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Controlling Analysis

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Controlling Analysis

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Queries can find only terms that actually exist in the inverted index, so it is important to ensure that the same analysis process is applied both to the document at index time, and to the query string at search time so that the terms in the query match the terms in the inverted index.

Although we say document, analyzers are determined per field. Each field can have a different analyzer, either by configuring a specific analyzer for that field or by falling back on the type, index, or node defaults. At index time, a field’s value is analyzed by using the configured or default analyzer for that field.

For instance, let’s add a new field to my_index:

PUT /my_index/_mapping/my_type
{
    "my_type": {
        "properties": {
            "english_title": {
                "type":     "string",
                "analyzer": "english"
            }
        }
    }
}

Now we can compare how values in the english_title field and the title field are analyzed at index time by using the analyze API to analyze the word Foxes:

GET /my_index/_analyze?field=my_type.title   
Foxes

GET /my_index/_analyze?field=my_type.english_title 
Foxes

Field title, which uses the default standard analyzer, will return the term foxes.

Field english_title, which uses the english analyzer, will return the term fox.

This means that, were we to run a low-level term query for the exact term fox, the english_title field would match but the title field would not.

High-level queries like the match query understand field mappings and can apply the correct analyzer for each field being queried. We can see this in action with the validate-query API:

GET /my_index/my_type/_validate/query?explain
{
    "query": {
        "bool": {
            "should": [
                { "match": { "title":         "Foxes"}},
                { "match": { "english_title": "Foxes"}}
            ]
        }
    }
}

which returns this explanation:

(title:foxes english_title:fox)

The match query uses the appropriate analyzer for each field to ensure that it looks for each term in the correct format for that field.

Default Analyzers

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While we can specify an analyzer at the field level, how do we determine which analyzer is used for a field if none is specified at the field level?

Analyzers can be specified at several levels. Elasticsearch works through each level until it finds an analyzer that it can use. At index time, the order is as follows:

  • The analyzer defined in the field mapping, else
  • The analyzer defined in the _analyzer field of the document, else
  • The default analyzer for the type, which defaults to
  • The analyzer named default in the index settings, which defaults to
  • The analyzer named default at node level, which defaults to
  • The standard analyzer

At search time, the sequence is slightly different:

  • The analyzer defined in the query itself, else
  • The analyzer defined in the field mapping, else
  • The default analyzer for the type, which defaults to
  • The analyzer named default in the index settings, which defaults to
  • The analyzer named default at node level, which defaults to
  • The standard analyzer

The two lines in italics in the preceding lists highlight differences in the index time sequence and the search time sequence. The _analyzer field allows you to specify a default analyzer for each document (for example, english, french, spanish) while the analyzer parameter in the query specifies which analyzer to use on the query string. However, this is not the best way to handle multiple languages in a single index because of the pitfalls highlighted in Dealing with Human Language.

Occasionally, it makes sense to use a different analyzer at index and search time. For instance, at index time we may want to index synonyms (for example, for every occurrence of quick, we also index fast, rapid, and speedy). But at search time, we don’t need to search for all of these synonyms. Instead we can just look up the single word that the user has entered, be it quick, fast, rapid, or speedy.

To enable this distinction, Elasticsearch also supports the index_analyzer and search_analyzer parameters, and analyzers named default_index and default_search.

Taking these extra parameters into account, the full sequence at index time really looks like this:

  • The index_analyzer defined in the field mapping, else
  • The analyzer defined in the field mapping, else
  • The analyzer defined in the _analyzer field of the document, else
  • The default index_analyzer for the type, which defaults to
  • The default analyzer for the type, which defaults to
  • The analyzer named default_index in the index settings, which defaults to
  • The analyzer named default in the index settings, which defaults to
  • The analyzer named default_index at node level, which defaults to
  • The analyzer named default at node level, which defaults to
  • The standard analyzer

And at search time:

  • The analyzer defined in the query itself, else
  • The search_analyzer defined in the field mapping, else
  • The analyzer defined in the field mapping, else
  • The default search_analyzer for the type, which defaults to
  • The default analyzer for the type, which defaults to
  • The analyzer named default_search in the index settings, which defaults to
  • The analyzer named default in the index settings, which defaults to
  • The analyzer named default_search at node level, which defaults to
  • The analyzer named default at node level, which defaults to
  • The standard analyzer

Configuring Analyzers in Practice

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The sheer number of places where you can specify an analyzer is quite overwhelming. In practice, though, it is pretty simple.

Use index settings, not config files

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The first thing to remember is that, even though you may start out using Elasticsearch for a single purpose or a single application such as logging, chances are that you will find more use cases and end up running several distinct applications on the same cluster. Each index needs to be independent and independently configurable. You don’t want to set defaults for one use case, only to have to override them for another use case later.

This rules out configuring analyzers at the node level. Additionally, configuring analyzers at the node level requires changing the config file on every node and restarting every node, which becomes a maintenance nightmare. It’s a much better idea to keep Elasticsearch running and to manage settings only via the API.

Keep it simple

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Most of the time, you will know what fields your documents will contain ahead of time. The simplest approach is to set the analyzer for each full-text field when you create your index or add type mappings. While this approach is slightly more verbose, it enables you to easily see which analyzer is being applied to each field.

Typically, most of your string fields will be exact-value not_analyzed fields such as tags or enums, plus a handful of full-text fields that will use some default analyzer like standard or english or some other language. Then you may have one or two fields that need custom analysis: perhaps the title field needs to be indexed in a way that supports find-as-you-type.

You can set the default analyzer in the index to the analyzer you want to use for almost all full-text fields, and just configure the specialized analyzer on the one or two fields that need it. If, in your model, you need a different default analyzer per type, then use the type level analyzer setting instead.

A common work flow for time based data like logging is to create a new index per day on the fly by just indexing into it. While this work flow prevents you from creating your index up front, you can still use index templates to specify the settings and mappings that a new index should have.

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