✨(backend) implement reranker

I implement a reranker to improve accuracy of search results

CHANGELOG.md

@@ -31,6 +31,7 @@ and this project adheres to
 - ✨(backend) add deletion endpoint
 - ✨(backend) add path filter
 - ✨(backend) add search_type param
+- ✨(backend) implement reranker #47
 
 ## Changed
 

src/backend/core/services/reranking.py

@@ -0,0 +1,73 @@
+"""Reranking utilities using rerankers library."""
+
+import logging
+from functools import cache
+from rerankers import Reranker, Document
+
+from django.conf import settings
+
+from core.services.indexing import format_document
+from core.utils import get_language_value
+
+logger = logging.getLogger(__name__)
+
+
+@cache
+def get_reranker():
+    """Get or create the reranker instance (singleton pattern for efficiency)."""
+    try:
+        logger.info("Initializing reranker model: %s", settings.RERANKER_MODEL_NAME)
+        reranker = Reranker("mixedbread-ai/mxbai-rerank-large-v1", model_type="cross-encoder")
+        logger.info("Reranker initialized successfully")
+    except Exception as e:  # noqa: BLE001
+        logger.error("Failed to initialize reranker: %s", str(e))
+        return None
+    
+    return reranker
+
+
+def rerank(query, results):
+    """
+    Rerank search results using the configured reranker model.
+    
+    Args:
+        query: The search query string
+        results: List of OpenSearch hit objects with _source containing title and content
+
+    Returns:
+        List of reranked results in the same format as input
+    """
+    if not settings.RERANKER_ENABLED:
+        logger.debug("Reranker disabled, returning original results")
+        return results
+
+    reranker = get_reranker()
+    if reranker is None:
+        logger.warning("Reranker not available, returning original results")
+        return results
+    
+    try:
+        documents = []
+        for hit in results:
+            title = get_language_value(hit["_source"], "title")
+            content = get_language_value(hit["_source"], "content")
+            documents.append(format_document(title, content))
+        
+        logger.info("Reranking %d results for query: %s", len(results), query)
+        reranked = reranker.rank(query=query, docs=documents)
+        
+        reranked_results = []
+        for reranked_result in reranked.results:
+            # doc_id is the index position in the original list
+            original_hit = results[reranked_result.doc_id]
+            # Add reranking score as metadata
+            original_hit["_rerank_score"] = reranked_result.score
+            reranked_results.append(original_hit)
+        
+        logger.info("Reranking completed, returned %d results", len(reranked_results))
+
+        return reranked_results
+        
+    except Exception as e:  # noqa: BLE001
+        logger.error("Reranking failed: %s, returning original results", str(e))
+        return results

src/backend/core/services/search.py

@@ -9,6 +9,7 @@ from core.enums import SearchTypeEnum
 
 from .embedding import embed_text
 from .opensearch import check_hybrid_search_enabled, opensearch_client
+from .reranking import rerank
 
 logger = logging.getLogger(__name__)
 
@@ -40,7 +41,7 @@ def search(  # noqa : PLR0913
         path=path,
         search_type=search_type,
     )
-    return opensearch_client().search(  # pylint: disable=unexpected-keyword-arg
+    response = opensearch_client().search(  # pylint: disable=unexpected-keyword-arg
         index=",".join(search_indices),
         body={
             "_source": enums.SOURCE_FIELDS,  # limit the fields to return
@@ -61,6 +62,15 @@ def search(  # noqa : PLR0913
         # ignore_unavailable is not in the method declaration
         ignore_unavailable=True,
     )
+    
+    # Apply reranking if enabled
+    if settings.RERANKER_ENABLED and q != "*":
+        response["hits"]["hits"] = rerank(
+            query=q,
+            results=response["hits"]["hits"],
+        )
+    
+    return response
 
 
 # pylint: disable=too-many-arguments, too-many-positional-arguments

src/backend/find/settings.py

@@ -319,6 +319,24 @@ class Base(Configuration):
         default=1024, environ_name="EMBEDDING_DIMENSION", environ_prefix=None
     )
 
+    # Reranking
+    RERANKER_ENABLED = values.BooleanValue(
+        default=False, environ_name="RERANKER_ENABLED", environ_prefix=None
+    )
+    RERANKER_MODEL_NAME = values.Value(
+        default=None,
+        environ_name="RERANKER_MODEL_NAME",
+        environ_prefix=None,
+    )
+    RERANKER_MODEL_TYPE = values.Value(
+        default=None,
+        environ_name="RERANKER_MODEL_TYPE",
+        environ_prefix=None,
+    )
+    RERANKER_API_KEY = values.Value(
+        default=None, environ_name="RERANKER_API_KEY", environ_prefix=None
+    )
+
     # CORS
     CORS_ALLOW_CREDENTIALS = True
     CORS_ALLOW_ALL_ORIGINS = values.BooleanValue(True)