k-Nearest Neighbor (k-NN) search in Amazon OpenSearch Service

Short for its associated k-nearest neighbors algorithm, k-NN for Amazon OpenSearch Service lets you search for points in a vector space and find the "nearest neighbors" for those points by Euclidean distance or cosine similarity. Use cases include recommendations (for example, an "other songs you might like" feature in a music application), image recognition, and fraud detection.

Use the following tables to find the version of the k-NN plugin running on your Amazon OpenSearch Service domain. Each k-NN plugin version corresponds to an OpenSearch or Elasticsearch version.

Getting started with k-NN

To use k-NN, you must create an index with the index.knn setting and add one or more fields of the knn_vector data type.

PUT my-index
{
  "settings": {
    "index.knn": true
  },
  "mappings": {
    "properties": {
      "my_vector1": {
        "type": "knn_vector",
        "dimension": 2
      },
      "my_vector2": {
        "type": "knn_vector",
        "dimension": 4
      }
    }
  }
}

The knn_vector data type supports a single list of up to 10,000 floats, with the number of floats defined by the required dimension parameter. After you create the index, add some data to it.

POST _bulk
{ "index": { "_index": "my-index", "_id": "1" } }
{ "my_vector1": [1.5, 2.5], "price": 12.2 }
{ "index": { "_index": "my-index", "_id": "2" } }
{ "my_vector1": [2.5, 3.5], "price": 7.1 }
{ "index": { "_index": "my-index", "_id": "3" } }
{ "my_vector1": [3.5, 4.5], "price": 12.9 }
{ "index": { "_index": "my-index", "_id": "4" } }
{ "my_vector1": [5.5, 6.5], "price": 1.2 }
{ "index": { "_index": "my-index", "_id": "5" } }
{ "my_vector1": [4.5, 5.5], "price": 3.7 }
{ "index": { "_index": "my-index", "_id": "6" } }
{ "my_vector2": [1.5, 5.5, 4.5, 6.4], "price": 10.3 }
{ "index": { "_index": "my-index", "_id": "7" } }
{ "my_vector2": [2.5, 3.5, 5.6, 6.7], "price": 5.5 }
{ "index": { "_index": "my-index", "_id": "8" } }
{ "my_vector2": [4.5, 5.5, 6.7, 3.7], "price": 4.4 }
{ "index": { "_index": "my-index", "_id": "9" } }
{ "my_vector2": [1.5, 5.5, 4.5, 6.4], "price": 8.9 }

Then you can search the data using the knn query type.

GET my-index/_search
{
  "size": 2,
  "query": {
    "knn": {
      "my_vector2": {
        "vector": [2, 3, 5, 6],
        "k": 2
      }
    }
  }
}

In this case, k is the number of neighbors you want the query to return, but you must also include the size option. Otherwise, you get k results for each shard (and each segment) rather than k results for the entire query. k-NN supports a maximum k value of 10,000.

If you mix the knn query with other clauses, you might receive fewer than k results. In this example, the post_filter clause reduces the number of results from 2 to 1.

GET my-index/_search
{
  "size": 2,
  "query": {
    "knn": {
      "my_vector2": {
        "vector": [2, 3, 5, 6],
        "k": 2
      }
    }
  },
  "post_filter": {
    "range": {
      "price": {
        "gte": 6,
        "lte": 10
      }
    }
  }
}

If you need to handle a large volume of queries while maintaining optimal performance, you can use the _msearch API to construct a bulk search with JSON and send a single request to perform multiple searches:

GET _msearch
{ "index": "my-index"}
{ "query": { "knn": {"my_vector2":{"vector": [2, 3, 5, 6],"k":2 }} } }
{ "index": "my-index", "search_type": "dfs_query_then_fetch"}
{ "query": { "knn": {"my_vector1":{"vector": [2, 3],"k":2 }} } }

The following video demonstrates how to set up bulk vector searches for K-NN queries.

k-NN differences, tuning, and limitations

OpenSearch lets you modify all k-NN settings using the _cluster/settings API. On OpenSearch Service, you can change all settings except knn.memory.circuit_breaker.enabled and knn.circuit_breaker.triggered. k-NN statistics are included as Amazon CloudWatch metrics.

In particular, check the KNNGraphMemoryUsage metric on each data node against the knn.memory.circuit_breaker.limit statistic and the available RAM for the instance type. OpenSearch Service uses half of an instance's RAM for the Java heap (up to a heap size of 32 GiB). By default, k-NN uses up to 50% of the remaining half, so an instance type with 32 GiB of RAM can accommodate 8 GiB of graphs (32 * 0.5 * 0.5). Performance can suffer if graph memory usage exceeds this value.

You can migrate a k-NN index created on version 2.x or later to UltraWarm or cold storage on a domain with version 2.17 or later.

Clear cache api and warmup apis for k-NN indices are blocked for warm indices. When the first query is initiated for the index, it downloads the graph files from Amazon S3 and loads the graph to memory. Similarly, when TTL is expired for the graphs, the files are automatically evicted from memory.