[RASCUNHO] Coleção de textos sobre como modificar a função de similaridade do ElasticSearch

It is necessary to extends the classes:
org.apache.lucene.search.similarities.Similarity;
org.elasticsearch.index.similarity.AbstractSimilarityProvider;

Since the ES 0.9 it is possible to change the similarity function to each field. This document explains how:
http://elasticsearchserverbook.com/elasticsearch-0-90-similarities/

General information:
http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/index-modules-similarity.html

About the tf/idf and BM25:
https://www.found.no/foundation/similarity/
http://stackoverflow.com/questions/19423423/simple-explanation-of-different-elasticsearch-similarity-algorithms
https://groups.google.com/forum/#!topic/elasticsearch/UZYa49_9AFg

Implementation of a custom function:
http://elasticsearch-users.115913.n3.nabble.com/How-to-use-ElasticSearch-Custom-Similarity-provider-classes-td4047683.html
https://github.com/awnuxkjy/es-custom-similarity-provider/tree/master/src/main/java/org/elasticsearch/index/similarity
http://elasticsearch-users.115913.n3.nabble.com/Configuring-a-Custom-Similarity-td4034063.html