feat: Add reciprocal rank fusion to MultiRetriever

docs-website/docs/pipeline-components/retrievers/multiretriever.mdx

@@ -2,12 +2,12 @@
 title: "MultiRetriever"
 id: multiretriever
 slug: "/multiretriever"
-description: "Runs multiple text retrievers in parallel and combines their deduplicated results."
+description: "Runs multiple text retrievers in parallel and combines their results using reciprocal rank fusion or deduplication."
 ---
 
 # MultiRetriever
 
-Runs multiple text retrievers in parallel and combines their deduplicated results.
+Runs multiple text retrievers in parallel and combines their results using reciprocal rank fusion or deduplication.
 
 :::warning[Experimental]
 
@@ -21,8 +21,9 @@ Runs multiple text retrievers in parallel and combines their deduplicated result
 | --- | --- |
 | **Most common position in a pipeline** | After query input, before a [`ChatPromptBuilder`](../builders/chatpromptbuilder.mdx) in RAG pipelines |
 | **Mandatory init variables** | `retrievers`: A dictionary mapping names to text retrievers (implementing the `TextRetriever` protocol) |
+| **Optional init variables** | `join_mode`: `"reciprocal_rank_fusion"` (default) or `"concatenate"` |
 | **Mandatory run variables** | `query`: A query string |
-| **Output variables** | `documents`: A deduplicated list of retrieved documents |
+| **Output variables** | `documents`: A merged list of retrieved documents |
 | **API reference** | [Retrievers](/reference/retrievers-api) |
 | **GitHub link** | https://github.com/deepset-ai/haystack/blob/main/haystack/components/retrievers/multi_retriever.py |
 | **Package name** | `haystack-ai` |
@@ -31,20 +32,27 @@ Runs multiple text retrievers in parallel and combines their deduplicated result
 
 ## Overview
 
-`MultiRetriever` composes any number of text retrievers into a single component. All retrievers are queried in parallel using a thread pool, and their results are deduplicated before being returned.
+`MultiRetriever` composes any number of text retrievers into a single component. All retrievers are queried in parallel using a thread pool, and their results are merged before being returned.
 
 The component:
 - Queries all retrievers concurrently for better performance
-- Automatically deduplicates results across retrievers
+- Merges results across retrievers using the configured `join_mode`
 - Supports selectively enabling retrievers at runtime via `active_retrievers`
 
 All retrievers passed to `MultiRetriever` must implement the `TextRetriever` protocol — their `run` method must accept a text `query`, `filters`, and `top_k`. Use [`TextEmbeddingRetriever`](textembeddingretriever.mdx) to wrap an embedding-based retriever so it can be used with this component.
 
+### Join modes
+
+The `join_mode` parameter controls how results from multiple retrievers are merged:
+
+- **`reciprocal_rank_fusion`** (default): Assigns scores based on each document's rank across retrieval lists using the [Reciprocal Rank Fusion](https://plg.uwaterloo.ca/~gvcormac/cormacksigir09-rrf.pdf) algorithm. Documents appearing highly ranked in multiple lists receive higher scores. Results are deduplicated and returned in descending score order. This is the recommended mode when combining retrievers with incomparable scores, such as BM25 and embedding retrievers.
+- **`concatenate`**: Combines all results into a single list and deduplicates.
+
 ## Usage
 
