You may have been wondering what consistently random scoring is, or why you would ever want to use it. The previous example provides a good use case. All results from the previous example would receive a final _score of 1, 2, 3, 4, or 5. Maybe there are only a few homes that score 5, but presumably there would be a lot of homes scoring 2 or 3.

As the owner of the website, you want to give your advertisers as much exposure as possible. With the current query, results with the same _score would be returned in the same order every time. It would be good to introduce some randomness here, to ensure that all documents in a single score level get a similar amount of exposure.

We want every user to see a different random order, but we want the same user to see the same order when clicking on page 2, 3, and so forth. This is what is meant by consistently random.

The random_score function, which outputs a number between 0 and 1, will produce consistently random results when it is provided with the same seed value, such as a user’s session ID:

GET /_search
{
  "query": {
    "function_score": {
      "filter": {
        "term": { "city": "Barcelona" }
      },
      "functions": [
        {
          "filter": { "term": { "features": "wifi" }},
          "weight": 1
        },
        {
          "filter": { "term": { "features": "garden" }},
          "weight": 1
        },
        {
          "filter": { "term": { "features": "pool" }},
          "weight": 2
        },
        {
          "random_score": { 
            "seed":  "the users session id" 
          }
        }
      ],
      "score_mode": "sum"
    }
  }
}

Of course, if you index new documents that match the query, the order of results will change regardless of whether you use consistent randomization or not.