ColBERT Provider

Multi-vector embedding generation using ColBERT models.

What is ColBERT?

ColBERT (Contextualized Late Interaction over BERT) produces multiple vectors per text - one for each token. This enables fine-grained, token-level matching.

Late Interaction: Query and document are encoded independently, then compared token-by-token using MaxSim scoring.

Requirements

# Using virtualenv with uv (recommended)
./scripts/setup_venv.sh
uv pip install transformers torch --python .venv/bin/python

# Or install manually
pip install transformers torch

Configuration

{ok, State} = barrel_embed:init(#{
    embedder => {colbert, #{
        python => "python3",                    % default, fallback only
        model => "colbert-ir/colbertv2.0",     % default
        timeout => 120000                       % default, ms
    }}
}).

Options

barrel_embed manages its own Python virtualenv automatically and installs transformers and torch into it on first use. See Python Virtualenv Setup for the managed venv API.

Supported Models

Multi-Vector Format

ColBERT returns a list of token vectors:

[
    [0.1, 0.2, ...],  % vector for token 1
    [0.3, 0.4, ...],  % vector for token 2
    [0.5, 0.6, ...],  % vector for token 3
    ...
]

API

Native Multi-Vector API

%% Single text - returns list of token vectors
{ok, TokenVecs} = barrel_embed_colbert:embed_multi(<<"document text">>, Config).

%% Batch
{ok, MultiVecs} = barrel_embed_colbert:embed_batch_multi(Texts, Config).

MaxSim Scoring

ColBERT uses MaxSim for scoring:

Score(Q, D) = Σ max(qi · dj) for all qi in Q, dj in D

%% Calculate MaxSim score between query and document
QueryVecs = ...,  % from embed_multi
DocVecs = ...,    % from embed_multi
Score = barrel_embed_colbert:maxsim_score(QueryVecs, DocVecs).

Dense API (Mean Pooling)

For compatibility, ColBERT can return a single vector via mean pooling:

%% Returns mean-pooled single vector
{ok, Vec} = barrel_embed:embed(<<"text">>, State).

Note

Mean pooling loses the fine-grained matching capability. Use embed_multi/2 for best results.

Example: Passage Retrieval

%% Initialize
{ok, State} = barrel_embed:init(#{embedder => {colbert, #{}}}).
{_, Config} = hd(maps:get(providers, State)).

%% Embed query and documents
{ok, QueryVecs} = barrel_embed_colbert:embed_multi(<<"What is machine learning?">>, Config).
{ok, Doc1Vecs} = barrel_embed_colbert:embed_multi(Doc1, Config).
{ok, Doc2Vecs} = barrel_embed_colbert:embed_multi(Doc2, Config).

%% Score and rank
Score1 = barrel_embed_colbert:maxsim_score(QueryVecs, Doc1Vecs).
Score2 = barrel_embed_colbert:maxsim_score(QueryVecs, Doc2Vecs).

Use Cases

• Passage retrieval: Fine-grained matching for QA systems
• Semantic search: When single-vector similarity isn't precise enough
• Long documents: Token-level matching captures local relevance