On Tue, Feb 28, 2012 at 1:47 PM, Emmanuel Bernard <emmanuel(a)hibernate.org>wrote:
On 28 févr. 2012, at 19:41, Seiya Kawashima wrote:
Thank you for your response.
Unfortunately I'm not quite sure the reason that the Voldemort project
doesn't push it to the maven repository. There are questions and
discussions about the usage of the repository on the Internet, but it seems
to me that there is no clear answer for the reason. Or I just have not
noticed the existence.
I have several questions about hibernate ogm core. These parts are that
particularly I would like you to take a look at. The implementations might
be awkward or incorrect.
1. To implement datastore providers for hibernate ogm, there are two
code styles. One is from MapBasedDatastoreProvider and the other is from
Infinispan and EhCache datastore providers. When we add other datastore
providers, which style should we follow ? I've been referencing
MapBasedDatastoreProvider from the beginning of my experiment on Hibernate
OGM, the implementation followed MapBasedDatastoreProvider.
By style you mean architectural approach or code style (tab vs space)? If
the former, then the MapBasedDatastoreProvider is a toy useful for us to
abstract Hibernate OGM engine from a specific provider. You should use
Infinispan as your example. But remember that Infinispan is full
transactional and which makes the dialect somewhat easier to implement.
Yes, I meant the architectural approach in MapBasedDatastoreProvider. I
should have mentioned about the relationship between MapBasedDialect and
MapBasedDatastoreProvider as well. Within MapBasedDatastoreProvider, all
the CRUD logic for entities and associations and setNextValue() method are
implemented and those methods are called from MapBasedDialect. As a result,
MapBasedDatastoreProvider mixes its task and MapBasedDialect's task.
However, InifispanDatastoreProvider implements its task by itself and those
CRUD logic for entities are moved to InfinispanDialect. I'll follow
Infinispan and change VoldemortDatastoreProvider and VoldemortDialect.
1. To implement VoldemortDatastoreProvider.setNextValue() method, Iwas confused a
little bit. I originally referenced other datastore
and implemented it, but on VoldemortDialectTest.testIsThreadSafe()which tests
concurrency on the method, my
original implementation <https://gist.github.com/1903794>
because it required an exclusive lock and datastore access every time
the method is called. And then I modified the method and got the
current implementation in VoldemortDatastoreProvider. However, it doesn't
quite reduce the number of accesses to the underlying datastore as I
wanted, but allows concurrency. As a result, I put a flag to store the
next value on the datastore or not. I'm not quite sure if this is the
right implementation or not.
Some NoSQL stores have very efficient next value operations and this
method is for them. You should try and make as efficient as possible but
make sure it's safe. Your flag does not look like safe but I have not had
time to investigate yet.
And don't worry too much about performance. Properly set, Hibernate OGM
will only call this method every 50 or so id generation required as we use
a hi/lo algorithm by default to generate sequential ids.
I see. I'll take a look at the implementation again and see what I can do
for the method. The flag is set in VoldemortDatastoreProvider.start()
method and is not set afterwards, so I assume that it's safe. But even
though it's safe, it's still a awkward flag and would like to do something
1. To implement GridDialect.getLockingStrategy() method, the returned
object whose type is LockingStratey represents locks on underlying datatore
according to the javadoc. How should we implement this method when the
underlying datastore doesn't explicitly expose locks as objects ? Looks
like that Infinispan exposes some lock objects, so they are instantiated
and returned from the method. Voldemort has the concept of optimistic lock
exposing a method called
Redis also has the concept exposing a method called
. I use Jedis
as Redis client.
Now that's clearly the hardest part to map and we will have to chat on IRC
about that. Hibernate has the notion of optimistic locking with this
definition: if somebody applies a change before a change I am applying,
then I lose. To do that it uses version number comparison and update where
Now applyUpdate seems to be slightly different in the sense that they try
and reapply a set of operations until they are not stale. That looks closer
to a transaction than the Hibernate optimistic locking. But I need to have
a chat with you to understand how people use it.
Watch seems more akeen to pessimistic locking which you have to implement
as well in LockingStrategy. But again, we will need to have a discussion
about how people use that. Pessimistic locking in Hibernate means that I am
guaranteed that noone will be able to do anything on my entry until I
release the lock (usually by committing the transaction.
I appreciate your explanation. I'll discuss on this matter with you to
implement GridDialect.getLockingStrategy() correctly for Voldemort and
other datastores as well. Before that, I'll familiarize myself with
LockingStrategy a little bit more.
Form this discussion it also seems that we might need to have
and dialect implement the Hibernate transaction object so that the
datastore can properly demarcate when isolation starts and when it ends.
But that's clearly not abstracted yet in Hibernate OGM.