[[cat-shards]]
== cat shards

The `shards` command is the detailed view of what nodes contain which
shards.  It will tell you if it's a primary or replica, the number of
docs, the bytes it takes on disk, and the node where it's located.

Here we see a single index, with three primary shards and no replicas:

[source,sh]
--------------------------------------------------
% curl 192.168.56.20:9200/_cat/shards
wiki1 0 p STARTED 3014 31.1mb 192.168.56.10 Stiletto
wiki1 1 p STARTED 3013 29.6mb 192.168.56.30 Frankie Raye
wiki1 2 p STARTED 3973 38.1mb 192.168.56.20 Commander Kraken
--------------------------------------------------

[float]
[[index-pattern]]
=== Index pattern

If you have many shards, you may wish to limit which indices show up
in the output.  You can always do this with `grep`, but you can save
some bandwidth by supplying an index pattern to the end.

[source,sh]
--------------------------------------------------
% curl 192.168.56.20:9200/_cat/shards/wiki*
wiki2 0 p STARTED 197 3.2mb 192.168.56.10 Stiletto
wiki2 1 p STARTED 205 5.9mb 192.168.56.30 Frankie Raye
wiki2 2 p STARTED 275 7.8mb 192.168.56.20 Commander Kraken
--------------------------------------------------


[float]
[[relocation]]
=== Relocation

Let's say you've checked your health and you see two relocating
shards.  Where are they from and where are they going?

[source,sh]
--------------------------------------------------
% curl 192.168.56.10:9200/_cat/health
1384315316 20:01:56 foo green 3 3 12 6 2 0 0
% curl 192.168.56.10:9200/_cat/shards | fgrep RELO
wiki1 0 r RELOCATING 3014 31.1mb 192.168.56.20 Commander Kraken -> 192.168.56.30 Frankie Raye
wiki1 1 r RELOCATING 3013 29.6mb 192.168.56.10 Stiletto -> 192.168.56.30 Frankie Raye
--------------------------------------------------

[float]
[[states]]
=== Shard states

Before a shard can be used, it goes through an `INITIALIZING` state.
`shards` can show you which ones.

[source,sh]
--------------------------------------------------
% curl -XPUT 192.168.56.20:9200/_settings -d'{"number_of_replicas":1}'
{"acknowledged":true}
% curl 192.168.56.20:9200/_cat/shards
wiki1 0 p STARTED      3014 31.1mb 192.168.56.10 Stiletto
wiki1 0 r INITIALIZING    0 14.3mb 192.168.56.30 Frankie Raye
wiki1 1 p STARTED      3013 29.6mb 192.168.56.30 Frankie Raye
wiki1 1 r INITIALIZING    0 13.1mb 192.168.56.20 Commander Kraken
wiki1 2 r INITIALIZING    0   14mb 192.168.56.10 Stiletto
wiki1 2 p STARTED      3973 38.1mb 192.168.56.20 Commander Kraken
--------------------------------------------------

If a shard cannot be assigned, for example you've overallocated the
number of replicas for the number of nodes in the cluster, the shard
will remain `UNASSIGNED` with the <<reason-unassigned,reason code>> `ALLOCATION_FAILED`.

[source,sh]
--------------------------------------------------
% curl -XPUT 192.168.56.20:9200/_settings -d'{"number_of_replicas":3}'
% curl 192.168.56.20:9200/_cat/health
1384316325 20:18:45 foo yellow 3 3 9 3 0 0 3
% curl 192.168.56.20:9200/_cat/shards
wiki1 0 p STARTED    3014 31.1mb 192.168.56.10 Stiletto
wiki1 0 r STARTED    3014 31.1mb 192.168.56.30 Frankie Raye
wiki1 0 r STARTED    3014 31.1mb 192.168.56.20 Commander Kraken
wiki1 0 r UNASSIGNED ALLOCATION_FAILED
wiki1 1 r STARTED    3013 29.6mb 192.168.56.10 Stiletto
wiki1 1 p STARTED    3013 29.6mb 192.168.56.30 Frankie Raye
wiki1 1 r STARTED    3013 29.6mb 192.168.56.20 Commander Kraken
wiki1 1 r UNASSIGNED ALLOCATION_FAILED
wiki1 2 r STARTED    3973 38.1mb 192.168.56.10 Stiletto
wiki1 2 r STARTED    3973 38.1mb 192.168.56.30 Frankie Raye
wiki1 2 p STARTED    3973 38.1mb 192.168.56.20 Commander Kraken
wiki1 2 r UNASSIGNED ALLOCATION_FAILED
--------------------------------------------------

[float]
[[reason-unassigned]]
=== Reasons for unassigned shard

These are the possible reasons for a shard be in a unassigned state:

[horizontal]
`INDEX_CREATED`::           Unassigned as a result of an API creation of an index.
`CLUSTER_RECOVERED`::       Unassigned as a result of a full cluster recovery.
`INDEX_REOPENED`::          Unassigned as a result of opening a closed index.
`DANGLING_INDEX_IMPORTED`:: Unassigned as a result of importing a dangling index.
`NEW_INDEX_RESTORED`::      Unassigned as a result of restoring into a new index.
`EXISTING_INDEX_RESTORED`:: Unassigned as a result of restoring into a closed index.
`REPLICA_ADDED`::           Unassigned as a result of explicit addition of a replica.
`ALLOCATION_FAILED`::       Unassigned as a result of a failed allocation of the shard.
`NODE_LEFT`::               Unassigned as a result of the node hosting it leaving the cluster.
`REROUTE_CANCELLED`::       Unassigned as a result of explicit cancel reroute command.
`REINITIALIZED`::           When a shard moves from started back to initializing, for example, with shadow replicas.
`REALLOCATED_REPLICA`::     A better replica location is identified and causes the existing replica allocation to be cancelled.