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neo4j-haystack

A Haystack Document Store for Neo4j.

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Table of Contents

Overview¤

An integration of Neo4j graph database with Haystack v2.0 by deepset. In Neo4j Vector search index is being used for storing document embeddings and dense retrievals.

The library allows using Neo4j as a DocumentStore, and implements the required Protocol methods. You can start working with the implementation by importing it from neo4_haystack package:

from neo4j_haystack import Neo4jDocumentStore

In addition to the Neo4jDocumentStore the library includes the following haystack components which can be used in a pipeline:

  • Neo4jEmbeddingRetriever - is a typical retriever component which can be used to query vector store index and find related Documents. The component uses Neo4jDocumentStore to query embeddings.
  • Neo4jDynamicDocumentRetriever is also a retriever component in a sense that it can be used to query Documents in Neo4j. However it is decoupled from Neo4jDocumentStore and allows to run arbitrary Cypher query to extract documents. Practically it is possible to query Neo4j same way Neo4jDocumentStore does, including vector search.

The neo4j-haystack library uses Python Driver and Cypher Queries to interact with Neo4j database and hide all complexities under the hood.

Neo4jDocumentStore will store Documents as Graph nodes in Neo4j. Embeddings are stored as part of the node, but indexing and querying of vector embeddings using ANN is managed by a dedicated Vector Index.

                                   +-----------------------------+
                                   |       Neo4j Database        |
                                   +-----------------------------+
                                   |                             |
                                   |      +----------------+     |
                                   |      |    Document    |     |
                write_documents    |      +----------------+     |
          +------------------------+----->|   properties   |     |
          |                        |      |                |     |
+---------+----------+             |      |   embedding    |     |
|                    |             |      +--------+-------+     |
| Neo4jDocumentStore |             |               |             |
|                    |             |               |index/query  |
+---------+----------+             |               |             |
          |                        |      +--------+--------+    |
          |                        |      |  Vector Index   |    |
          +----------------------->|      |                 |    |
               query_embeddings    |      | (for embedding) |    |
                                   |      +-----------------+    |
                                   |                             |
                                   +-----------------------------+

In the above diagram:

  • Document is a Neo4j node (with "Document" label)
  • properties are Document attributes stored as part of the node. In current implementation meta attributes are stored on the same level as the rest of Document fields.
  • embedding is also a property of the Document node (just shown separately in the diagram for clarity) which is a vector of type LIST[FLOAT].
  • Vector Index is where embeddings are getting indexed by Neo4j as soon as those are updated in Document nodes.

Neo4jDocumentStore by default creates a vector index if it does not exist. Before writing documents you should make sure Documents are embedded by one of the provided embedders. For example SentenceTransformersDocumentEmbedder can be used in indexing pipeline to calculate document embeddings before writing those to Neo4j.

Installation¤

neo4j-haystack can be installed as any other Python library, using pip:

pip install --upgrade pip # optional
pip install sentence-transformers # required in order to run pipeline examples given below
pip install neo4j-haystack

Warning The neo4j-haystack package currently uses Haystack 2.0-Beta, an unstable version of what will eventually become Haystack 2.0. It will be updated and tested with the latest changes periodically until a stable version of the Haystack is released.

Usage¤

Running Neo4j¤

You will need to have a running instance of Neo4j database to use components from the package (in-memory version of Neo4j is not supported). There are several options available:

  • Docker, other options available in the same Operations Manual
  • AuraDB - a fully managed Cloud Instance of Neo4j
  • Neo4j Desktop client application

The simplest way to start database locally will be with Docker container:

docker run \
    --restart always \
    --publish=7474:7474 --publish=7687:7687 \
    --env NEO4J_AUTH=neo4j/passw0rd \
    neo4j:5.15.0

As of Neo4j 5.13, the vector search index is no longer a beta feature, consider using a version of the database ">= 5.13". In the example above version 5.15.0 is being used to start a container. You could explore Known issues and Limitations in the documentation.

The NEO4J_AUTH environment variable sets default credentials (username/password) for authentication.

Note Assuming you have a docker container running navigate to http://localhost:7474 to open Neo4j Browser to explore graph data and run Cypher queries.

