentialy everynode will have copy of each nodes meta-info like node_name,
ip, port, no_of_sessions ..etc

2. But as the nodes increase then the chatter among each nodes are going to
increase a lot. If there are N nodes then each node will have N no of copies
to maintain.

   Does logical partitioning slove this problem? Like u divide the 10 nodes
into groups of 3,3 & 4, where they interact only between the group.
   Does WADI (or Active Cluster) provide this logical partitioning yet allow
to view the cluster as a whole.

   If this is the case I guess the load balancer should be aware of this
configuration and keep requests within the respective groups. Is this
possible with current load balances ??

3. So basically each node makes the decesions for themselves instead of a
central command and control center??? I guess distributed decesion making is
good bcos otherwise there will be election stratergies involved in electing
a leader if the leader_node goes down.

4. Lets say there is need for a system admin to monitor the cluster. And u
did mention that u can plug in listerners to the underlying AC code.
   Basically what I like is a  management API where I can monitor the node
via listners or by registering my self as a dummy node and provide that
infomation in  human readable format.
   You mentioned this is under construction at the moment. (I really like
that feature and see a lot of value in it for an end user)

By the way when are u guys moving WADI into incubation so we can implement
some of the ideas we talked about??

Sorry about the long list, but more questions I ask the better understanding
I get (of course at the expense of your time :) )

Regards,

Rajith.

