Elasticsearch is used to retrieve data used in many pages. When data is created, updated, or deleted, and index of this data is saved into elastic search. This is denormlised data, which for us means it's saved into elastic search in a structure ready to be displayed onto our pages. Data in postgres, on the other hand, is saved in a different structure, that ensures data integrity.
For example, a crop needs a default photo. The photo used is the most "liked" photo of plantings, seeds or harvests for that crop. To get this from postgresql you'd need to look up the crops tables, the plantings,seeds,harvests, the photos, and then the likes. Instead of doing that on every display of a crop photo, we calculate this at data change and saved into the Elasticsearch index for crops.
Currently we use elastic search 7. You can check if this is still true by
looking for the ELASTIC_SEARCH_VERSION variable in the .travis.yml file
To install ES on a debian/ubuntu machine you can use the same script used in travis-ci:
export ELASTIC_SEARCH_VERSION="7.5.1-amd64" ./script/install_elasticsearch.sh
You can check that it started happily using:
./script/check_elasticsearch.sh