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Elastic Docs ›Elasticsearch: The Definitive Guide [2.x] ›Search in Depth ›Controlling Relevance

Scoring with Scripts

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Scoring with Scripts

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Finally, if none of the function_score's built-in functions suffice, you can implement the logic that you need with a script, using the script_score function.

For an example, let’s say that we want to factor our profit margin into the relevance calculation. In our business, the profit margin depends on three factors:

  • The price per night of the vacation home.
  • The user’s membership level—​some levels get a percentage discount above a certain price per night threshold.
  • The negotiated margin as a percentage of the price-per-night, after user discounts.

The algorithm that we will use to calculate the profit for each home is as follows:

if (price < threshold) {
  profit = price * margin
} else {
  profit = price * (1 - discount) * margin;
}

We probably don’t want to use the absolute profit as a score; it would overwhelm the other factors like location, popularity and features. Instead, we can express the profit as a percentage of our target profit. A profit margin above our target will have a positive score (greater than 1.0), and a profit margin below our target will have a negative score (less than 1.0):

if (price < threshold) {
  profit = price * margin
} else {
  profit = price * (1 - discount) * margin
}
return profit / target

The default scripting language in Elasticsearch is Groovy, which for the most part looks a lot like JavaScript. The preceding algorithm as a Groovy script would look like this:

price  = doc['price'].value 
margin = doc['margin'].value 

if (price < threshold) { 
  return price * margin / target
}
return price * (1 - discount) * margin / target

The price and margin variables are extracted from the price and margin fields in the document.

The threshold, discount, and target variables we will pass in as params.

Finally, we can add our script_score function to the list of other functions that we are already using:

GET /_search
{
  "function_score": {
    "functions": [
      { ...location clause... }, 
      { ...price clause... }, 
      {
        "script_score": {
          "params": { 
            "threshold": 80,
            "discount": 0.1,
            "target": 10
          },
          "script": "price  = doc['price'].value; margin = doc['margin'].value;
          if (price < threshold) { return price * margin / target };
          return price * (1 - discount) * margin / target;" 
        }
      }
    ]
  }
}

The location and price clauses refer to the example explained in The Closer, The Better.

By passing in these variables as params, we can change their values every time we run this query without having to recompile the script.

JSON cannot include embedded newline characters. Newline characters in the script should either be escaped as \n or replaced with semicolons.

This query would return the documents that best satisfy the user’s requirements for location and price, while still factoring in our need to make a profit.

The script_score function provides enormous flexibility. Within a script, you have access to the fields of the document, to the current _score, and even to the term frequencies, inverse document frequencies, and field length norms (see Text scoring in scripts).

That said, scripts can have a performance impact. If you do find that your scripts are not quite fast enough, you have three options:

  • Try to precalculate as much information as possible and include it in each document.
  • Groovy is fast, but not quite as fast as Java. You could reimplement your script as a native Java script. (See Native Java Scripts).
  • Use the rescore functionality described in Rescoring Results to apply your script to only the best-scoring documents.
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