 ### On its own
 
-This example sets up a `MultiRetriever` combining a BM25 retriever and an embedding-based retriever (wrapped with `TextEmbeddingRetriever`). Both are queried in parallel and the deduplicated results are returned.
+This example sets up a `MultiRetriever` combining a BM25 retriever and an embedding-based retriever (wrapped with `TextEmbeddingRetriever`). Both are queried in parallel and the results are merged using reciprocal rank fusion.
 
 ```python
 from haystack import Document

haystack/components/joiners/document_joiner.py

@@ -11,6 +11,7 @@
 
 from haystack import Document, component, default_from_dict, default_to_dict, logging
 from haystack.core.component.types import Variadic
+from haystack.utils.misc import _reciprocal_rank_fusion
 
 logger = logging.getLogger(__name__)
 
@@ -117,7 +118,7 @@ def __init__(
         join_mode_functions = {
             JoinMode.CONCATENATE: DocumentJoiner._concatenate,
             JoinMode.MERGE: self._merge,
-            JoinMode.RECIPROCAL_RANK_FUSION: self._reciprocal_rank_fusion,
+            JoinMode.RECIPROCAL_RANK_FUSION: self._rrf,
             JoinMode.DISTRIBUTION_BASED_RANK_FUSION: DocumentJoiner._distribution_based_rank_fusion,
         }
         self.join_mode_function = join_mode_functions[join_mode]
@@ -193,35 +194,11 @@ def _merge(self, document_lists: list[list[Document]]) -> list[Document]:
 
         return [replace(doc, score=scores_map[doc.id]) for doc in documents_map.values()]
 
-    def _reciprocal_rank_fusion(self, document_lists: list[list[Document]]) -> list[Document]:
+    def _rrf(self, document_lists: list[list[Document]]) -> list[Document]:
         """
         Merge multiple lists of Documents and assign scores based on reciprocal rank fusion.
-
-        The constant k is set to 61 (60 was suggested by the original paper,
-        plus 1 as python lists are 0-based and the paper used 1-based ranking).
         """
-        # This check prevents a division by zero when no documents are passed
-        if not document_lists:
-            return []
-
-        k = 61
-
-        scores_map: dict = defaultdict(int)
-        documents_map = {}
-        weights = self.weights if self.weights else [1 / len(document_lists)] * len(document_lists)
-
-        # Calculate weighted reciprocal rank fusion score
-        for documents, weight in zip(document_lists, weights, strict=True):
-            for rank, doc in enumerate(documents):
-                scores_map[doc.id] += (weight * len(document_lists)) / (k + rank)
-                documents_map[doc.id] = doc
-
-        # Normalize scores. Note: len(results) / k is the maximum possible score,
-        # achieved by being ranked first in all doc lists with non-zero weight.
-        for _id in scores_map:
-            scores_map[_id] /= len(document_lists) / k
-
-        return [replace(doc, score=scores_map[doc.id]) for doc in documents_map.values()]
+        return _reciprocal_rank_fusion(document_lists, weights=self.weights)
 
     @staticmethod
     def _distribution_based_rank_fusion(document_lists: list[list[Document]]) -> list[Document]:

haystack/components/retrievers/multi_retriever.py

@@ -4,14 +4,15 @@
 
 import asyncio
 from concurrent.futures import ThreadPoolExecutor, as_completed
-from typing import Any
+from math import inf
+from typing import Any, Literal
 
 from haystack import component, default_from_dict, default_to_dict
 from haystack.components.retrievers.types.protocol import TextRetriever
 from haystack.core.serialization import component_from_dict, component_to_dict, import_class_by_name
 from haystack.dataclasses import Document
 from haystack.utils.experimental import _experimental
-from haystack.utils.misc import _deduplicate_documents
+from haystack.utils.misc import _deduplicate_documents, _reciprocal_rank_fusion
 
 
 @_experimental
@@ -82,6 +83,7 @@ def __init__(
         filters: dict[str, Any] | None = None,
         top_k: int = 10,
         max_workers: int = 4,
+        join_mode: Literal["concatenate", "reciprocal_rank_fusion"] = "reciprocal_rank_fusion",
     ) -> None:
         """
         Create the MultiRetriever component.
