Support multi-search (_msearch)

[chrispappas]

Elasticsearch supports a _msearch API, which allows you to send multiple queries with one request.

https://www.elastic.co/guide/en/elasticsearch/reference/current/search-multi-search.html

The request body looks like an ES bulk index request, with a header and body line for each request.

GET my-index-000001/_msearch
{ }
{"query" : {"match" : { "message": "this is a test"}}}
{"index": "my-index-000002"}
{"query" : {"match_all" : {}}}

The response JSON includes a top-level responses key with an array of sub-response objects:

{
  "took" : 3,
  "responses" : [
    {
      "took" : 1,
      "timed_out" : false,
      "_shards" : {
        "total" : 1,
        "successful" : 1,
        "skipped" : 0,
        "failed" : 0
      },
      "hits" : {
        "total" : {
          "value" : 10000,
          "relation" : "gte"
        },
        "max_score" : 0.0,
        "hits" : [
          {
      ...
    },
    {
      "took" : 1,
      "timed_out" : false,
      "_shards" : {
    ...

Since this library doesn't handle the response, all it would need to do is support building an array of requests and running it via the _msearch API.

If this has not been implemented yet, I'd like to take a crack at it with a MR when I can get to it. Of course, if the project is not interested in such a feature, I won't spend the time on it.

[chrispappas]

PR is up for this feature:
#64