Hi Bongjae Chang
Thanks for posting your questions. Please see inline for my responses.
Thanks
Shreedhar
Bongjae Chang wrote:
> Hi all. I'm a beginner in Shoal Project.
> Recently, I have reviewed Shoal's source code because I am interested
> in Generic Clustering Framework.
Very glad to know of this. Please let us know of your use cases and we
can surely help.
> I know DSC usespoint-to-point communication(JXTA) with all instances
> for reliablity.
Up until now DSC implementation was using UDP broadcasts which are
inherently unreliable. We are moving to make any message intended to be
sent synchronously to use TCP based unicast for reliability and ordering
for all messaging in Shoal with the current service provider. That is in
the works and not yet complete. That should be happening next week.
> following Shoal's source code isa message sending part in
> GroupCommunicationProviderimpl.java
> ---
> public void sendMessage(...) {
> ...
> List<String> currentMembers =
> getGMSContext().getGroupHandle().getAllCurrentMembers();
> for (String currentMember : currentMembers) {
> ...
> clusterManager.send(id, message);
> }
> ...
> }
> ---
The above code snippet is for applications that want to send messages to
other members' application layer. For corresponding DSC send message
snippet, look here:
https://shoal.dev.java.net/source/browse/shoal/gms/src/java/com/sun/enterprise/ee/cms/impl/jxta/DistributedStateCacheImpl.java?rev=1.17&view=markup
> The above, I have a question in DSC(distributed state cache, default
> implementation)
> DoesShoal DSC guarantee data consistency for same group?
The answer is not a clear yes or no and is "it depends" as total
consistency requires total ordering. The service provider impl does not
provide total ordering as we have not seen a requirement for it. For
DSC, at this point, the lightweight nature of its intended usage does
not require total ordering and consistency provided by the transport
layer has been sufficient so far in our tests.
As Shoal's adoption increases, we are beginning to see requirements that
would help us prioritize our efforts to provide such aspects.
> for example,
> Current members consist ofA, B and C.
> WhenA send a message(key="a", value="1")tothe group,
> B send another message(but same key, key="a", value="2")to the group
> at the same time.
> At this time, can the group memers(A, B, C)view same value about
> key="a" in DSC? (all"1" or all"2" )
There is definitely a small timing factor to be considered in any
distributed system of this nature. Assuming the network is not a
limiting factor, the data should be consistently available in the
instances of DSC in each member. More below:
> I think it doesn't matter aboutvalue(whether "1" or not), butit's
> important whethergroup membershave same view concurrently or not.
The members are expected to have identical membership view
consistently.As a result, messages are delivered to all members in a
current view.
> And does DSC guarantee Atomicity?(All sent or not)
The current DSC implementation is a simple shared concurrent hashmap. A
write from one member gets disseminated to all members so that reads are
local. A message sent is not transactional in that there is no notion of
a 2-phase commit equivalent.
We have not gone to the level of guaranteeing each of the ACID
properties as yet.
For instance, in your example above, with the current implementation,
when members A and B write data to the same key, the last member writing
data at that moment would overwrite the previous value in the group.
That said, I would even venture to say that there is no code in the DSC
implementation that would ensure that a single write blocks all other
writes for the same key.
Please let us know more details of what your requirements are and we
will surely look into providing extensions for such a use case. We are
currently in discussions on doing a full fledged ShoalCache and would
like to open this up to the community for contributions both for
discussions, input and for code.
> If DSC guarantees consistency and atomicity, please explain the
> mechanism briefly :-)
> Orplease let me knowthe web link about this information.
> I need your helps. Thanks!
Would be glad to help. Keep sending us your questions, enhancement
requests and any contributions you may have.
> --
> Bongjae Chang