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From "Peddi, Praveen" <pe...@amazon.com>
Subject Re: auto_boorstrap when a node is down
Date Thu, 31 Mar 2016 14:34:50 GMT
Hi Carlos,
In our case, old node is dead and is not accessible. So I am not sure if we can use rsync
in this case.

Praveen


From: Carlos Alonso <info@mrcalonso.com<mailto:info@mrcalonso.com>>
Reply-To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Date: Thursday, March 31, 2016 at 10:31 AM
To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Subject: Re: auto_boorstrap when a node is down

If that's your use case I've developed a quick disk based replacement procedure.

Basically all it involves is rsyncing the data from the old node to the new node and bring
the new one as if it was the old one (only the IP will change). Step by step details here:
http://mrcalonso.com/cassandra-instantaneous-in-place-node-replacement/

We've been using it for a while and works nicely and avoids the time, resources and baby-sitting
consumption of streaming data across nodes.

Regards

Carlos Alonso | Software Engineer | @calonso<https://twitter.com/calonso>

On 31 March 2016 at 15:26, Peddi, Praveen <peddi@amazon.com<mailto:peddi@amazon.com>>
wrote:
Hi Paulo,
Thanks a lot for detailed explanation. Our usecase is that, when one node goes down, a new
node in the same AZ comes up immediately (5 to 10 mins) and it is safe to assume that no other
nodes in another AZ are down at this point of time. So based on your explanation, using -Dcassandra.consistent.rangemovement=false
seems like the way to go for our usecase. I will test it with that option.

Thanks again.

Praveen





From: Paulo Motta <pauloricardomg@gmail.com<mailto:pauloricardomg@gmail.com>>
Reply-To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Date: Wednesday, March 30, 2016 at 10:55 AM
To: "user@cassandra.apache.org<mailto:user@cassandra.apache.org>" <user@cassandra.apache.org<mailto:user@cassandra.apache.org>>
Subject: Re: auto_boorstrap when a node is down

When you add a node it will take over the range of an existing node, and thus it should stream
data from it to maintain consistency. If the existing node is unavailable, the new node may
fetch the data from a different replica, which may not have some of data from the node which
you are taking the range for, what may break consistency.

For example, imagine a ring with nodes A, B and C, RF=3. The row X=1 maps to node A and is
replicated in nodes B and C, so the initial arrangement will be:

A(X=1), B(X=1) and C(X=1)

Node B is down and you write X=2 to A, which replicates the data only to C since B is down
(and hinted handoff is disabled). The write succeeds at QUORUM. The new arragement becomes:

A(X=2), B(X=1), C(X=2) (if any of the nodes is down, any read at QUORUM will fetch the correct
value of X=2)

Now imagine you add a new node D between A and B. If D streams data from A, the new replica
group will become:

A, D(X=2), B(X=1), C(X=2) (if any of the nodes is down, any read at QUORUM will fetch the
correct value of X=2)

But if A is down when you bootstrap D and you have -Dcassandra.consistent.rangemovement=false,
D may stream data from B, so the new replica group will be:

A, D(X=1), B(X=1), C(X=2)

Now, if C becomes down, reads at QUORUM will succeed but return the stale value of X=1, so
consistency is broken.

If you're continuously running repair, have hinted handoff and read repair enabled, the probability
of something like this happening will decrease, but it may still happen. If this is not a
problem you may use option -Dcassandra.consistent.rangemovement=false to bootstrap a node
when another node is down. See CASSANDRA-2434 for more background.

2016-03-30 11:14 GMT-03:00 Peddi, Praveen <peddi@amazon.com<mailto:peddi@amazon.com>>:
Hello all,
We just upgraded to 2.2.4 (from 2.0.9) and we noticed one issue when new nodes are added.
When we add a new node when no nodes are down in the cluster, everything works fine but when
we add new node while 1 node is down, I am seeing following error. My understanding was when
auto_bootstrap is enabled, bootstrapping process uses QUORUM consistency so it should work
when one node is down. Is that not correct? Is there a way to add a new node with bootstrapping,
but not using replace address option? We use auto scaling and new node gets added automatically
when one node goes down and since its all scripted I can't use replace address in cassandra-env.sh
file as a one-time option.

One fallback mechanism we could use is to disable auto bootstrap and let read repairs populate
the data over time but its not ideal. Is this even a good alternative to this failure?

ERROR 20:30:45 Exception encountered during startup
java.lang.RuntimeException: A node required to move the data consistently is down (/xx.xx.xx.xx).
If you wish to move the data from a potentially inconsistent replica, restart the node with
-Dcassandra.consistent.rangemovement=false

Praveen



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