On 1/22/06, Jules Gosnell <jules@coredevelopers.net> wrote:
>
> Rajith Attapattu wrote:
>
> > >>However if we use 100% session affinity then the chance
> > Sorry it should be asume not use. :)
> >
> > Don't think of it as a battle, it's a disscussion for me (and perhaps
> > for you) to understand both sides of the coin. Unless u think my
> > questions are stupid :)
>
> Good - I'm glad that we see eye to eye on this one. It is indeed an
> interesting technical debate. I'm just a little paranoid about upsetting
> people at the moment, because I seem to be doing a lot of it :-)
>
> >
> > >>Sorry to put you down again - I don't want to, but when I think it
> > all through, I just can't see a good reason to go this route.....
> >
> > >>I am planning to refactor this area in WADI to be pluggable, so you
> > wil have the option of writing something like this if you really want it
> >
> > Well u haven't put me down, u have agreed to refactor this area in
> > WADI to be pluggable (Thats what I wanted).
> > All I am saying is we don't have to go ahead and implement the
> > scenario, but we shouldn't close the door either !!!
> > So what your doing will allow a somebody the provision to use it they
> > really want :)
>
> OK - so, we understand each others position much more clearly and have
> what we want. Cool. I have found this thread particularly beneficial
> because you have forced me to crystallise what I felt into hard reason,
> which is a very useful thing to be forced to do. As a result, I now
> understand my position much better and have have noticed a couple of
> further optimisations that I can make :-)
>
> >
> > Ok, so finnaly I have achived what I wanted, that is to open up the
> > API :) and thanks for that.
> >
> > State-Balancing
> > ----------------------------
> > Ok thats what I was looking for. My concern was that if we use session
> > affinity can a server get overloaded. Sorry if never asked about this
> > directly and all credit to you for identifying what my real concern
> > was wrt session affinity.
> >
> > It's cool if we have a mechanism if the ClusterManager is aware of no
> > of sessions in each node and make sure new requests are redirected to
> > another node if the current server has already reached the threshold.
>
> I was already intending for each node to publish a rough indication of
> the number of session that it carries, because I figured that it would
> come in useful - we now have a good usecase for it.
>
> >
> > Does WADI provide a management API for this kind of thing??
>
> Not yet - look at the LoadBalancer strategy stuff - it will be extended
> to hang off here somewhere.
>
> >
> > also doe it have some sort of mechanism to get info about a particular
> > node, no of nodes, no of nodes in a session at given time...etc ????
>
> ActiveCluster allows each node to maintain a small amount of published
> data about itself. It can write this data. All other nodes can read it.
> I use it, currently, to advertise human-readable-node-name, http-port,
> shutting-down?, etc... - I figure that if if e.g. the number of sessions
> that you are carrying fluctuates by more than 10% since the last
> published figure, you publish the new number etc. We want to throttle
> publishing a little because each publishing causes a 1->all message...
>
> >
> > I assume u still expose the listerners in ActiveCluster for interested
> > parties to get events like node joined, left ...etc
>
> This area of the code is still undergoing a lot of change, but it should
> be possible to get the ActiveCluster Cluster object from it and attach a
> ClusterListener.
>
> Jules
>
> >
> > Regards,
> >
> > Rajith.
> >
> > On 1/19/06, *Jules Gosnell* <jules@coredevelopers.net
> > <mailto:jules@coredevelopers.net>> wrote:
> >
> >     Rajith Attapattu wrote:
> >
> >     >
> >     > Ok, I am not fixed on multiple-active-sessions.
> >     > But my concern is high availability with single-active-session
> >     model
> >     > under high load conditions.
> >     >
> >     > As u pointed out,
> >     > >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
> >     >
> >     > However if we use 100% session affinity then the chance of one
> >     server
> >     > getting too many hits is possible (due to that being the primary
> >     and
> >     > the LB indiscrimantely maintaining session affinity without due
> >     > consideration for load).
> >     > Thus a compromise of service quality is inevitable. The service
> will
> >     > have to drop requests or degrade the service (provide only some
> >     of the
> >     > services which are not expensive).
> >     >
> >     > So sometimes the cost of maintaining multiple-active-sessions may
> be
> >     > less compared to the exceptional cost that has to be paid with a
> >     > server crash thus increasing the load within the remaining nodes
> of
> >     > the cluster.
> >
> >     I haven't heard this argument before - lets take a look at it...
> >
> >     I'm not aware of loadbalancers that allow you to ask for e.g. 50%
> >     affinity - do they exist ? So, lets look at the extremes (100% and
> >     0%),
> >     this is often the best way of seeing how something will actually
> work.
> >
> >     100%
> >     - all requests are routed to the correct node, regardless of
> >     cluster size
> >     - no replicants ever need to be deserialised because they are
> >     never hit
> >     - if a node collects too many sessions, it may get overloaded
> >
> >     0%
> >     - the amount of extra deserialisation that you will have to
> >     perform will
> >     increase in line with the number of nodes in your cluster as the
> >     chance
> >     of hitting the one deserialised copy decreases.
> >     - this extra deserialisation cost will mean that total work done
> >     for the
> >     same load will be higher and may overload nodes.
> >
> >     So, I guess we want to choose the solution that leads to the least
> >     work
> >     being done in the cluster - 100% affinity - and figure out how to
> >     avoid
> >     so many clients getting stuck to one node that they overload it -
> >     solution - state-balancing.
> >
> >     Each node needs to be aware of roughly how many sessions the other
> >     nodes
> >     are carrying. If it feels that it has more than everyone else, it
> >     could
> >     (load-balancer  integration permitting) offload sessions to its
> peers
> >     and relocate subsequent requests for them to their new location, or
> >     simply relocate requests that look like they might create a new
> >     session
> >     to peers that are carrying fewer sessions - or use both of these
> >     algorithms to keep state equally balanced across the cluster.
> >
> >     how does that sound ? It shouldn't be too hard in WADI, because
> >     most of
> >     the stuff to do this sort of thing is already there...
> >
> >     >
> >     > The cost in terms of money value of loosing revenue due to service
> >     > un-availability could be higher than providing more memory,
> >     high-speed
> >     > network infra which could handle the cost of
> >     > serialization/desirialization of replicas and the overhead of a
> >     > distributed locking mechanism without compromising performance.
> >     >
> >     > Thats why I said that we should provide both stratergies and the
> >     > end-user can make an informed decesion based on there business
> >     > requirments and load conditions within there cluster.
> >     >
> >     > We should avoid making those decesions before hand.
> >     >
> >
> >     > Also allowing the idea of configurable active replicas will
> >     allow the
> >     > end-user the flexibility of trying out both
> multiple-active-session
> >     > and single-active-session models and see what works best for them.
> >     >
> >     > I would strongly advocate the idea of a Replication mgt
> abstraction
> >     > API especially with some of the ideas Gianny provided on the
> thread.
> >     >
> >     > What do u think about that?? Have I made a case??
> >
> >     Hmmm...
> >
> >     I don't want to make a battle out of this, but I really cannot see
> >     any
> >     advantage to multiple live copies. All you will do is increase the
> >     work
> >     that your cluster is having to perform. If you choose 1->few
> >     replication, you will also miss your session and its copies with
> >     increasing frequency as you add nodes, creating even more work. If
> >     you
> >     choose 1->all replication you will suffer from increasing workload
> and
> >     memory requirements on all nodes as you add new ones, and you will
> >     quite
> >     quickly hit a ceiling in terms of scale... Affinity is the silver
> >     bullet
> >     as far as clustering sessions in concerned. I can't understand why
> >     anyone would want to run without affinity turned up as high as it
> >     will go.
> >
> >     I am planning to refactor this area in WADI to be pluggable, so
> >     you will
> >     have the option of writing something like this if you really want
> >     it...but I am not convinced that I should provide it...
> >
> >     Sorry to put you down again - I don't want to, but when I think it
> all
> >     through, I just can't see a good reason to go this route.....
> >
> >     Jules
> >
> >     >
> >     > Regards,
> >     >
> >     > Rajith.
> >     >
> >     >
> >     > On 1/18/06, *Jules Gosnell* <jules@coredevelopers.net
> >     <mailto:jules@coredevelopers.net>
> >     > <mailto: jules@coredevelopers.net
> >     <mailto:jules@coredevelopers.net>>> wrote:
> >     >
> >     >     Jules Gosnell wrote:
> >     >
> >     >     > 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,
> >     >     >
> >     >     BTW - if we do assume that we can rely on affinity 100% in
> >     the EJB
> >     >     tier
> >     >     then I am still not sure that I see any real advantage in
> >     holding
> >     >     multiple active copies of a session. I guess you will have to
> >     >     explain to
> >     >     me exactly why you would want to do this.
> >     >
> >     >     Finally, the locking system that WADI currently uses will
> >     only incur
> >     >     extra work, taking distributed locks, if affinity breaks
> >     down, so the
> >     >     cost of applying it to the SFSBs where, we hope for 100%
> >     affinity,
> >     >     should be 0.
> >     >
> >     >     Jules
> >     >
> >     >     >
> >     >     > 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>
> >     >     <mailto: jules@coredevelopers.net
> >     <mailto:jules@coredevelopers.net>>
> >     >     >>> <mailto: jules@coredevelopers.net
> >     <mailto:jules@coredevelopers.net>
> >     >     <mailto: 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>
> >     >     < http://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 <http://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 <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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