JVector

JVector is a pure Java embedded vector search engine, used by DataStax Astra DB and (soon) Apache Cassandra.

What is JVector?

• Algorithmic-fast. JVector uses state of the art graph algorithms inspired by DiskANN and related research that offer high recall and low latency.
• Implementation-fast. JVector uses the Panama SIMD API to accelerate index build and queries.
• Memory efficient. JVector compresses vectors using product quantization so they can stay in memory during searches. (As part of our PQ implementation, our SIMD-accelerated kmeans class is 5x faster than the one in Apache Commons Math.)
• Disk-aware. JVector’s disk layout is designed to do the minimum necessary iops at query time.
• Concurrent. Index builds scale linearly to at least 32 threads. Double the threads, half the build time.
• Incremental. Query your index as you build it. No delay between adding a vector and being able to find it in search results.
• Easy to embed. API designed for easy embedding, by people using it in production.

JVector performance, visualized

JVector vs Lucene searching the Deep100M dataset (about 35GB of vectors and 25GB index):

JVector scales updates linearly to at least 32 threads:

Upgrading from 1.0.x to 2.0.x

See UPGRADING.md.

JVector basics

Adding to your project. Replace ${latest-version} with . Example <version>1.0.1</version>:

<dependency>        
    <groupId>io.github.jbellis</groupId>          
    <artifactId>jvector</artifactId>
    <!-- Use the latest version from https://central.sonatype.com/artifact/io.github.jbellis/jvector -->
    <version>${latest-version}</version>
</dependency>

Building the index:

• GraphIndexBuilder is the entry point for building a graph. You will need to implement RandomAccessVectorValues to provide vectors to the builder; ListRandomAccessVectorValues is a good starting point.
• If all your vectors are in the provider up front, you can just call build() and it will parallelize the build across all available cores. Otherwise you can call addGraphNode as you add vectors; this is non-blocking and can be called concurrently from multiple threads.
• Call GraphIndexBuilder.cleanup when you are done adding vectors. This will optimize the index and make it ready to write to disk. (Graphs that are in the process of being built can be searched at any time; you do not have to call cleanup first.)

Searching the index:

• GraphSearcher is the entry point for searching. Results come back as a SearchResult object that contains node IDs and scores, in descending order of similarity to the query vector. GraphSearcher objects are re-usable, so unless you have a very simple use case you should use GraphSearcher.Builder to create them; GraphSearcher.search is also available with simple defaults, but calling it will instantiate a new GraphSearcher every time so performance will be worse.
• JVector represents vectors in the index as the ordinal (int) corresponding to their index in the RandomAccessVectorValues you provided. You can get the original vector back with GraphIndex.getVector, if necessary, but since this is a disk-backed index you should design your application to avoid doing so unnecessarily.

DiskANN and Product Quantization

JVector implements DiskANN-style search, meaning that vectors can be compressed using product quantization so that searches can be performed using the compressed representation that is kept in memory. You can enable this with the following steps:

• Create a VectorCompressor object with your vectors using either ProductQuantization.compute
• or BinaryQuantization.compute. PQ is more flexible than BQ and is less lossy: even at the same compressed size, in the datasets tested by Bench, only the ada002 vectors in the wikipedia dataset are large enough and/or overparameterized enough to benefit from BQ while achieving recall competitive with PQ. However, if you are dealing with very large vectors and/or your recall requirement is not strict, you may still want to try BQ since it is MUCH faster to both compute and search with.
• Use VectorCompressor::encode or encodeAll to encode your vectors, then call VectorCompressor::createCompressedVectors to create a CompressedVectors object.
• Call CompressedVectors::approximateScoreFunctionFor to create a NeighborSimilarity.ApproximateScoreFunction for your query that uses the compressed vectors to accelerate search, and pass this to the GraphSearcher.search method.

