8:47 p.m.
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.
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.
To implement VoldemortDatastoreProvider.setNextValue() method, I was
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 ran poorly 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.
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 voldemort.client.StoreClient.applyUpdate and
Redis also has the concept exposing a method called watch and the description . I use
Jedis 2.0.0 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 style operations.
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.
Form this discussion it also seems that we might need to have datastores 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.
Emmanuel