@@ -95,13 +97,30 @@ def __init__(
             The maximum number of documents to return per retriever.
         :param max_workers:
             The maximum number of threads to use for parallel retrieval.
+        :param join_mode:
+            How to merge results from multiple retrievers. Available modes:
+            - `concatenate`: Combines all results into a single list and deduplicates.
+            - `reciprocal_rank_fusion`: Deduplicates and assigns scores based on reciprocal rank fusion.
         """
         self.retrievers = retrievers
         self.filters = filters
         self.top_k = top_k
         self.max_workers = max_workers
+        self.join_mode = join_mode
         self._is_warmed_up = False
 
+    def _merge_results(self, document_lists: list[list[Document]]) -> list[Document]:
+        """
+        Merge per-retriever result lists according to `join_mode`.
+
+        In `concatenate` mode, all lists are flattened and deduplicated. In `reciprocal_rank_fusion` mode, results
+        are deduplicated and re-scored using RRF, then returned in descending score order.
+        """
+        if self.join_mode == "reciprocal_rank_fusion":
+            documents = _reciprocal_rank_fusion(document_lists)
+            return sorted(documents, key=lambda d: d.score if d.score is not None else -inf, reverse=True)
+        return _deduplicate_documents([doc for docs in document_lists for doc in docs])
+
     def _resolve_retrievers(self, active_retrievers: list[str] | None) -> dict[str, TextRetriever]:
         """
         Returns the subset of retrievers to run based on the active_retrievers list.
@@ -171,7 +190,7 @@ def run(
 
         retrievers_to_run = self._resolve_retrievers(active_retrievers)
 
-        all_documents: list[Document] = []
+        results_by_name: dict[str, list[Document]] = {}
         with ThreadPoolExecutor(max_workers=self.max_workers) as executor:
             future_to_name = {
                 executor.submit(retriever.run, query=query, filters=resolved_filters, top_k=resolved_top_k): name
@@ -180,11 +199,12 @@ def run(
             for future in as_completed(future_to_name):
                 name = future_to_name[future]
                 try:
-                    all_documents.extend(future.result().get("documents", []))
+                    results_by_name[name] = future.result().get("documents", [])
                 except Exception as e:
                     raise RuntimeError(f"Retriever '{name}' failed: {e}") from e
 
-        return {"documents": _deduplicate_documents(all_documents)}
+        document_lists = [results_by_name[name] for name in retrievers_to_run]
+        return {"documents": self._merge_results(document_lists)}
 
     @component.output_types(documents=list[Document])
     async def run_async(
@@ -238,13 +258,8 @@ async def _run_one(name: str, retriever: TextRetriever) -> list[Document]:
             except Exception as e:
                 raise RuntimeError(f"Retriever '{name}' failed: {e}") from e
 
-        results = await asyncio.gather(*[_run_one(name, r) for name, r in retrievers_to_run.items()])
-
-        all_documents: list[Document] = []
-        for docs in results:
-            all_documents.extend(docs)
-
-        return {"documents": _deduplicate_documents(all_documents)}
+        document_lists = list(await asyncio.gather(*[_run_one(name, r) for name, r in retrievers_to_run.items()]))
+        return {"documents": self._merge_results(document_lists)}
 
     def to_dict(self) -> dict[str, Any]:
         """
@@ -259,6 +274,7 @@ def to_dict(self) -> dict[str, Any]:
             filters=self.filters,
             top_k=self.top_k,
             max_workers=self.max_workers,
+            join_mode=self.join_mode,
         )
 
     @classmethod

haystack/utils/misc.py

@@ -5,6 +5,8 @@
 import json
 import mimetypes
 import tempfile
+from collections import defaultdict
+from dataclasses import replace
 from math import inf
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Literal, overload
@@ -145,6 +147,41 @@ def _deduplicate_documents(documents: list["Document"]) -> list["Document"]:
     return list(highest_scoring_docs.values())
 
 
