Indices Statsedit
Indices level stats provide statistics on different operations happening on an index. The API provides statistics on the index level scope (though most stats can also be retrieved using node level scope).
The following returns high level aggregation and index level stats for all indices:
curl localhost:9200/_stats
Specific index stats can be retrieved using:
curl localhost:9200/index1,index2/_stats
By default, all stats are returned, returning only specific stats can be specified as well in the URI. Those stats can be any of:

The number of docs / deleted docs (docs not yet merged out). Note, affected by refreshing the index. 

The size of the index. 

Indexing statistics, can be combined with a comma
separated list of 

Get statistics, including missing stats. 

Search statistics. You can include statistics for custom groups by adding
an extra 

Completion suggest statistics. 

Fielddata statistics. 

Flush statistics. 

Merge statistics. 

Shard query cache statistics. 

Refresh statistics. 

Suggest statistics. 

Warmer statistics. 

Translog statistics. 
Some statistics allow per field granularity which accepts a list commaseparated list of included fields. By default all fields are included:

List of fields to be included in the statistics. This is used as the default list unless a more specific field list is provided (see below). 

List of fields to be included in the Completion Suggest statistics. 

List of fields to be included in the Fielddata statistics. 
Here are some samples:
# Get back stats for merge and refresh only for all indices curl 'localhost:9200/_stats/merge,refresh' # Get back stats for type1 and type2 documents for the my_index index curl 'localhost:9200/my_index/_stats/indexing?types=type1,type2 # Get back just search stats for group1 and group2 curl 'localhost:9200/_stats/search?groups=group1,group2
The stats returned are aggregated on the index level, with
primaries
and total
aggregations, where primaries
are the values for only the
primary shards, and total
are the cumulated values for both primary and replica shards.
In order to get back shard level stats, set the level
parameter to shards
.
Note, as shards move around the cluster, their stats will be cleared as they are created on other nodes. On the other hand, even though a shard "left" a node, that node will still retain the stats that shard contributed to.