# WIP - Scaling Down Elasticsearch Clusters To Single Node

This is a template documentation about scaling an es cluster to a single node without any errors and minimum downtime. Feel free to post recommendations using the comment section below.

First, run this command to exclude node(s). You can use a comma-separated list of IP addresses; (Note: Explain why and what is this setting)

```bash
$ curl -XPUT X.X.X.X:9200/_cluster/settings -H 'Content-Type: application/json' -d '{
  "persistent" :{
      "cluster.routing.allocation.exclude._ip" : "Y.Y.Y.Y,Z.Z.Z.Z"
   }
}';echo
```

Check cluster settings if nodes are excluded correctly;

```plaintext
$ curl -XGET 'X.X.X.X:9200/_cluster/settings?pretty'
```

It should look similar to this:

```json
{
  "persistent" : {
    "cluster" : {
      "routing" : {
        "allocation" : {
          "exclude" : {
            "_ip" : "Y.Y.Y.Y,Z.Z.Z.Z"
            }
          }
        }
      }
    },
  "transient" : { }
}
```

Wait until you see `status: green` and `relocating shards: 0` when you run the following command. This process may take some time depending on the replication count setting in the indexes.

```bash
$ curl -XGET 'X.X.X.X:9200/_cluster/health?pretty';
```

```json
{
  "cluster_name" : "cluster-name",
  "status" : "green",
  "timed_out" : false,
  "number_of_nodes" : 3,
  "number_of_data_nodes" : 3,
  "active_primary_shards" : 60,
  "active_shards" : 120,
  "relocating_shards" : 0,
  "initializing_shards" : 0,
  "unassigned_shards" : 0,
  "delayed_unassigned_shards" : 0,
  "number_of_pending_tasks" : 0,
  "number_of_in_flight_fetch" : 0,
  "task_max_waiting_in_queue_millis" : 0,
  "active_shards_percent_as_number" : 100.0
}
```

After that, you should edit the `elasticsearch.yml` file on the last server standing so that there is no other elastic node in the list. In my case the relevant lines are:

```yaml
discovery.zen.ping.unicast.hosts: ["X.X.X.X", "Y.Y.Y.Y", "Z.Z.Z.Z"]
discovery.zen.minimum_master_nodes: 2
```

I've commented out the `minimum_master_node` line as it is. I also left only one IP in the `ping.unicast.hosts` list;

```yaml
discovery.zen.ping.unicast.hosts: ["X.X.X.X"]
#discovery.zen.minimum_master_nodes: 2
```

Then you have to restart the elasticsearch service. You can now safely close the nodes that you have excluded from the cluster.

You might want to check the status of the shards to make sure everything is correct. If you see an `UNASSIGNED` shard in the list, it is likely because elastic does not have a node left to replicate this shard.

```bash
$ curl -XGET 'http://X.X.X.X:9200/_cat/shards?v';
```

```plaintext
index                            shard prirep state   docs   store ip            node
index-name                       3     r      STARTED   42  74.4kb X.X.X.X localhost.localdomain
index-name                       3     r      UNASIGNED
```

This causes the cluster health to appear yellow and is simple to fix;

```bash
$ curl -XPUT X.X.X.X:9200/_settings -H 'Content-Type: application/json' -d '{
  "index" :{
      "number_of_replicas": 0
   }
}';echo
```

After this step, all shards should be listed as `STARTED`

Currently, all data is on the server with the `X.X.X.X` address and is not trying to communicate with other nodes. If you follow the steps in reverse, you can restore the cluster.