Re: [jgroups-users] Protocols' questions

On 17/08/18 17:49, Questions/problems related to using JGroups wrote:
> Hello,
> 
> I am trying to make my mind up regarding some of the protocols JGroups provides - I don't have a lot of experience with it  therefore forgive me if some questions are blatantly naïve - I did read the docs a few times but I am still unsure.

OK


> It seems to me that many of the protocols have an UDP "mindset", namely they make most sense when used upon UDP as transport protocol, I am debugging an issue
> Whose jgroups' stack involve the following (more or less):
> 
> <FRAG>
> <STABLE>
> <NACK2>
> <UNICAST2>
> <TCP>

This is upside-down, I assume you're just listing the protocols? And 
this config won't work, as you're missing protocols and NACK2 is not 
known...

> I am investigating if I could simplify it and I few questions popped up
> 
> Reading the <UNICAST2> documentation:
> 
>> On top of a reliable transport, such as TCP, UNICAST is not really needed. However, concurrent delivery of messages from the same sender is prevented by UNICAST by acquiring a lock on the >sender's retransmission table, so unless concurrent delivery is desired,
> 
> What is it actually meant by "concurrent delivery" ?

If P sends m1 and m2, by default m1 is delivered _before_ m2 
('delivered' = receive() callback returns). If m1 and/or m2 are OOB, 
then they'll be delivered concurrently. Same if UNICAST2 is missing and 
the messages are point-to-point.

> Does it refer to the fact that JChannel.send() is not synchronized therefore, when sending an bunch messages we could end up with a catastrophic
> mess where bits of messages get scattered on the stream and the whole thing becomes un-intelligible to the receiver ?

No, that won't be the case, but ordering might be completely undefined.

> Does it make sense to have one among UNICAST{1,2,3} and NACKAK{1,2} in the stack ? Wouldn't that be redundant ?

You need both, take udp.xml or tcp.xml as starting point.

> Does STABLE make sense if we are using TCP to deliver messages to the cluster's members ?

Yes, as it guarantees a message M is only purged (to be GC'ed later) 
once it has been delivered by everyone.

> Does FRAG make sense if, by using a connection-oriented protocol such as TCP, we don't have a max datagram size to stick to ?

Yes, for reasons of flow control. So, with TCP, the only protocol we 
actually *can* remove is UFC

> Thanks,
> Pietro
> 
> 
> 
> Pietro Paolini
> Consultant
> 
> Alfa
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-- 
Bela Ban | http://www.jgroups.org


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