Vespa is a fully featured search engine and vector database. It supports vector search (ANN), lexical search, and search in structured data, all in the same query.
This notebook shows how to use Vespa.ai as a LangChain retriever. In order to create a retriever, we use pyvespa to create a connection a Vespa service. 
%pip install --upgrade --quiet  pyvespa

from vespa.application import Vespa

vespa_app = Vespa(url="https://doc-search.vespa.oath.cloud")
This creates a connection to a Vespa service, here the Vespa documentation search service. Using pyvespa package, you can also connect to a Vespa Cloud instance or a local Docker instance. After connecting to the service, you can set up the retriever: 
from langchain_community.retrievers import VespaRetriever

vespa_query_body = {
    "yql": "select content from paragraph where userQuery()",
    "hits": 5,
    "ranking": "documentation",
    "locale": "en-us",
}
vespa_content_field = "content"
retriever = VespaRetriever(vespa_app, vespa_query_body, vespa_content_field)
This sets up a LangChain retriever that fetches documents from the Vespa application. Here, up to 5 results are retrieved from the content field in the paragraph document type, using doumentation as the ranking method. The userQuery() is replaced with the actual query passed from LangChain. Please refer to the pyvespa documentation for more information. Now you can return the results and continue using the results in LangChain. 
retriever.invoke("what is vespa?")