Newbie help

Elastic Stack Elasticsearch
1

Hi,
I am quite new to ES and currently doing some experiments to evaluate it. I
am trying to figure following case:

Assume That

  1. we started two nodes on two different PC
  2. We create a index with 2 shards and 1 replicas
  3. One of the primary shard goes to first node and other goes to second
    node
  4. Two application writing data to these nodes. And also assume that
    applications run on two different PC.
  5. If the connection between these nodes goes away each ES nodes will
    declare their replicas as primary and application will continue write the
    data to their local nodes
  6. Now, we have two nodes with different data on each

Now, I want to ask some questions:

What will happen if connection comes back? Will the nodes discover each
other? (I read from user list that one of them should be restarted to
rediscover each other)
What will happen if I restart one of the nodes after connection comes back?
What will happen to data written on restarted node?

Mustafa Sener

2

Hi Mustafa

 1. Now, we have two nodes with different data on each

This is known as a split brain.

What will happen if connection comes back? Will the nodes discover
each other? (I read from user list that one of them should be
restarted to rediscover each other)

That is correct

What will happen if I restart one of the nodes after connection comes
back? What will happen to data written on restarted node?

The data on the other node will be lost and will need to be reindexed.

Currently there is no way of merging these two clusters back together.

This issue in master allows you to configure your cluster to reduce the
likelihood of this situation, by preventing writes unless a quorum of
nodes is present:

This doesn't solve the issue, but at least will make you aware that
there IS an issue, because your writes will start failing.

clint