Okamoto Hidetaka

This plugin gives you Related Item Widget and function.
If you want to show Related Item List, you can do following actions:
<ul>
<li>Add <code>Elasticommerce Related Widgets</code> in your widgets area.</li>
<li>Add <code>escr_related_item();</code> function in your theme or plugins.</li>
<li>If you only want to get the data, you can use <code>escr_get_related_item();</code> function.</li>
</ul>
Notice 
This plugin requires WooCommerce Plugin , so please install it. If WooCommerce Plugin is not installed or activated, this plugin will not work.
Required 
This Plugin cannot use Elasticsearch version 2.x. Please use Elasticsearch version 1.5 instead.

Elasticommerce Related Items

You can get good Related Item List powered by Elasticsearch.

Paris Holley

This plugin is NOT a simple drop-in, it is expected you have some understanding of what an ElasticSearch server is and how it works. The goals/features of this plugin are:
<ul>
<li>Replace default wordpress search functionality with that of an ElasticSearch server.</li>
<li>Ability to specify what data points should be indexed and what the relevancy of those points are.</li>
<li>Fall back to default wordpress search if ElasticSearch server is not responsive.</li>
<li>Update ElasticSearch server when posts are removed/added/unpublished.</li>
<li>Provide a faceting API for building your own custom searches</li>
</ul>
Please submit bugs or contributions to the github location and not on wordpress’ system:
https://github.com/parisholley/wordpress-fantastic-elasticsearch/
API/filter/faceting documentation can be found on the wiki:
https://github.com/parisholley/wordpress-fantastic-elasticsearch/wiki

Fantastic ElasticSearch

Improve wordpress search performance/accuracy and enable faceted search by leveraging an ElasticSearch server.

Elasticommerce Related Items vs Fantastic ElasticSearch