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From Purushotham Pushpavanthar <pushpavant...@gmail.com>
Subject Re: Unable to modify flow when one of the nodes in a cluster is disconnected
Date Sat, 29 Jun 2019 18:53:54 GMT
Hi Mark,

I thank you for your time and descriptive insights. However, the concern I
raised was regarding the allowable changes like changing the run status of
the processors. I couldn't stop or start a processor in the cluster when
one of the nodes was disconnected. The warning panel displayed is attached
to the initial mail in this thread.


*Now, there are some changes that we do allow, and the node will still
re-join. For instance, if the positions of elements change, elements are
startedor stopped, etc. In these cases, the new node will just inherit the
flow from the cluster and take on those changes.*

 Regarding certain kind of changes you mentioned in your previous mail,
could you please throw some light on which release this it supported from?


Regards,
Purushotham Pushpavanth



On Thu, 27 Jun 2019 at 19:34, Mark Payne <markap14@hotmail.com> wrote:

> Purushotham,
>
> If the node is disconnected and then attempts to reconnect, flow election
> does not occur. Rather, the node obtains a copy of the flow
> from the cluster, determines whether or not it matches, and if so rejoins.
> If the flow does not match, it disconnects and stops trying to
> reconnect.
>
> There are a few reasons that the node doesn't just inherit the cluster's
> flow blindly. Firstly, if a user were to delete a connection, and the
> re-joining node had data in that connection, it would lose the data. This
> is probably the most important reason - we never want to
> design for data loss.
>
> Secondly, when a node is disconnected from the cluster, the user is able
> to make changes. There are times when users will disconnect a
> particular node from the cluster and make some changes to the dataflow for
> diagnostic purposes. For example, they may want to temporarily
> send data to a new endpoint for sampling. When this happens, we don't want
> to just blindly lose those changes, because the user may not
> have wanted those changes lost. And if an admin is managing several
> systems, it's possible that they could accidentally configure the node
> to point to the wrong cluster, in which case it could potentially lose the
> entire dataflow. Perhaps not a problem if the dataflow exists on other
> nodes, but if this is a standalone node being converted into cluster, it
> could be devastating for the user.
>
> Now, there are some changes that we do allow, and the node will still
> re-join. For instance, if the positions of elements change, elements are
> started
> or stopped, etc. In these cases, the new node will just inherit the flow
> from the cluster and take on those changes.
>
> I think it would probably be advantageous to allow the node to back up its
> own flow before inheriting from the cluster, and then apply any changes from
> the cluster that do not result in data loss (i.e., if any connection is
> removed and the node has data in that connection, then fail, else inherit).
> The big down
> side there, honestly, is that it's just a huge amount of effort that would
> be required in order to make that work properly.
>
> So to make a long story short: there are reasons that we don't just
> inherit the flow, but we could work around those problems. There are
> definitely
> areas where we could improve, but it's just not been taken on yet by
> anyone in the community.
>
> Thanks
> -Mark
>
>
> On Jun 27, 2019, at 3:37 AM, Purushotham Pushpavanthar <
> pushpavanthar@gmail.com<mailto:pushpavanthar@gmail.com>> wrote:
>
> Hi,
>
> I'm having a 3 nodes( ver 1.9.2) cluster running in production. As infra
> is unreliable due to various factors, our nodes go down often. We don't
> have distinction between dev and prod cluster. We modify, deploy, test in
> the same cluster. However, when one of the node goes down NiFi restricts us
> to modify the state of the flow by throwing warning window in the
> attachment.
>
> I read<
> https://nifi.apache.org/docs/nifi-docs/html/administration-guide.html#flow-election>
> that if a node in the cluster is disconnected and comes back again, flow
> election happens. I would like to understand the motivation for not
> allowing the change of flow in the above scenario.
> I was thinking why can't the latest node joining to the cluster pull a
> most elected flow.xml.gz from the cluster and apply it to itself?
>
> Regards,
> Purushotham Pushpavanth
>
>
>

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