Index-Time Search-as-You-Type

The first step is to configure a custom edge_ngram token filter, which we will call the autocomplete_filter:

{
    "filter": {
        "autocomplete_filter": {
            "type":     "edge_ngram",
            "min_gram": 1,
            "max_gram": 20
        }
    }
}

This configuration says that, for any term that this token filter receives, it should produce an n-gram anchored to the start of the word of minimum length 1 and maximum length 20.

Then we need to use this token filter in a custom analyzer, which we will call the autocomplete analyzer:

{
    "analyzer": {
        "autocomplete": {
            "type":      "custom",
            "tokenizer": "standard",
            "filter": [
                "lowercase",
                "autocomplete_filter" 
            ]
        }
    }
}

This analyzer will tokenize a string into individual terms by using the standard tokenizer, lowercase each term, and then produce edge n-grams of each term, thanks to our autocomplete_filter.

The full request to create the index and instantiate the token filter and analyzer looks like this:

PUT /my_index
{
    "settings": {
        "number_of_shards": 1, 
        "analysis": {
            "filter": {
                "autocomplete_filter": { 
                    "type":     "edge_ngram",
                    "min_gram": 1,
                    "max_gram": 20
                }
            },
            "analyzer": {
                "autocomplete": {
                    "type":      "custom",
                    "tokenizer": "standard",
                    "filter": [
                        "lowercase",
                        "autocomplete_filter" 
                    ]
                }
            }
        }
    }
}

You can test this new analyzer to make sure it is behaving correctly by using the analyze API:

GET /my_index/_analyze
{
  "analyzer": "autocomplete",
  "text": "quick brown"
}

The results show us that the analyzer is working correctly. It returns these terms:

To use the analyzer, we need to apply it to a field, which we can do with the update-mapping API:

PUT /my_index/_mapping/my_type
{
    "my_type": {
        "properties": {
            "name": {
                "type":     "string",
                "analyzer": "autocomplete"
            }
        }
    }
}

Now, we can index some test documents:

POST /my_index/my_type/_bulk
{ "index": { "_id": 1            }}
{ "name": "Brown foxes"    }
{ "index": { "_id": 2            }}
{ "name": "Yellow furballs" }

If you test out a query for “brown fo” by using a simple match query

GET /my_index/my_type/_search
{
    "query": {
        "match": {
            "name": "brown fo"
        }
    }
}

you will see that both documents match, even though the Yellow furballs doc contains neither brown nor fo:

{

  "hits": [
     {
        "_id": "1",
        "_score": 1.5753809,
        "_source": {
           "name": "Brown foxes"
        }
     },
     {
        "_id": "2",
        "_score": 0.012520773,
        "_source": {
           "name": "Yellow furballs"
        }
     }
  ]
}

As always, the validate-query API shines some light:

GET /my_index/my_type/_validate/query?explain
{
    "query": {
        "match": {
            "name": "brown fo"
        }
    }
}

The explanation shows us that the query is looking for edge n-grams of every word in the query string:

name:b name:br name:bro name:brow name:brown name:f name:fo

The name:f condition is satisfied by the second document because furballs has been indexed as f, fu, fur, and so forth. In retrospect, this is not surprising. The same autocomplete analyzer is being applied both at index time and at search time, which in most situations is the right thing to do. This is one of the few occasions when it makes sense to break this rule.

We want to ensure that our inverted index contains edge n-grams of every word, but we want to match only the full words that the user has entered (brown and fo). We can do this by using the autocomplete analyzer at index time and the standard analyzer at search time. One way to change the search analyzer is just to specify it in the query:

GET /my_index/my_type/_search
{
    "query": {
        "match": {
            "name": {
                "query":    "brown fo",
                "analyzer": "standard" 
            }
        }
    }
}

Alternatively, we can specify the analyzer and search_analyzer in the mapping for the name field itself. Because we want to change only the search_analyzer, we can update the existing mapping without having to reindex our data:

PUT /my_index/my_type/_mapping
{
    "my_type": {
        "properties": {
            "name": {
                "type":            "string",
                "analyzer":  "autocomplete", 
                "search_analyzer": "standard" 
            }
        }
    }
}

If we were to repeat the validate-query request, it would now give us this explanation:

name:brown name:fo

Repeating our query correctly returns just the Brown foxes document.

Because most of the work has been done at index time, all this query needs to do is to look up the two terms brown and fo, which is much more efficient than the match_phrase_prefix approach of having to find all terms beginning with fo.

The edge n-gram approach can also be used for structured data, such as the postcodes example from earlier in this chapter. Of course, the postcode field would need to be analyzed instead of not_analyzed, but you could use the keyword tokenizer to treat the postcodes as if they were not_analyzed.

This example uses the keyword tokenizer to convert the postcode string into a token stream, so that we can use the edge n-gram token filter:

{
    "analysis": {
        "filter": {
            "postcode_filter": {
                "type":     "edge_ngram",
                "min_gram": 1,
                "max_gram": 8
            }
        },
        "analyzer": {
            "postcode_index": { 
                "tokenizer": "keyword",
                "filter":    [ "postcode_filter" ]
            },
            "postcode_search": { 
                "tokenizer": "keyword"
            }
        }
    }
}