Document Store¤

Once you have the package installed and the database running, you can start using Neo4jDocumentStore as any other document stores that support embeddings.

from neo4j_haystack import Neo4jDocumentStore

document_store = Neo4jDocumentStore(
    url="bolt://localhost:7687",
    username="neo4j",
    password="passw0rd",
    database="neo4j",
    embedding_dim=384,
    embedding_field="embedding",
    index="document-embeddings", # The name of the Vector Index in Neo4j
    node_label="Document", # Providing a label to Neo4j nodes which store Documents
)

Alternatively, Neo4j connection properties could be specified using a dedicated Neo4jClientConfig class:

from neo4j_haystack import Neo4jClientConfig, Neo4jDocumentStore

client_config = Neo4jClientConfig(
    url="bolt://localhost:7687",
    username="neo4j",
    password="passw0rd",
    database="neo4j",
)

document_store = Neo4jDocumentStore(client_config=client_config, embedding_dim=384)

Assuming there is a list of documents available and a running Neo4j database you can write/index those in Neo4j, e.g.:

from haystack import Document

documents = [Document(content="My name is Morgan and I live in Paris.")]

document_store.write_documents(documents)

If you intend to obtain embeddings before writing documents use the following code:

from haystack import Document

# import one of the available document embedders
from haystack.components.embedders import SentenceTransformersDocumentEmbedder 

documents = [Document(content="My name is Morgan and I live in Paris.")]

document_embedder = SentenceTransformersDocumentEmbedder(model="sentence-transformers/all-MiniLM-L6-v2")
document_embedder.warm_up() # will download the model during first run
documents_with_embeddings = document_embedder.run(documents)

document_store.write_documents(documents_with_embeddings.get("documents"))

Make sure embedding model produces vectors of same size as it has been set on Neo4jDocumentStore, e.g. setting embedding_dim=384 would comply with the "sentence-transformers/all-MiniLM-L6-v2" model.

Note Most of the time you will be using Haystack Pipelines to build both indexing and querying RAG scenarios.

It is important to understand how haystack Documents are stored in Neo4j after you call write_documents.

from random import random

sample_embedding = [random() for _ in range(384)]  # using fake/random embedding for brevity here to simplify example
document = Document(
    content="My name is Morgan and I live in Paris.", embedding=sample_embedding, meta={"num_of_years": 3}
)
document.to_dict()

The above code converts a Document to a dictionary and will render the following output:

>>> output:
{
    "id": "11c255ad10bff4286781f596a5afd9ab093ed056d41bca4120c849058e52f24d",
    "content": "My name is Morgan and I live in Paris.",
    "dataframe": None,
    "blob": None,
    "score": None,
    "embedding": [0.025010755222666936, 0.27502931836911926, 0.22321073814882275, ...], # vector of size 384
    "num_of_years": 3,
}

The data from the dictionary will be used to create a node in Neo4j after you write the document with document_store.write_documents([document]). You could query it with Cypher, e.g. MATCH (doc:Document) RETURN doc. Below is a json representation of the node in Neo4j:

{
  "identity": 0,
  "labels": [
    "Document" // label name is specified in the Neo4jDocumentStore.node_label argument
  ],
  "properties": { // this is where Document data is stored
    "id": "11c255ad10bff4286781f596a5afd9ab093ed056d41bca4120c849058e52f24d",
    "embedding": [0.6394268274307251, 0.02501075528562069,0.27502933144569397, ...], // vector of size 384
    "content": "My name is Morgan and I live in Paris.",
    "num_of_years": 3
  },
  "elementId": "4:8bde9fb3-3975-4c3e-9ea1-3e10dbad55eb:0"
}

Note Metadata (num_of_years) is serialized to the same level as rest of attributes (flatten). It is expected by current implementation as Neo4j node's properties can not have nested structures.

The full list of parameters accepted by Neo4jDocumentStore can be found in API documentation.

