You may have heard about synonyms—words that have the same or similar meaning. Sometimes, you will want to have some words match when one of those words is entered into the search box. Let's recall our sample data from The example data section of Chapter 3, Searching Your Data; there was a book called Crime and Punishment. What if we want that book to be matched not only when the words crime or punishment are used, but also when using words like criminality and abuse. To perform this, we will use synonyms.
In order to use the synonym filter, we need to define our own analyzer. Our analyzer will be called synonym and will use the whitespace tokenizer and a single filter called synonym. Our filter's type property needs to be set to synonym, which tells Elasticsearch that this filter is a synonym filter. In addition to that, we want to ignore case so that upper- and lowercase synonyms will be treated equally (set the ignore_case property to true...