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From Bip Thelin <...@razorfish.com>
Subject Re: [PROPOSAL Tomcat 4.x] Cluster
Date Mon, 07 May 2001 17:34:03 GMT
Kief Morris wrote:
> 
> [...]
>
> My point is that the Manager/Cluster needs to know when the session is in
> use by another instance of Catalina. A locking mechanism must be
> implemented by the Cluster (or whatever) to prevent a session from being
> used by multiple instances at once. This mechanism will require the
> Manager/Cluster system to know when a request  begins using
> a session, and when it has finished.
> 
> >If we say that only one JVM at a time can manipulate a sessions since
> >a sessions only belongs to one machine at a time the only time a session
> >needs to be replicated is when it's created/changed/destroyed.
> 
> Yes. But putting the session into the Cluster at creation time is
> unnecessary. It should be put in at the end of the request when
> it is created (other Catalina instances can't use it before then
> anyway), and updated at the end of each subsequent request.
> So we need to have the end of a request call into the Manager
> to indicate that the session can be sent to the Cluster and
> unlocked for use.

Do we really need to lock a session for each request and then replicate it?
Sorry I might be confused, you mean a request for a session or a request
as in generating a new request object(http request). If we assume that a session
is only in use in one JVM at a time(which I think we can assume) then that
session doesn't need to be locked it just needs replication when it's changed.

> >I'd rather see the replication be implemented in a Manager(i.e.
> >DistributedManager or
> >maybe change name to MulticastDistributedManager) thus making it possible to
> >run any Store with the DistributedManager(i.e. FileStore).
> 
> OK, I take your point that extending Store isn't the way to go. But
> I don't think we should have a different Manager implementation for
> each available distribution mechanism - MulticastDistributedManager,
> JMSDistributedManager, JavaSpacesDistributedManager,
> JCacheDistributedManager, etc. We should use the same pattern
> that Manager/Store uses: a single DistributedManager should be
> implemented which is independent of the actual session sharing
> mechanism. It should be able to use any implementation of the
> Cluster interface.

Yes, sorry I was clear as mudd in my last email, so if you look at the
DistributedManager.java I checked in 14h ago(as of writing) it now uses the
new API's I created which is common for any distribuition protocol you might implement.

> I'd like to keep the possibility open to implement different distribution
> strategies. The strategy we're looking at now is for each instance of the
> application to hold copies of every session. An alternative Cluster strategy
> would keep the sessions in a central location such as a database: when
> a request comes in for a session not found in the current instance, the
> Cluster checks the database to see if it's there. This isn't too different
> from simply using JDBCStore. A third way is to have just two instances
> of the application holding a given session: when instance A creates
> the session, the Cluster chooses instance B to hold a backup copy
> in case A goes down: if a request comes in to C, B still has it available.

One way that you could simply go with the cluster is to group them. So there
is an option now to specify the name/port/address of the cluster. What I was
thinking is that you could specify a cluster that this.jvm belongs too and then
specify a cluster it should replicate too.

> Not that we need to implement all of these, but the architecture we
> build now should allow these possibilities and others. Other people
> can try out different ideas, and users can choose the system best
> suited to their needs.

yes, agree.

> I'm also not sure about the issues with using persistence and distribution
> simultaneously. If we simply use PersistentManager with this distribution
> code, each instance will save its own copy of every session to persistent
> storage. This might be desirable in some cases - I can see using FileStore,
> for instance. But if you use JDBCStore and the Multicast distribution, it's
> wasteful - with a 4 server farm, we have 4 copies of each session in the
> database. So how should this be addressed? Cluster ought to have some
> mechanism which (optionally) ensures that each session is only
> persisted once. This may mean having Cluster override Store functionality,
> which is why I was thinking of combining the two.

Yes, that's a good point, at first I was thinking that each machine in a Cluster
is having it's own unique key, so when you generate an session id machine 1 would
get something like: A1KDSFNRKIFLKMFDSFDSA where:
--------------------|| <-- Are the two letters that identifies the machine, so when
you know which machine that "owns" the session all machines that have the session replicated
know that it doesn't belong to them so they shouldn't save in a Store. It's also
useful for an eventuall tomcat dispatcher frontend to know which machine the session
origins from. However some complications occur when you replicate a session and
the machine that "owned" the session dies so another machine takes it over. Should that
machine then take the role as Machine A1.

	Cheers, Bip

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