The entity bean architecture were designed such that the implementor
of an EJB Container can make *any* entity bean highly available by
using some form of server failover. The EJB spec however does not
specify how the EJB Container should implement the failover--this is
a value added by the EJB Container vendor. The main point is that
the bean developer does not have to code the bean differently to
enable failover.
Note that this failover is possible by the fact that the entity bean architecture
carefully separates the entity object's state from the instance of the
entity bean class. The state of an entity object is managed by a resource
manager. Note that the key to implementing an HA (highly available) EJB
server is to store the state of entity objects in an HA resource manager.
Vlada
----- Original Message -----
From: "dan benanav" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Saturday, March 18, 2000 12:08 AM
Subject: Re: Entity Bean failover
> Assaf Arkin wrote:
>
> > D Swaminathan wrote:
> > >
> > > Hi,
> > >
> > > I have a query regarding entity bean failover. To
> > > begin with, is there any such thing as entity bean
> > > failover? The scenario is such that the EJB client has
> > > obtained a reference to the bean (EJBObject) through a
> > > finder method and quite possibly invoked a few methods
> > > on the bean. At this stage, the server/container
> > > crashes. What would happen for subsequent method
> > > invocations on the bean ( assuming that we have a
> > > clustered server environment)?
> >
> > The EJB server will deal with that at the stub level by looking up the
> > same entity on a different server if you are not within a transaction.
>
> Depends on the server. I don't think theres is anything in the spec about that.
> WLS does not fail over automatically.
>
> >
> > If you are within a transaction, then in all probability your
> > transaction just failed.
>
> Why can't the container failover in this case?
>
>
>
> >
> >
> > > Also, how can the EJB container achieve this? Would
> > > this imply that to have entity bean failover all
> > > entity beans would have to be replicated across the
> > > containers? What would be the implication of such
> > > replication on concurrent access to the entity beans?
> >
> > The EJB container achieves that by reloading that entity bean on the
> > other server. It may, or may not, and that might depend on the bean (and
> > certainly the EJB container) replicate the entity bean. If your entity
> > bean simply holds information retrieved from the database, just
> > reloading it on the other server will do the trick.
> >
> > arkin
> >
> > > We are trying to come up with a set of test suites to
> > > compare the different EJB servers and this is one of
> > > the things we needed to check.
> > >
> > > Prasad
> > > __________________________________________________
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> > >
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> >
> > --
> > ----------------------------------------------------------------------
> > Assaf Arkin www.exoffice.com
> > CTO, Exoffice Technologies, Inc. www.exolab.org
> >
> > ===========================================================================
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>
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