Standard k-NN search methods compute similarity using a brute-force approach that measures the nearest distance between a query and a number of points, which produces exact results. This works well in many applications. However, in the case of extremely large datasets with high dimensionality, this creates a scaling problem that reduces the efficiency of the search. Approximate k-NN search methods can overcome this by employing tools that restructure indexes more efficiently and reduce the dimensionality of searchable vectors. Using this approach requires a sacrifice in accuracy but increases search processing speeds appreciably.
The Approximate k-NN search methods leveraged by OpenSearch use approximate nearest neighbor (ANN) algorithms from the [nmslib](https://github.com/nmslib/nmslib), [faiss](https://github.com/facebookresearch/faiss), and [Lucene](https://lucene.apache.org/) libraries to power k-NN search. These search methods employ ANN to improve search latency for large datasets. Of the three search methods the k-NN plugin provides, this method offers the best search scalability for large datasets. This approach is the preferred method when a dataset reaches hundreds of thousands of vectors.
For details on the algorithms the plugin currently supports, see [k-NN Index documentation]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-index#method-definitions).
{: .note}
The k-NN plugin builds a native library index of the vectors for each knn-vector field/Lucene segment pair during indexing, which can be used to efficiently find the k-nearest neighbors to a query vector during search. To learn more about Lucene segments, see the [Apache Lucene documentation](https://lucene.apache.org/core/8_9_0/core/org/apache/lucene/codecs/lucene87/package-summary.html#package.description). These native library indexes are loaded into native memory during search and managed by a cache. To learn more about preloading native library indexes into memory, refer to the [warmup API]({{site.url}}{{site.baseurl}}/search-plugins/knn/api#warmup-operation). Additionally, you can see which native library indexes are already loaded in memory. To learn more about this, see the [stats API section]({{site.url}}{{site.baseurl}}/search-plugins/knn/api#stats).
Because the native library indexes are constructed during indexing, it is not possible to apply a filter on an index and then use this search method. All filters are applied on the results produced by the approximate nearest neighbor search.
### Recommendations for engines and cluster node sizing
Each of the three engines used for approximate k-NN search has its own attributes that make one more sensible to use than the others in a given situation. You can follow the general information below to help determine which engine will best meet your requirements.
In general, nmslib outperforms both faiss and Lucene on search. However, to optimize for indexing throughput, faiss is a good option. For relatively smaller datasets (up to a few million vectors), the Lucene engine demonstrates better latencies and recall. At the same time, the size of the index is smallest compared to the other engines, which allows it to use smaller AWS instances for data nodes.
Also, the Lucene engine uses a pure Java implementation and does not share any of the limitations that engines using platform-native code experience. However, one exception to this is that the maximum dimension count for the Lucene engine is 1,024, compared with 16,000 for the other engines. Refer to the sample mapping parameters in the following section to see where this is configured.
When considering cluster node sizing, a general approach is to first establish an even distribution of the index across the cluster. However, there are other considerations. To help make these choices, you can refer to the OpenSearch managed service guidance in the section [Sizing domains](https://docs.aws.amazon.com/opensearch-service/latest/developerguide/sizing-domains.html).
To use the k-NN plugin's approximate search functionality, you must first create a k-NN index with `index.knn` set to `true`. This setting tells the plugin to create native library indexes for the index.
In the example above, both `knn_vector` fields are configured from method definitions. Additionally, `knn_vector` fields can also be configured from models. You can learn more about this in the [knn_vector data type]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-index#knn_vector-data-type) section.
The `knn_vector` data type supports a vector of floats that can have a dimension count of up to 16,000 for the nmslib and faiss engines, as set by the dimension mapping parameter. The maximum dimension count for the Lucene library is 1,024.
In OpenSearch, codecs handle the storage and retrieval of indexes. The k-NN plugin uses a custom codec to write vector data to native library indexes so that the underlying k-NN search library can read it.
For some of the algorithms that we support, the native library index needs to be trained before it can be used. It would be expensive to training every newly created segment, so, instead, we introduce the concept of a *model* that is used to initialize the native library index during segment creation. A *model* is created by calling the [Train API]({{site.url}}{{site.baseurl}}/search-plugins/knn/api#train-model), passing in the source of training data as well as the method definition of the model. Once training is complete, the model will be serialized to a k-NN model system index. Then, during indexing, the model is pulled from this index to initialize the segments.
To train a model, we first need an OpenSearch index with training data in it. Training data can come from
any `knn_vector` field that has a dimension matching the dimension of the model you want to create. Training data can be the same data that you are going to index or have in a separate set. Let's create a training index:
If you use the `knn` query alongside filters or other clauses (e.g. `bool`, `must`, `match`), you might receive fewer than `k` results. In this example, `post_filter` reduces the number of results from 2 to 1:
A space corresponds to the function used to measure the distance between two points in order to determine the k-nearest neighbors. From the k-NN perspective, a lower score equates to a closer and better result. This is the opposite of how OpenSearch scores results, where a greater score equates to a better result. To convert distances to OpenSearch scores, we take 1 / (1 + distance). The k-NN plugin the spaces the plugin supports are below. Not every method supports each of these spaces. Be sure to check out [the method documentation]({{site.url}}{{site.baseurl}}/search-plugins/knn/knn-index#method-definitions) to make sure the space you are interested in is supported.
