Sorting using script

ElasticSearch provides scripting support for sorting functionality. In real-world applications, there is often a need to modify the score using an algorithm that depends on the context and some external variables. Some common scenarios are as follows:

  • Sorting places near a point
  • Sorting by most read articles
  • Sorting items by custom customer logic
  • Sorting items with more revenue

Getting ready

You need a working ElasticSearch cluster and an index populated with the script used for facet processing, available in the online code.

How to do it...

For sorting using scripting, we will perform the following steps:

  1. If we want to order our documents by the price field multiplied by a factor parameter (usually VAT), the search will be as shown in the following code:
     curl -XGET 'http://127.0.0.1:9200/test-index/test-type/_search?&pretty=true&size=3' -d '{
  "query": {
    "match_all": {}
  },
  "sort": {
    "_script" : {
      "script" : "doc["price"].value * factor",
      "type" : "number",
      "ignore_unmapped" : true,
      "params" : {
        "factor" : 1.1
      },
            "order" : "asc"
        }
    }
}'

    In this case we have used a match_all query and a sort script.

  2. If everything is correct, the result returned by ElasticSearch should be as shown in the following code:
    {
  "took" : 7,
  "timed_out" : false,
  "_shards" : {
    "total" : 5,
    "successful" : 5,
    "failed" : 0
  },
  "hits" : {
    "total" : 1000,
    "max_score" : null,
    "hits" : [ {
      "_index" : "test-index",
      "_type" : "test-type",
      "_id" : "161",
      "_score" : null, "_source" : …,
      "sort" : [ 0.0278578661440021 ]
    }, {
      "_index" : "test-index",
      "_type" : "test-type",
      "_id" : "634",
      "_score" : null, "_source" : …,
      "sort" : [ 0.08131364254827411 ]
    }, {
      "_index" : "test-index",
      "_type" : "test-type",
      "_id" : "465",
      "_score" : null, "_source" : …,
      "sort" : [ 0.1094966959069832 ]
    } ]
  }
}

How it works...

The sort parameter, which we discussed in Chapter 5, Search, Queries, and Filters, can be extended with the help of scripting.

The sort scripting allows defining several parameters, such as:

  • order (default "asc") ("asc" or "desc"): This determines whether the order must be ascending or descending.
  • script: This contains the code to be executed.
  • type: This defines the type to convert the value.
  • params (optional, a JSON object): This defines the parameters to be passed to.
  • ignore_unmapped (optional): This ignores unmapped fields in sort. This flag allows us to skip problems due to missing fields in shards.

Extending the sort with scripting allows the use of a broader approach in scoring your hits.

Tip

ElasticSearch scripting permits the use of every code that you want. You can create custom complex algorithms for scoring your documents.

There's more...

MVEL provides a lot of built-in functions that can be used in scripts such as:

Function

Description

time()

The current time in milliseconds

sin(a)

Returns the trigonometric sine of an angle

cos(a)

Returns the trigonometric cosine of an angle

tan(a)

Returns the trigonometric tangent of an angle

asin(a)

Returns the arc sine of a value

acos(a)

Returns the arc cosine of a value

atan(a)

Returns the arc tangent of a value

toRadians(angdeg)

Converts an angle measured in degrees to an approximately equivalent angle measured in radians

toDegrees(angrad)

Converts an angle measured in radians to an approximately equivalent angle measured in degrees

exp(a)

Returns Euler's number raised to the power of a value

log(a)

Returns the natural logarithm (base e) of a value

log10(a)

Returns the base 10 logarithm of a value

sqrt(a)

Returns the correctly rounded positive square root of a value

cbrt(a)

Returns the cube root of a double value

IEEEremainder(f1, f2)

Computes the remainder operation on two arguments as prescribed by the IEEE 754 standard

ceil(a)

Returns the smallest (closest to negative infinity) value that is greater than or equal to the argument and is equal to a mathematical integer

floor(a)

Returns the largest (closest to positive infinity) value that is less than or equal to the argument and is equal to a mathematical integer

rint(a)

Returns the value that is closest in value to the argument and is equal to a mathematical integer

atan2(y, x)

Returns the angle, theta from the conversion of rectangular coordinates (x,y_) to polar coordinates (r,_theta)

pow(a, b)

Returns the value of the first argument raised to the power of the second argument

round(a)

Returns the closest integer to the argument

random()

Returns a random double value

abs(a)

Returns the absolute value of a value

max(a, b)

Returns the greater of two values

min(a, b)

Returns the smaller of two values

ulp(d)

Returns the size of the unit in the last place of the argument

signum(d)

Returns the signum function of the argument

sinh(x)

Returns the hyperbolic sine of a value

cosh(x)

Returns the hyperbolic cosine of a value

tanh(x)

Returns the hyperbolic tangent of a value

hypot(x,y)

Returns sqrt(x^2+y^2) without intermediate overflow or underflow

acos(a)

Returns the arc cosine of a value

atan(a)

Returns the arc tangent of a value

If you want to retrieve records in a random order, you can use a script with a random method as shown in the following code:

curl -XGET 'http://127.0.0.1:9200/test-index/test-type/_search?&pretty=true&size=3' -d '{
    "query": {
        "match_all": {}
    },
    "sort": {
        "_script" : {
            "script" : "Math.random()",
            "type" : "number",
            "params" : {}
        }
    }
}'
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