After connecting Redis and MongoDB to Solr, I figured it'd be interesting to do the same with ElasticSearch. Here's the result of my experiments:

We'll be implementing this using AbstractSearchScript, which is roughly ElasticSearch's version of Solr's FunctionQuery.

ES' NativeScriptFactory corresponds loosely to Solr's ValueSourceParser, and AbstractSearchScript to ValueSource.

public class RedisNativeScriptFactory implements NativeScriptFactory {
  @Override public ExecutableScript newScript(@Nullable Map<String, Object> params) {
    return new RedisScript(params);
  }
}

public class RedisScript extends AbstractFloatSearchScript {
  private String idField;
  private String redisKey;
  private String redisValue;
  private final Jedis jedis;
  private JSONObject obj;

  public RedisScript(Map<String, Object> params) {
    this.idField = (String) params.get("idField");
    this.redisKey = (String) params.get("redisKey");
    this.redisValue = (String) params.get("redisValue");
    jedis = new Jedis("localhost");
    String v = jedis.hget(redisKey, redisValue);
    if (v != null) {
      obj = (JSONObject) JSONValue.parse(v);
    } else {
      obj = new JSONObject();
    }
  }

  @Override public float runAsFloat() {
    String id = doc().field(idField).stringValue();
    Object v = obj.get(id);
    if (v != null) {
      try {
        return Float.parseFloat(v.toString());
      } catch (NumberFormatException e) {
        return 0;
      }
    }
    return 0;
  }
}

Now in config/elasticsearch.yml, add this:

script.native:
  redis.type: org.supermind.es.redis.RedisNativeScriptFactory

Change redis to whatever you want the script name to be, and change the class name accordingly too.

Now, to use this:

curl -XGET 'http://localhost:9200/electronics/product/_search' -d '{
  "query" :{
     "custom_score": {
       "query" : { "match_all": {}},
       "script" : "redis",
       "params" :{
          "idField": "id",
          "redisKey": "bar",
          "redisValue" : "500"
       },
       "lang": "native"
     }
  }
}'

PS: My implementation of RedisScript assumes a Redis hash has been populated with a json object corresponding to an idField. Here's a class populating the redis hash. JSON objects are created with the json-smart package, but you can plugin your favourite json lib:

public static void main(String[] args) {
    Jedis jedis = new Jedis("localhost");
    int num = 100000;
    Random r = new Random();
    for(int i=0;i< num;++i) {
      JSONObject o = new JSONObject();
      int numberOfEntries = r.nextInt(100);
      for(int j=0;j< numberOfEntries;++j) {
        o.put("es" + j, r.nextInt(100));
      }
      String json = o.toJSONString(JSONStyle.MAX_COMPRESS);
      jedis.hset("bar", Integer.toString(i), json);
    }
  }