Implementing reranking in ZenML

Learn how to implement reranking in ZenML.

We already have a working RAG pipeline, so inserting a reranker into the pipeline is relatively straightforward. The reranker will take the retrieved documents from the initial retrieval step and reorder them in terms of the query that was used to retrieve them.

How and where to add reranking

We'll use the rerankers package to handle the reranking process in our RAG inference pipeline. It's a relatively low-cost (in terms of technical debt and complexity) and lightweight dependency to add into our pipeline. It offers an interface to most of the model types that are commonly used for reranking and means we don't have to worry about the specifics of each model.

This package provides a Reranker abstract class that you can use to define your own reranker. You can also use the provided implementations to add reranking to your pipeline. The reranker takes the query and a list of retrieved documents as input and outputs a reordered list of documents based on the reranking scores. Here's a toy example:

from rerankers import Reranker

ranker = Reranker('cross-encoder')

texts = [
    "I like to play soccer",
    "I like to play football",
    "War and Peace is a great book"
    "I love dogs",
    "Ginger cats aren't very smart",
    "I like to play basketball",

results = ranker.rank(query="What's your favorite sport?", docs=texts)

And results will look something like this:

        Result(doc_id=5, text='I like to play basketball', score=-0.46533203125, rank=1),
        Result(doc_id=0, text='I like to play soccer', score=-0.7353515625, rank=2),
        Result(doc_id=1, text='I like to play football', score=-0.9677734375, rank=3),
        Result(doc_id=2, text='War and Peace is a great book', score=-5.40234375, rank=4),
        Result(doc_id=3, text='I love dogs', score=-5.5859375, rank=5),
        Result(doc_id=4, text="Ginger cats aren't very smart", score=-5.94921875, rank=6)
    query="What's your favorite sport?",

We can see that the reranker has reordered the documents based on the reranking scores, with the most relevant document appearing at the top of the list. The texts about sport are at the top and the less relevant ones about animals are down at the bottom.

We specified that we want a cross-encoder reranker, but you can also use other reranker models from the Hugging Face Hub, use API-driven reranker models (from Jina or Cohere, for example), or even define your own reranker model. Read their documentation to see how to use these different configurations.

In our case, we can simply add a helper function that can optionally be invoked when we want to use the reranker:

def rerank_documents(
    query: str, documents: List[Tuple], reranker_model: str = "flashrank"
) -> List[Tuple[str, str]]:
    """Reranks the given documents based on the given query."""
    ranker = Reranker(reranker_model)
    docs_texts = [f"{doc[0]} PARENT SECTION: {doc[2]}" for doc in documents]
    results = ranker.rank(query=query, docs=docs_texts)
    # pair the texts with the original urls in `documents`
    # `documents` is a tuple of (content, url)
    # we want the urls to be returned
    reranked_documents_and_urls = []
    for result in results.results:
        # content is a `rerankers` Result object
        index_val = result.doc_id
        doc_text = result.text
        doc_url = documents[index_val][1]
        reranked_documents_and_urls.append((doc_text, doc_url))
    return reranked_documents_and_urls

This function takes a query and a list of documents (each document is a tuple of content and URL) and reranks the documents based on the query. It returns a list of tuples, where each tuple contains the reranked document text and the URL of the original document. We use the flashrank model from the rerankers package by default as it appeared to be a good choice for our use case during development.

This function then gets used in tests in the following way:

def query_similar_docs(
    question: str,
    url_ending: str,
    use_reranking: bool = False,
    returned_sample_size: int = 5,
) -> Tuple[str, str, List[str]]:
    """Query similar documents for a given question and URL ending."""
    embedded_question = get_embeddings(question)
    db_conn = get_db_conn()
    num_docs = 20 if use_reranking else returned_sample_size
    # get (content, url) tuples for the top n similar documents
    top_similar_docs = get_topn_similar_docs(
        embedded_question, db_conn, n=num_docs, include_metadata=True

    if use_reranking:
        reranked_docs_and_urls = rerank_documents(question, top_similar_docs)[
        urls = [doc[1] for doc in reranked_docs_and_urls]
        urls = [doc[1] for doc in top_similar_docs]  # Unpacking URLs

    return (question, url_ending, urls)

We get the embeddings for the question being passed into the function and connect to our PostgreSQL database. If we're using reranking, we get the top 20 documents similar to our query and rerank them using the rerank_documents helper function. We then extract the URLs from the reranked documents and return them. Note that we only return 5 URLs, but in the case of reranking we get a larger number of documents and URLs back from the database to pass to our reranker, but in the end we always choose the top five reranked documents to return.

Now that we've added reranking to our pipeline, we can evaluate the performance of our reranker and see how it affects the quality of the retrieved documents.

Code Example

To explore the full code, visit the Complete Guide repository and for this section, particularly the file.

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