What's the minimum number of nodes that assures the state of the cluster is green?

1
votes

I have a cluster of two nodes, and set the configs for shard number and replica number as following:

index.number_of_shards: 10
index.number_of_replicas: 1

The master node is elected automatically.

Before I index data, the state of the cluster is green. After I index data, the state of the cluster becomes yellow. Some replica shards are not allocated.

But if I add another node and now the cluster consists of three nodes, then I index data and the state of the cluster becomes green.

The version of ES that I use is 1.1.

Question 1:
If the replica number is 1, do I have to have at least 3 nodes to assure the state of the cluster is green?

Question 2:
I have observed that one (not the master node) of the three nodes just has primary nodes. What's the mechanism ES uses to allocate primary/replia shards?

Thanks in advance.

UPDATE

The first problem is caused by different versions of ES. In my previous test, one node use ES-1.1.1 and the other uses ES-1.1.2. After every node of the cluster uses the same version of ES, then the first problem disappears.

1
elasticsearch

1 Answers

2
votes

Answer 1) If you mention 

 index.number_of_shards: 10
 index.number_of_replicas: 1

It means your index is split into 10 shards and 1 replica means there is one replica shard for each shard of index.therefore you will have 20 shards totally.

IN ES the primary shard and replica shard are not allocated in same node.So if you have 1 replica you must have 2 nodes to keep the node status green.

 Number of nodes to keep the cluster state green=NO of replica+1

Answer 2) Among shards of index, ES choose one primary shards where writes are done.It is not necessary, that master node should contain primary shard.Shards are distributed among nodes.If a node fails that contains primary shard ,then corresponding replica shard become as primary.You data will be saved.

To understand basics of ES Refer

HOpe it Helps..!