Depending on the environment where an Elasticsearch cluster is running, the (default) index settings might need to be changed. Especially if the cluster only exists with a one-node setup (which is nothing abnormal) indexes with a replica setting of 1 or more will report unassigned shards.
This can be verified with the check_es_system Elasticsearch monitoring plugin:
root@elasticsearch:~# /usr/lib/nagios/plugins/check_es_system.sh -H localhost -t status
ES SYSTEM WARNING - Elasticsearch Cluster "mycluster" is yellow (1 nodes, 1 data nodes, 3 shards, 0 relocating shards, 0 initializing shards, 3 unassigned shards, 10934 docs)
The reason can be spotted when listing all the available shards:
root@elasticsearch:~# curl -q -s "http://localhost:9200/_cat/shards"
magento2_product_1_v204 0 p STARTED 3639 6.2mb 127.0.0.1 elasticsearch
magento2_product_1_v204 0 r UNASSIGNED
magento2_product_2_v204 0 p STARTED 3639 7.1mb 127.0.0.1 elasticsearch
magento2_product_2_v204 0 r UNASSIGNED
magento2_product_3_v204 0 p STARTED 3656 6.2mb 127.0.0.1 elasticsearch
magento2_product_3_v204 0 r UNASSIGNED
All the unassigned shards have "r" as second flag - which stands for replicated shard. As this is a single node cluster, Elasticsearch does not find any other nodes where to replicate this shard to -> UNASSIGNED.
By looking at the settings of these three indexes, especially the number_of_replicas setting, all indexes have a (default) setting of 1 replica defined:
root@elasticsearch:~# curl -q -s "http://localhost:9200/magento2_product_1_v204/_settings" | jq -r '.magento2_product_1_v204.settings.index.number_of_replicas'
1
root@elasticsearch:~# curl -q -s
"http://localhost:9200/magento2_product_2_v204/_settings" | jq -r
'.magento2_product_2_v204.settings.index.number_of_replicas'
1
root@elasticsearch:~# curl -q -s
"http://localhost:9200/magento2_product_3_v204/_settings" | jq -r
'.magento2_product_2_v204.settings.index.number_of_replicas'
1
Fortunately this setting can be changed without downtime. With a PUT request the index settings can be updated:
root@elasticsearch:~# curl -X PUT -H "Content-Type: application/json" "http://localhost:9200/magento2_product_1_v204/_settings" -d '{ "number_of_replicas": 0 }'
{"acknowledged":true}
Verification of the current setting value:
root@elasticsearch:~# curl -q -s "http://localhost:9200/magento2_product_1_v204/_settings" | jq -r '.magento2_product_1_v204.settings.index.number_of_replicas'
0
Yes, it worked!
Now looking at the shards again, we can see there are only two unassigned shards left:
root@elasticsearch:~# curl -q -s "http://localhost:9200/_cat/shards"
magento2_product_1_v204 0 p STARTED 3639 6.2mb 127.0.0.1 elasticsearch
magento2_product_2_v204 0 p STARTED 3639 7.1mb 127.0.0.1 elasticsearch
magento2_product_2_v204 0 r UNASSIGNED
magento2_product_3_v204 0 p STARTED 3656 6.2mb 127.0.0.1 elasticsearch
magento2_product_3_v204 0 r UNASSIGNED
The number_of_replicas setting is now also changed on the two remaining indexes:
root@elasticsearch:~# curl -X PUT -H "Content-Type: application/json" "http://localhost:9200/magento2_product_2_v204/_settings" -d '{ "number_of_replicas": 0 }'
{"acknowledged":true}
root@elasticsearch:~# curl -X PUT -H "Content-Type: application/json" "http://localhost:9200/magento2_product_3_v204/_settings" -d '{ "number_of_replicas": 0 }'
{"acknowledged":true}
And voilĂ , no more unassigned shards are shown:
root@elasticsearch:~# curl -q -s "http://localhost:9200/_cat/shards"
magento2_product_1_v204 0 p STARTED 3639 6.2mb 127.0.0.1 elasticsearch
magento2_product_2_v204 0 p STARTED 3639 7.1mb 127.0.0.1 elasticsearch
magento2_product_3_v204 0 p STARTED 3656 6.2mb 127.0.0.1 elasticsearch
Now with all unassigned shards gone, the Elasticsearch status immediately changed to green:
root@elasticsearch:~# /usr/lib/nagios/plugins/check_es_system.sh -H localhost -t status
ES SYSTEM OK - Elasticsearch Cluster "mycluster" is green (1 nodes, 1 data nodes, 3 shards, 10934 docs)|total_nodes=1;;;; data_nodes=1;;;; total_shards=3;;;; relocating_shards=0;;;; initializing_shards=0;;;; unassigned_shards=0;;;; docs=10934;;;;
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