When will Master Node push replica to new join data node?

Elastic Stack Elasticsearch
1

Hi I'd like to know the whole process of primary shard & replica shard actual interaction process.
I have a single node and setting with 2 shards & 1 replica. When I start to index document,

  1. since there is only one node (it is mater node), where does replica shard exist?

  2. if a new node join as data node, does master node knows and when push shard to new node as replica?

I trace the source code (1.7.2) and I can see primaryPhase and replicaPhase. but if I have only one node, seems nothing happended on replica.

2

since there is only one node (it is mater node), where does replica shard exist?

Nowhere. ES refuses to allocate a replica shard on the same node as the primary shard. It's therefore useful to try to have replicas in a single-node cluster.

3

@magnusbaeck thank.
But what if later I setup a new node to join?
Will master start to replicate data to new node(with all indexed docs before new node join) or stat to allocate replica from new node join?

4

Replica shards will always be in sync with the primary shards.

5

When the new node joins ES will try and allocate unallocated shard copies
to it. That means if you have a single node cluster and you make an index
with number_of_replicas: 1 and you add another node, then that other node
will get a second copy of everything on the first node.