Index and query vectors

Learn how to index and query vector embeddings with Redis

Redis Query Engine lets you index vector fields in hash or JSON objects (see the Vectors reference page for more information). Among other things, vector fields can store text embeddings, which are AI-generated vector representations of the semantic information in pieces of text. The vector distance between two embeddings indicates how similar they are semantically. By comparing the similarity of an embedding generated from some query text with embeddings stored in hash or JSON fields, Redis can retrieve documents that closely match the query in terms of their meaning.

The example below uses the HuggingFace model all-MiniLM-L6-v2 to generate the vector embeddings to store and index with Redis Query Engine. The code is first demonstrated for hash documents with a separate section to explain the differences with JSON documents.

Note:
From v6.0.0 onwards, Jedis uses query dialect 2 by default. Redis query engine methods such as ftSearch() will explicitly request this dialect, overriding the default set for the server. See Query dialects for more information.

Initialize

If you are using Maven, add the following dependencies to your pom.xml file:

<dependency>
    <groupId>redis.clients</groupId>
    <artifactId>jedis</artifactId>
    <version>6.1.0</version>
</dependency>
<dependency>
    <groupId>ai.djl.huggingface</groupId>
    <artifactId>tokenizers</artifactId>
    <version>0.33.0</version>
</dependency>
<dependency>
    <groupId>ai.djl.pytorch</groupId>
    <artifactId>pytorch-model-zoo</artifactId>
    <version>0.33.0</version>
</dependency>
<dependency>
    <groupId>ai.djl</groupId>
    <artifactId>api</artifactId>
    <version>0.33.0</version>
</dependency>

If you are using Gradle, add the following dependencies to your build.gradle file:

implementation 'redis.clients:jedis:6.1.0'
implementation 'ai.djl.huggingface:tokenizers:0.33.0'
implementation 'ai.djl.pytorch:pytorch-model-zoo:0.33.0'
implementation 'ai.djl:api:0.33.0'

Import dependencies

Import the following classes in your source file:

Define a helper method

The embedding model represents the vectors as an array of float values, which is the format required by Redis Query Engine. Also, when you store vectors in a hash object, you must encode the vector array as a byte string. To simplify this situation, you can declare a helper method floatsToByteString() that takes the float array that the embedding model returns and encodes it as a byte string:

Create a tokenizer instance

The next step is to generate the embeddings using the all-MiniLM-L6-v2 model. The vectors that represent the embeddings have 384 components, regardless of the length of the input text, but note that the input is truncated to 256 tokens (see Word piece tokenization

Create the index

Connect to Redis and delete any index previously created with the name vector_idx. (The ftDropIndex() call throws an exception if the index doesn't already exist, which is why you need the try...catch block.)

Next, create the index. The schema in the example below includes three fields: the text content to index, a tag field to represent the "genre" of the text, and the embedding vector generated from the original text content. The embedding field specifies HNSW indexing, the L2 vector distance metric, Float32 values to represent the vector's components, and 384 dimensions, as required by the all-MiniLM-L6-v2 embedding model.

The FTCreateParams object specifies hash objects for storage and a prefix doc: that identifies the hash objects to index.

Add data

You can now supply the data objects, which will be indexed automatically when you add them with hset(), as long as you use the doc: prefix specified in the index definition.

Use the predict() method of the Predictor object as shown below to create the embedding that represents the content field. The predict() method returns a float[] array which is then converted to a byte string using the helper method. Use the byte string representation when you are indexing hash objects (as in this example), but use the array of float directly for JSON objects (see Differences with JSON objects below). Note that when you set the embedding field, you must use an overload of hset() that requires byte arrays for each of the key, the field name, and the value, which is why you must include the getBytes() calls on the strings.

Run a query

After you have created the index and added the data, you are ready to run a query. To do this, you must create another embedding vector from your chosen query text. Redis calculates the vector distance between the query vector and each embedding vector in the index as it runs the query. You can request the results to be sorted to rank them in order of ascending distance.

The code below creates the query embedding using the predict() method, as with the indexing, and passes it as a parameter when the query executes (see Vector search for more information about using query parameters with embeddings). The query is a K nearest neighbors (KNN) search that sorts the results in order of vector distance from the query vector.

Assuming you have added the code from the steps above to your source file, it is now ready to run, but note that it may take a while to complete when you run it for the first time (which happens because the tokenizer must download the all-MiniLM-L6-v2 model data before it can generate the embeddings). When you run the code, it outputs the following result text:

Results:
ID: doc:1, Distance: 0.114169836044, Content: That is a very happy person
ID: doc:2, Distance: 0.610845506191, Content: That is a happy dog
ID: doc:3, Distance: 1.48624765873, Content: Today is a sunny day

Note that the results are ordered according to the value of the distance field, with the lowest distance indicating the greatest similarity to the query. As expected, the text "That is a very happy person" is the result judged to be most similar in meaning to the query text "That is a happy person".

Differences with JSON documents

Indexing JSON documents is similar to hash indexing, but there are some important differences. JSON allows much richer data modeling with nested fields, so you must supply a path in the schema to identify each field you want to index. However, you can declare a short alias for each of these paths (using the as() option) to avoid typing it in full for every query. Also, you must specify IndexDataType.JSON when you create the index.

The code below shows these differences, but the index is otherwise very similar to the one created previously for hashes:

An important difference with JSON indexing is that the vectors are specified using arrays of float instead of binary strings, so you don't need a helper method to convert the array to a binary string.

Use jsonSet() to add the data instead of hset(). Use instances of JSONObject to supply the data instead of Map, as you would for hash objects.

The query is almost identical to the one for the hash documents. This demonstrates how the right choice of aliases for the JSON paths can save you having to write complex queries. An important thing to notice is that the vector parameter for the query is still specified as a binary string (created using the floatsToByteString() method), even though the data for the embedding field of the JSON was specified as an array.

Apart from the jdoc: prefixes for the keys, the result from the JSON query is the same as for hash:

Results:
ID: jdoc:1, Distance: 0.114169836044, Content: That is a very happy person
ID: jdoc:2, Distance: 0.610845506191, Content: That is a happy dog
ID: jdoc:3, Distance: 1.48624765873, Content: Today is a sunny day

Learn more

See Vector search for more information about the indexing options, distance metrics, and query format for vectors.

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