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+# Search Design Document
+
+## Task
+Given a query $q$, the system aims to retrieve the most relevant concept or document $d$ from a candidate set $\mathcal{D}$. In our context, this task necessitates identifying the relevant ontological concepts to a given entity (e.g., in Named Entity Recognition scenarios) or to a broader text input, enabling semantic grounding and structured representation of the extracted information.
+
+## Current Situation
+
+Currently, we use **BioPortal** as our ontology database. BioPortal is a well-established, community-trusted platform that hosts and manages a large number of ontologies. However, these benefits come with several trade-offs:
+
+1. **Dependency on BioPortal** — If BioPortal is unavailable (for example, during upgrades), our use case is directly impacted.  
+2. **API rate limits** — Rate limiting can slow down API calls. While this is understandable given BioPortal’s design and shared usage model, it affects performance.  
+3. **Implementation dependency** — We rely on BioPortal’s implementations (e.g., search), which may not always be optimal or fully aligned with our specific use case.
+
+## Overview & Requirements
+
+Before further detail, let’s first understand the steps involved. This task typically consists of two main stages:**retrieval** and **reranking**.
+
+In the first stage, **retrieval**, the objective is to identify a subset of potentially relevant candidates from $\mathcal{D}$. This is achieved by maximizing a scoring function $f(q, d)$, which estimates the relevance between the query and each candidate document:
+
+$$
+d^* = \arg\max_{d \in \mathcal{D}} f(q, d)
+$$
+
+Note at this stage we want to prioritize high recall and computational efficiency. 
+
+In the second stage, **reranking**, the retrieved candidates $\mathcal{D}_K$ are re-evaluated using a more expressive (and often computationally expensive) relevance model $g(q, d)$. The goal is to refine the initial ordering by more precisely estimating relevance:
+
+$$
+d^{**} = \arg\max_{d \in \mathcal{D}_K} g(q, d)
+$$
+
+With this in mind, we define the following search requirements (focused on algorithms not system design):
+
+1. **Contextualized retrieval** — The implementation must overcome the limitations of sparse retrieval methods such as BM25 and basic similarity scoring, which lack contextual understanding. This includes support for contextualized approaches such as cross-encoders, dual-encoders and late interaction models (e.g., ColBERT).
+2. **Keyword-based retrieval** — The system must also support fast and efficient keyword-based search.
+3. **Generalizability** — The implementation should be easily adaptable to other use cases with minimal or no additional effort.
+
+## Proposed approach
+The figure below presents a high-level overview of the proposed approach. Note that not all techniques shown will be used simultaneously; the final selection depends on trade-offs such as accuracy versus computational cost. For example, cross-encoding techniques offer high accuracy but are computationally expensive. Dual-encoder (or bi-encoder) techniques, on the other hand, provide a better balance between accuracy and computational efficiency.
+
+```mermaid
+flowchart TB
+ subgraph R1["Retrieval"]
+        B{"Candidate Retrieval"}
+        C1["BM25 / Inverted Index"]
+        C2["Vector Search / ANN Index"]
+        D["Candidate Set (d*)"]
+  end
+ subgraph R2["Re-ranking"]
+        G{"Scoring / Re-rank"}
+        N3["Dual Encoder"]
+        H1["Late-Interaction Encoder"]
+        H2["Cross-Encoder / LLM Reranker"]
+  end
+    B -- Keyword (BM25) --> C1
+    B -- Dense Embeddings --> C2
+    C1 --> D
+    C2 --> D
+    G -- <br> --> N3
+    G --> H1
+    G -- </br> --> H2
+    A["Input Text"] --> B
+    D --> G
+    N3 --> I["Final Results (d**)"]
+    H1 --> I
+    H2 --> I
+```
+
+
+## Implementation
+
+The system will be implemented as an API-first service. Agents and external clients will consume the same API endpoints, ensuring a unified interface, consistent behavior, and eliminating duplicate implementations across tool and service layers.
+
+The API will encapsulate the full retrieval and ranking pipeline, including candidate retrieval, and reranking. The architecture will remain modular to allow interchangeable ranking components while preserving a stable external interface.
+
+### Ranking and Retrieval Strategy
+
+The implementation will prioritize a hybrid retrieval and reranking approach, combining fast lexical retrieval with dense and late-interaction models for improved accuracy. The following models and methods will be evaluated and integrated where appropriate:
+
+- Li, Y., Li, J., Yu, M., Ding, G., Lin, Z., Wang, W. and Zhou, J., 2026. Query-focused and Memory-aware Reranker for Long Context Processing. arXiv preprint arXiv:2602.12192.
+
+- Lù, X.H., 2024. Bm25s: Orders of magnitude faster lexical search via eager sparse scoring. arXiv preprint arXiv:2407.03618.
+
+- Jha, R., Wang, B., Günther, M., Mastrapas, G., Sturua, S., Mohr, I., Koukounas, A., Wang, M.K., Wang, N. and Xiao, H., 2024, November. Jina-colbert-v2: A general-purpose multilingual late interaction retriever. In Proceedings of the Fourth Workshop on Multilingual Representation Learning (MRL 2024) (pp. 159-166).
+
+- Zhang, Y., Li, M., Long, D., Zhang, X., Lin, H., Yang, B., Xie, P., Yang, A., Liu, D., Lin, J. and Huang, F., 2025. Qwen3 embedding: Advancing text embedding and reranking through foundation models. arXiv preprint arXiv:2506.05176.
+
+- BiomedBERT Reranker - [https://huggingface.co/NeuML/biomedbert-base-reranker](https://huggingface.co/NeuML/biomedbert-base-reranker)
+
+### Design Principles
+
+- API-first architecture with a single service interface
+- Modular retrieval and ranking components
+- Support for sparse, dense, and late-interaction methods
+- Configurable reranking layer (neural or LLM-based)
+- Scalability for large candidate concept sets
+- Extensibility for domain-specific models
+- **Provenance by design**, ensuring that model versions, configurations, scoring methods, and retrieval metadata are tracked and reproducible
+
+This design enables flexible experimentation with retrieval and reranking strategies while maintaining a stable and production-ready service interface.