How to create unique constraint in Elasticsearch database?

One solution will be to use uniqueId field value for specifying document ID and use op_type=create while storing the documents in ES. With this you can make sure your uniqueId field will have unique value and will not be overridden by another same valued document.

For this, the elasticsearch document says:

The index operation also accepts an op_type that can be used to force a create operation, allowing for "put-if-absent" behavior. When create is used, the index operation will fail if a document by that id already exists in the index.

Here is an example of using the op_type parameter:

$ curl -XPUT 'http://localhost:9200/es_index/es_type/unique_a?op_type=create' -d  '{
    "user" : "kimchy",
    "uniqueId" : "unique_a"
}'

If you run the above request it is ok, but running it the next time will give you an error.

You'd need to use the field that is supposed to be unique as id for your documents. By default a new document with existing id would override the existing document with same id, but you can switch to op_type=create in order to get back an error if a document with same id already exists.

There's no way to have the same behaviour with arbitrary fields though, only the _id field works that way. I would probably consider handling this logic in the application layer instead of within elasticsearch.

You can use the _id in the column you want to have unique contraint on. Here is the sample river that uses postgresql. Yo can change the Database Driver/DB-URL according to your usage.

curl -XPUT localhost:9200/_river/simple_jdbc_river/_meta -d "{\"type\":\"jdbc\",\"jdbc\":{\"strategy\":\"simple\",\"poll\":\"1s\",\"driver\":\"org.postgresql.Driver\",\"url\":\"jdbc:postgresql://DB-URL/DB-INSTANCE\",\"user\":\"USERNAME\",\"password\":\"PASSWORD\",\"sql\":\"select t.id as _id,t.name from topic as t \",\"digesting\" : true},\"index\":{\"index\":\"jdbc\",\"type\":\"topic_jdbc_river1\"}}"