Indexing documents¤

With Haystack you can use DocumentWriter component to write Documents into a Document Store. In the example below we construct pipeline to write documents to Neo4j using Neo4jDocumentStore:

from haystack import Document
from haystack.components.embedders import SentenceTransformersDocumentEmbedder
from haystack.components.writers import DocumentWriter
from haystack.pipeline import Pipeline

from neo4j_haystack import Neo4jDocumentStore

documents = [Document(content="This is document 1"), Document(content="This is document 2")]

document_store = Neo4jDocumentStore(
    url="bolt://localhost:7687",
    username="neo4j",
    password="passw0rd",
    database="neo4j",
    embedding_dim=384,
    embedding_field="embedding",
    index="document-embeddings",
    node_label="Document",
)
embedder = SentenceTransformersDocumentEmbedder(model="sentence-transformers/all-MiniLM-L6-v2")
document_writer = DocumentWriter(document_store=document_store)

indexing_pipeline = Pipeline()
indexing_pipeline.add_component(instance=embedder, name="embedder")
indexing_pipeline.add_component(instance=document_writer, name="writer")

indexing_pipeline.connect("embedder", "writer")
indexing_pipeline.run({"embedder": {"documents": documents}})

>>> output:
`{'writer': {'documents_written': 2}}`

Retrieving documents¤

Neo4jEmbeddingRetriever component can be used to retrieve documents from Neo4j by querying vector index using an embedded query. Below is a pipeline which finds documents using query embedding as well as metadata filtering:

from typing import List

from haystack import Document, Pipeline
from haystack.components.embedders import SentenceTransformersDocumentEmbedder, SentenceTransformersTextEmbedder

from neo4j_haystack import Neo4jDocumentStore, Neo4jEmbeddingRetriever

document_store = Neo4jDocumentStore(
    url="bolt://localhost:7687",
    username="neo4j",
    password="passw0rd",
    database="neo4j",
    embedding_dim=384,
    index="document-embeddings",
)

documents = [
    Document(content="My name is Morgan and I live in Paris.", meta={"num_of_years": 3}),
    Document(content="I am Susan and I live in Berlin.", meta={"num_of_years": 7}),
]

# Same model is used for both query and Document embeddings
model_name = "sentence-transformers/all-MiniLM-L6-v2"

document_embedder = SentenceTransformersDocumentEmbedder(model=model_name)
document_embedder.warm_up()
documents_with_embeddings = document_embedder.run(documents)

document_store.write_documents(documents_with_embeddings.get("documents"))

print("Number of documents written: ", document_store.count_documents())

pipeline = Pipeline()
pipeline.add_component("text_embedder", SentenceTransformersTextEmbedder(model=model_name))
pipeline.add_component("retriever", Neo4jEmbeddingRetriever(document_store=document_store))
pipeline.connect("text_embedder.embedding", "retriever.query_embedding")

result = pipeline.run(
    data={
        "text_embedder": {"text": "What cities do people live in?"},
        "retriever": {
            "top_k": 5,
            "filters": {"field": "num_of_years", "operator": "==", "value": 3},
        },
    }
)

documents: List[Document] = result["retriever"]["documents"]

>>> output:
[Document(id=3930326edabe6d172031557556999e2f8ba258ccde3c876f5e3ac7e66ed3d53a, content: 'My name is Morgan and I live in Paris.', meta: {'num_of_years': 3}, score: 0.8348373770713806)]

Note You can learn more about how a given metadata filter is converted into Cypher queries by looking at documentation of the Neo4jQueryConverter class.

Retrieving documents using Cypher¤

In certain scenarios you might have an existing graph in Neo4j database which was created by custom scripts or data ingestion pipelines. The schema of the graph could be complex and not exactly fitting into Haystack Document model. Moreover in many situations you might want to leverage existing graph data to extract more context for grounding LLMs. To make it possible with Haystack we have Neo4jDynamicDocumentRetriever component - a flexible retriever which can run arbitrary Cypher query to obtain documents. This component does not require Document Store to operate.

The above example of Neo4jEmbeddingRetriever could be rewritten without usage of Neo4jDocumentStore in the retrieval pipeline:

from typing import List

from haystack import Document, Pipeline
from haystack.components.embedders import SentenceTransformersDocumentEmbedder, SentenceTransformersTextEmbedder

from neo4j_haystack import Neo4jClientConfig, Neo4jDocumentStore, Neo4jDynamicDocumentRetriever

client_config = Neo4jClientConfig(
    url="bolt://localhost:7687",
    username="neo4j",
    password="passw0rd",
    database="neo4j",
)

documents = [
    Document(content="My name is Morgan and I live in Paris.", meta={"num_of_years": 3}),
    Document(content="I am Susan and I live in Berlin.", meta={"num_of_years": 7}),
]

