Use this free bayesian A/B testing calculator to find out if your test results are statistically significant.

Use this free bayesian A/B testing calculator to find out if your test results are statistically significant. 
https://marketing.dynamicyield.com/bayesian-calculator/

Learning to rank problems. Well explained

Learn to rank and its various optimizations - http://opensourceconnections.com/blog/2017/08/03/search-as-machine-learning-prob/

http://opensourceconnections.com/blog/2017/04/01/learning-to-rank-linear-models/

What works in e-commerce - a meta-analysis of 6700 online experiments

http://www.qubit.com/sites/default/files/pdf/qubit_meta_analysis.pdf

Good short overview of caching in modern versions of ES - Caching In Elasticsearch

On two types of suggestions

Basic overview of two types of search suggestions: content && search query based suggestions.

https://www.searchblox.com/autosuggest-search-query-based-vs-content-based

Spatial search algorithms and data scructures

https://medium.com/@agafonkin/a-dive-into-spatial-search-algorithms-ebd0c5e39d2a

How to deal with the synonyms. Awesome examples

http://opensourceconnections.com/blog/2016/12/02/solr-elasticsearch-synonyms-better-patterns-keyphrases/