+def _reciprocal_rank_fusion(
+    document_lists: list[list["Document"]], weights: list[float] | None = None
+) -> list["Document"]:
+    """
+    Merge multiple ranked lists of Documents using Reciprocal Rank Fusion, deduplicating across lists.
+
+    See the original paper: https://plg.uwaterloo.ca/~gvcormac/cormacksigir09-rrf.pdf
+
+    The constant k is set to 61 (60 was suggested by the original paper, plus 1 as python lists are 0-based and the
+    paper used 1-based ranking).
+
+    :param document_lists: A list of ranked document lists to fuse.
+    :param weights: Optional per-list weights. Defaults to equal weights.
+    :returns:
+        Deduplicated list of documents with updated RRF scores.
+    """
+    if not document_lists:
+        return []
+
+    k = 61
+    scores_map: dict = defaultdict(int)
+    documents_map: dict = {}
+    resolved_weights = weights if weights else [1 / len(document_lists)] * len(document_lists)
+
+    for documents, weight in zip(document_lists, resolved_weights, strict=True):
+        for rank, doc in enumerate(documents):
+            scores_map[doc.id] += (weight * len(document_lists)) / (k + rank)
+            documents_map[doc.id] = doc
+
+    for _id in scores_map:
+        scores_map[_id] /= len(document_lists) / k
+
+    return [replace(doc, score=scores_map[doc.id]) for doc in documents_map.values()]
+
+
 @overload
 def _parse_dict_from_json(
     text: str, expected_keys: list[str] | None = ..., raise_on_failure: Literal[True] = ...

releasenotes/notes/multi-retriever-rrf-effc174d0a440788.yaml

@@ -0,0 +1,8 @@
+---
+enhancements:
+  - |
+    Add ``join_mode`` parameter to the experimental ``MultiRetriever`` component, supporting
+    ``"reciprocal_rank_fusion"`` (default) and ``"concatenate"``. Reciprocal Rank Fusion merges
+    the ranked result lists from all retrievers into a single deduplicated list ordered by RRF score.
+    The underlying RRF logic is extracted into a shared utility ``_reciprocal_rank_fusion`` in
+    ``haystack.utils.misc``, which is now also used by ``DocumentJoiner``.

test/components/joiners/test_document_joiner.py

@@ -270,43 +270,3 @@ def test_output_documents_not_sorted_by_score(self):
         documents_2 = [Document(content="d", score=0.2)]
         output = joiner.run([documents_1, documents_2])
         assert output["documents"] == documents_1 + documents_2
-
-    def test_test_score_norm_with_rrf(self):
-        """
-        Verifies reciprocal rank fusion (RRF) of the DocumentJoiner component with various weight configurations.
-        It creates a set of documents, forms them into two lists, and then applies multiple DocumentJoiner
-        instances with distinct weights to these lists. The test checks if the resulting
-        joined documents are correctly sorted in descending order by score, ensuring the RRF ranking works as
-        expected under different weighting scenarios.
-        """
-        num_docs = 6
-        docs = []
-
-        for i in range(num_docs):
-            docs.append(Document(content=f"doc{i}"))
-
-        docs_2 = [docs[0], docs[4], docs[2], docs[5], docs[1]]
-        document_lists = [docs, docs_2]
-
-        joiner_1 = DocumentJoiner(join_mode="reciprocal_rank_fusion", weights=[0.5, 0.5])
-
-        joiner_2 = DocumentJoiner(join_mode="reciprocal_rank_fusion", weights=[7, 7])
-
-        joiner_3 = DocumentJoiner(join_mode="reciprocal_rank_fusion", weights=[0.7, 0.3])
-
-        joiner_4 = DocumentJoiner(join_mode="reciprocal_rank_fusion", weights=[0.6, 0.4])
-
-        joiner_5 = DocumentJoiner(join_mode="reciprocal_rank_fusion", weights=[1, 0])
-
-        joiners = [joiner_1, joiner_2, joiner_3, joiner_4, joiner_5]
-
-        for joiner in joiners:
-            join_results = joiner.run(documents=document_lists)
-            is_sorted = all(
-                join_results["documents"][i].score >= join_results["documents"][i + 1].score
-                for i in range(len(join_results["documents"]) - 1)
-            )
-
-            assert is_sorted, (
-                "Documents are not sorted in descending order by score, there is an issue with rff ranking"
-            )