# Same model is used for both query and Document embeddings
model_name = "sentence-transformers/all-MiniLM-L6-v2"

document_embedder = SentenceTransformersDocumentEmbedder(model=model_name)
document_embedder.warm_up()
documents_with_embeddings = document_embedder.run(documents)

document_store = Neo4jDocumentStore(client_config=client_config, embedding_dim=384)
document_store.write_documents(documents_with_embeddings.get("documents"))

# Same model is used for both query and Document embeddings
model_name = "sentence-transformers/all-MiniLM-L6-v2"

cypher_query = """
            CALL db.index.vector.queryNodes($index, $top_k, $query_embedding)
            YIELD node as doc, score
            MATCH (doc) WHERE doc.num_of_years = $num_of_years
            RETURN doc{.*, score}, score
            ORDER BY score DESC LIMIT $top_k
        """

embedder = SentenceTransformersTextEmbedder(model="sentence-transformers/all-MiniLM-L6-v2")
retriever = Neo4jDynamicDocumentRetriever(
    client_config=client_config, runtime_parameters=["query_embedding"], doc_node_name="doc"
)

pipeline = Pipeline()
pipeline.add_component("text_embedder", embedder)
pipeline.add_component("retriever", retriever)
pipeline.connect("text_embedder.embedding", "retriever.query_embedding")

result = pipeline.run(
    data={
        "text_embedder": {"text": "What cities do people live in?"},
        "retriever": {
            "query": cypher_query,
            "parameters": {"index": "document-embeddings", "top_k": 5, "num_of_years": 3},
        },
    }
)

documents: List[Document] = result["retriever"]["documents"]

>>> output:
[Document(id=4014455c3be5d88151ba12d734a16754d7af75c691dfc3a5f364f81772471bd2, content: 'My name is Morgan and I live in Paris.', meta: {'num_of_years': 3}, score: 0.6696747541427612, embedding: vector of size 384)]

Please notice how query parameters are being used in the cypher_query:

  • runtime_parameters is a list of parameter names which are going to be input slots when connecting components in a pipeline. In our case query_embedding input is connected to the text_embedder.embedding output.
  • pipeline.run specifies additional parameters to the retriever component which can be referenced in the cypher_query, e.g. top_k and num_of_years.

In some way Neo4jDynamicDocumentRetriever resembles the DynamicPromptBuilder component, only instead of prompt it constructs a Cypher query using parameters. In the example above documents retrieved by running the query, the RETURN doc{.*, score} part returns back found documents with scores. Which node variable is going to be used to construct haystack Document is specified in the doc_node_name parameter (see above doc_node_name="doc").

You have options to enhance your RAG pipeline with data having various schemas, for example by first finding nodes using vector search and then expanding query to search for nearby nodes using appropriate Cypher syntax. It is possible to implement "Parent-Child" chunking strategy with such approach. Before that you have to ingest/index data into Neo4j accordingly by building an indexing pipeline or a custom ingestion script. A simple schema is shown below:

┌────────────┐                ┌─────────────┐
│   Child    │                │   Parent    │
│            │  :HAS_PARENT   │             │
│   content  ├────────────────┤   content   │
│  embedding │                │             │
└────────────┘                └─────────────┘

The following Cypher query is an example of how Neo4jDynamicDocumentRetriever can first search embeddings for Child document chunks and then return Parent documents which have larger context window (text length) for RAG applications:

// Query Child documents by $query_embedding
CALL db.index.vector.queryNodes($index, $top_k, $query_embedding)
YIELD node as child_doc, score

// Find Parent document for previously retrieved child (e.g. extend RAG context)
MATCH (child_doc)-[:HAS_PARENT]->(parent:Parent)
WITH parent, max(score) AS score // deduplicate parents
RETURN parent{.*, score}

As you might have guessed, the value for the doc_node_name parameter should be equal to parent according to the query above.

More examples¤

You can find more examples in the implementation repository:

More technical details available in the Code Reference documentation. For example, in real world scenarios there could be requirements to tune connection settings to Neo4j database (e.g. request timeout). Neo4jDocumentStore accepts an extended client configuration using Neo4jClientConfig class.

License¤

neo4j-haystack is distributed under the terms of the MIT license.