Re: Network partition and re-merge

Pablo,

I apologize that I forgot to respond to your earlier questions on this.  Pl=
ease feel free to prod me multiple times if I=92m unresponsive in the futur=
e again.

> what happens if SPREAD reports a (false) network failure, and the new vie=
w does not include one of the old-view member, but it is really operational=
? The other "surviving" members should deliver CAUSED_BY_NETWORK membership=
 messages and their high level application surely do not consider the "dead=
" member. They continue exchanging messages and, thus changing the global s=
tate.

Yes, that=92s certainly possible.

> What happens with the "false-dead" member? Because it is considered as a =
"dead" member it doesn't receive the token anymore. Therefore, a token time=
out should expires and  surely  it delivers a CAUSED_BY_NETWORK membership =
messages.

Yes, it too will eventually see that the ring has broken and deliver such a=
 message.  It may deliver a membership with only its own members (a singlet=
on daemon membership), or, depending on the exact timing of events it might=
 skip right to the next part ...

> How the "false-dead" member could be included in the group again? Does SP=
READ report it as a member again in the membership information? =


The daemons will eventually notice one another again.  Either by seeing eac=
h other=92s traffic or through periodic probe messages they each send out. =
 Then they will attempt to form a new ring and, if successful, will eventua=
lly all report a different CAUSED_BY_NETWORK membership that remerges the d=
aemons=92 group members.

> Suppose that the "false-dead" member gets an resource exclusively. As the=
 other members considered that member as "dead", one of them allocates that=
 "false-free" resource. A difficult situation could arise.   =


Yes.  Typically, the way such universal exclusivity is handled is through q=
uorums.  As a simple example, a weighted majority of the potential system m=
ust be present and agree for any of those daemons or that quorum to utilize=
 such a resource.  It is not possible for multiple quorums to exist virtual=
ly in parallel, so you can ensure exclusivity this way.  It=92s not simple =
to get 100% correct and in some cases, no system quorum exists for extended=
 periods of time.

Cheers!

-----
John Lane Schultz
Spread Concepts LLC
Cell: 443 838 2200

On Aug 7, 2014, at 9:03 PM, Pablo Pessolani <[email protected]> wrote:

Does any body has experience about handling network partition and re-merge =
with spread?
 =

What happened if spread reports false-positive network failures? =

 =

Thanks in advance.
PAP
 =

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