Clustering some general questions:

Question

1. Can dbIsShared be set to false if clustering is used?
          2. Why does Weblogic not automatically provide failover for entity
          beans? Many other servers including WebSphere and iPlanet provide
          that. In other words I would like to be able to set up a clustered
          environment without having to change any code.
          

Answer 1

dan benanav wrote:
          >
          > see inline comments
          >
          > Rob Woollen wrote:
          >
          > > I'm not sure I follow you here. If you want pessimistic concurrency in
          > > a cluster, set the isolation level in the database to serializable.
          > >
          >
          > Setting the isolation level to serializable does not cause blocking (at least in Oracle). It
          > would be helpful if you read up on that to get a better > >understanding of what I am saying.
          I'm very familiar with Oracle, and you are correct it will not cause
          blocking. I should have been clearer in my response.
          > Setting the mode to serializable will only cause an exception to be generated if a transaction
          > attempts to update a row that was updated by another transaction that started after the current
          > transaction.
          >
          > Bottom line is, and I am sure about this, setting the isolation level in the database does not
          > yield the same effect as pessimistic concurrency (it does not block).
          >
          > Oracle is obviously one of the major database vendors so one would expect that there is a way to
          > have the account entity bean work with that.
          >
          > So can you offer a solution?
          >
          If you design your application to only work with pessimistic locks in
          the EJB container, then your design is flawed. You are depending on
          container internal behavior for your application to work. It certainly
          won't be portable to other J2EE platforms.
          Depending on serializable behavior for your transactions is fine, and
          that's what setting the transaction's isolation level will do.
          Can you give us some more information on why you require a global
          pessimistic lock in a cluster? I am interested in why this is needed in
          your application.
          -- Rob
          > dan
          >
          > >
          > > Robert was offering another solution which is to use optimistic
          > > concurrency. You could read the data in a single transaction and then
          > > do the conditional update in a second transaction.
          >
          > With a clustered system there is no choice but to use optimistic concurrency (from the entity
          > bean point of view).
          >
          > >
          > >
          > > -- Rob
          > >
          > > dan benanav wrote:
          > > >
          > > > The way you suggest is not the best approach, nor does it provide the same benefits as
          > > > pessimistic concurrency. It can also be done automatically (without a timestamp, etc.. )
          > > > by setting the mode to transaction_serializable. (If Oracle is being used). Your approach
          > > > makes the programming on the client side more complicated. You will need to catch the
          > > > exception and try again until you succeed.
          > > >
          > > > What we really want is to block somehow. How would you do that? Perhaps people have been
          > > > doing this in the client/server world. I would like to see how it would be handled with
          > > > entity beans. I am not convinced there is an easy way to do it.
          > > >
          > > > dan
          > > >
          > > > Robert Patrick wrote:
          > > >
          > > > >
          > > > > There are several ways to do this. People have been solving these problems for years
          > > > > in the client/server world. This is just one possible way of doing it...
          > > > >
          > > > > public int balance;
          > > > > public int oldBalance;
          > > > >
          > > > > public void ejbLoad()
          > > > > {
          > > > > balance = read from database;
          > > > > oldBalance = balance;
          > > > > }
          > > > >
          > > > > public void ejbStore()
          > > > > {
          > > > > // probably should use a version number or timestamp rather than the balance field
          > > > > // to detect if the bean has changed since the data was loaded...
          > > > > //
          > > > > update accounts set balance = :balance where accoutNumber = :accountNumber and
          > > > > balance = :oldBalance;
          > > > > if (numRowsAffected == 0) {
          > > > > ejbContext.setRollbackOnly();
          > > > > throw new ConcurrentModificationException("account was modified by someone
          > > > > else");
          > > > > }
          > > > > }
          > > > >
          > > > > public deposit(int amount)
          > > > > {
          > > > > balance += amount;
          > > > > }
          > > > >
          > > > > Hope this helps,
          > > > > Robert