You will need an up-and-running Elasticsearch installation as used in the Downloading and installing Elasticsearch recipe in Chapter 2, Downloading and Setup.

To execute curl via a command line, you need to install curl for your operating system.

To correctly execute the following commands, you will need an index populated with the chapter_05/populate_query.sh script available in the online code.

In order to execute a counting query, we will perform the following steps:

The result is composed of the count result (a long type) and the shard status at the time of the query.

The query is interpreted in the same way as for searching. The count action is processed and distributed in all the shards, in which is executed as a low-level Lucene count call. Every hit shard returns a count that is aggregated and returned to the user.

The HTTP method to execute a count is GET (but also POST works), and the REST endpoints are as follows:

http://<server>/_count

http://<server>/<index_name(s)>/_count

http://<server>/<index_name(s)>/<type_name(s)>/_count

Multi indices and types are comma separated. If an index or a type is defined, the search is limited only to them. An alias can be used as an index name.

Typically, a body is used to express a query, but for simple queries, the q (query argument) can be used. For example, look at the following code:

    curl -XGET 'http://127.0.0.1:9200/test-index/test-type/_count?
    q=uuid:11111'

In a previous version of Elasticsearch, the count API call (_count REST entrypoint) was implemented as a custom action, but in Elasticsearch version 5.x, it's removed. Internally the previous count API is implemented as a standard search with the size set to 0.

Using this trick, it not only speeds up the searching, but reduces networking.

You can use this approach to execute aggregations (we will see them in Chapter 8, Aggregations) without returning hits.

The previous query can be also executed as follows:

    curl -XGET 'http://127.0.0.1:9200/test-index/test-type/_search?
    pretty&size=0' -d '{"query":{"match_all":{}}}'

And the result returned by Elasticsearch, if everything works, should be as follows:

{ 
  "took" : 32, 
  "timed_out" : false, 
  "_shards" : { 
    "total" : 5, 
    "successful" : 5, 
    "failed" : 0 
  }, 
  "hits" : { 
    "total" : 3, 
    "max_score" : 0.0, 
    "hits" : [ ] 
  } 
} 

The count result (a long type) is available in the hits.total.