Saving and loading indexes

• OnDiskGraphIndex and CompressedVectors have write() methods to save state to disk. They initialize from disk using their constructor and load() methods, respectively. Writing just requires a DataOutput, but reading requires an implementation of RandomAccessReader and the related ReaderSupplier to wrap your preferred i/o class for best performance. See SimpleMappedReader and SimpleMappedReaderSupplier for an example.
• Building a graph does not technically require your RandomAccessVectorValues object to live in memory, but it will perform much better if it does. OnDiskGraphIndex, by contrast, is designed to live on disk and use minimal memory otherwise.
• You can optionally wrap OnDiskGraphIndex in a CachingGraphIndex to keep the most commonly accessed nodes (the ones nearest to the graph entry point) in memory.

Advanced configuration

• JVector heavily utilizes the Panama Vector API(SIMD) for ANN indexing and search. We have seen cases where the memory bandwidth is saturated during indexing and product quantization and can cause the process to slow down. To avoid this, index and PQ builds use a PhysicalCoreExecutor to limit the amount of operations to the physical core count. The default value is 1/2 the processor count seen by Java. This may not be correct in all setups (e.g. no hyperthreading or hybrid architectures) so if you wish to override the default use the -Djvector.physical_core_count property.

Sample code

• The SiftSmall class demonstrates how to put all of the above together to index and search the "small" SIFT dataset of 10,000 vectors.
• The Bench class performs grid search across the GraphIndexBuilder parameter space to find the best tradeoffs between recall and throughput. You can use plot_output.py to graph the pareto-optimal points found by Bench.

Some sample KNN datasets for testing based on ada-002 embeddings generated on wikipedia data are available in ivec/fvec format for testing at:

aws s3 ls s3://astra-vector/wikipedia_squad/ --no-sign-request
                           PRE 100k/
                           PRE 1M/
                           PRE 4M/

Bench (see below) automatically downloads the 100k dataset to the ./fvec directory

Developing and Testing

This project is organized as a multimodule Maven build. The intent is to produce a multirelease jar suitable for use as a dependency from any Java 11 code. When run on a Java 20+ JVM with the Vector module enabled, optimized vector providers will be used. In general, the project is structured to be built with JDK 20+, but when JAVA_HOME is set to Java 11 -> Java 19, certain build features will still be available.

Base code is in jvector-base and will be built for Java 11 releases, restricting language features and APIs appropriately. Code in jvector-twenty will be compiled for Java 20 language features/APIs and included in the final multirelease jar targeting supported JVMs. jvector-multirelease packages jvector-base and jvector-twenty as a multirelease jar for release. jvector-examples is an additional sibling module that uses the reactor-representation of jvector-base/jvector-twenty to run example code. jvector-tests contains tests for the project, capable of running against both Java 11 and Java 20+ JVMs.

To run tests, use mvn test. To run tests against Java 20+, use mvn test. To run tests against Java 11, use mvn -Pjdk11 test. To run a single test class, use the Maven Surefire test filtering capability, e.g., mvn -Dtest=TestNeighborArray test. You may also use method-level filtering and patterns, e.g., mvn -Dtest=TestNeighborArray#testRetain* test.

You can run SiftSmall and Bench directly to get an idea of what all is going on here. Bench will automatically download required datasets to the fvec and hdf5 directories. The files used by SiftSmall can be found in the siftsmall directory in the project root.

To run either class, you can use the Maven exec-plugin via the following incantations:

mvn compile exec:exec@bench

or for Sift:

mvn compile exec:exec@sift

Bench takes an optional benchArgs argument that can be set to a list of whitespace-separated regexes. If any of the provided regexes match within a dataset name, that dataset will be included in the benchmark. For example, to run only the glove and nytimes datasets, you could use:

mvn compile exec:exec@bench -DbenchArgs="glove nytimes"

To run Sift/Bench without the JVM vector module available, you can use the following invocations:

mvn -Pjdk11 compile exec:exec@bench

mvn -Pjdk11 compile exec:exec@sift

The ... -Pjdk11 invocations will also work with JAVA_HOME pointing at a Java 11 installation.

To release, configure ~/.m2/settings.xml to point to OSSRH and run mvn -Prelease clean deploy.

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