SPLADE Deep Dive

SPLADE vs BM25 in One Sentence

BM25 scores documents by raw term frequency weighted by IDF. SPLADE uses a masked-language-model head to predict a learned weight for every term in the vocabulary — including terms that never appeared in the document.

This is called learned term expansion: a document about "token refresh" gets non-zero weights for "oauth", "bearer", "jwt", "access" even if those words are absent in the surface text.

The FLOPS Regularizer

Without constraint, SPLADE would fill the whole vocabulary with small non-zero weights, which is slow to index and search. SPLADE adds a FLOPS loss that penalizes the expected number of non-zero dot products at query time:

L = L_rank + lambda_q * FLOPS(q) + lambda_d * FLOPS(d)

lambda_q > lambda_d produces short, precise queries with slightly fatter documents — the right tradeoff for inverted-index serving. Published SPLADE++ (splade-cocondenser-ensembledistil) averages ~120 non-zero terms per document.

When SPLADE Wins

Rule of thumb: SPLADE replaces BM25 in a hybrid pipeline when you have a GPU at ingest. BM25 is still the right choice if you cannot afford GPU encoding.

Encoding with Hugging Face Transformers

# pip install transformers torch
from transformers import AutoTokenizer, AutoModelForMaskedLM
import torch

MODEL_ID = "naver/splade-cocondenser-ensembledistil"  # SPLADE++; MIT license
tokenizer = AutoTokenizer.from_pretrained(MODEL_ID)
model = AutoModelForMaskedLM.from_pretrained(MODEL_ID).eval()

if torch.cuda.is_available():
    model = model.to("cuda").half()

@torch.inference_mode()
def splade_encode(text: str, max_length: int = 256) -> dict[int, float]:
    tok = tokenizer(text, return_tensors="pt", truncation=True, max_length=max_length)
    if torch.cuda.is_available():
        tok = {k: v.to("cuda") for k, v in tok.items()}
    logits = model(**tok).logits  # (1, seq_len, vocab)
    attn = tok["attention_mask"].unsqueeze(-1)
    weights = torch.max(
        torch.log1p(torch.relu(logits)) * attn, dim=1
    ).values.squeeze(0)  # (vocab,)
    idx = torch.nonzero(weights).squeeze(-1)
    return {int(i): float(weights[i]) for i in idx.tolist()}

For batching (ingest-time), replace with a real DataLoader; one-at-a-time will bottleneck on GPU launch overhead.

Efficient Batching for Ingest

from torch.utils.data import DataLoader

def collate(batch):
    return tokenizer(batch, padding=True, truncation=True, max_length=256,
                     return_tensors="pt")

@torch.inference_mode()
def batch_splade(texts: list[str], batch_size: int = 32) -> list[dict[int, float]]:
    loader = DataLoader(texts, batch_size=batch_size, collate_fn=collate)
    out = []
    for tok in loader:
        tok = {k: v.to("cuda") for k, v in tok.items()}
        logits = model(**tok).logits
        attn = tok["attention_mask"].unsqueeze(-1)
        w = torch.max(torch.log1p(torch.relu(logits)) * attn, dim=1).values
        for row in w:
            idx = torch.nonzero(row).squeeze(-1)
            out.append({int(i): float(row[i]) for i in idx.tolist()})
    return out

At 32-batch on an A10G, SPLADE++ encodes ~400-800 docs/sec depending on length.

Indexing in Qdrant (native sparse vectors)

from qdrant_client import QdrantClient, models

client = QdrantClient(url="http://localhost:6333", prefer_grpc=True)
client.create_collection(
    "splade-kb",
    vectors_config={},  # no dense needed if pure SPLADE
    sparse_vectors_config={
        "splade": models.SparseVectorParams(
            index=models.SparseIndexParams(on_disk=False, full_scan_threshold=5000),
        ),
    },
)

def to_sparse(weights: dict[int, float]) -> models.SparseVector:
    items = sorted(weights.items())
    return models.SparseVector(indices=[i for i, _ in items],
                               values=[v for _, v in items])

client.upsert(
    "splade-kb",
    points=[models.PointStruct(id=i, vector={"splade": to_sparse(w)}, payload={"text": t})
            for i, (w, t) in enumerate(zip(doc_weights, docs))],
)

# Query
q_weights = splade_encode("token refresh 403")
result = client.query_points(
    "splade-kb",
    query=to_sparse(q_weights),
    using="splade",
    limit=10,
)

Indexing in Elasticsearch (rank_features)

Elasticsearch's rank_features field accepts a map of token-string -> weight; SPLADE terms map to BERT WordPiece tokens. Use the tokenizer's convert_ids_to_tokens to stringify.

from elasticsearch import Elasticsearch, helpers

es = Elasticsearch("http://localhost:9200")
es.indices.create(
    index="splade",
    mappings={"properties": {"splade_weights": {"type": "rank_features"}}},
)

def stringify(weights: dict[int, float]) -> dict[str, float]:
    return {tokenizer.convert_ids_to_tokens(i): float(v) for i, v in weights.items()}

def actions(docs, weights):
    for i, (d, w) in enumerate(zip(docs, weights)):
        yield {"_index": "splade", "_id": i,
               "_source": {"text": d, "splade_weights": stringify(w)}}

helpers.bulk(es, actions(docs, doc_weights))

# Query: each non-zero term becomes a rank_feature query
def splade_query(q_weights: dict[int, float]):
    return {
        "query": {
            "bool": {
                "should": [
                    {"rank_feature": {"field": f"splade_weights.{tokenizer.convert_ids_to_tokens(i)}",
                                      "saturation": {}, "boost": float(v)}}
                    for i, v in q_weights.items()
                ],
            },
        },
    }

Elasticsearch 8.11+ also has sparse_vector field type which is faster — use it if available.

Hybrid SPLADE + Dense (RRF)

SPLADE replaces BM25 in the hybrid pattern. Rerun with RRF or alpha fusion.

from qdrant_client.models import Prefetch, FusionQuery, Fusion

client.query_points(
    "hybrid-kb",
    prefetch=[
        Prefetch(query=dense_q.tolist(), using="dense", limit=50),
        Prefetch(query=to_sparse(q_weights), using="splade", limit=50),
    ],
    query=FusionQuery(fusion=Fusion.RRF),
    limit=10,
)

See rag/hybrid-search for fusion algorithms.

Efficiency Tradeoffs

SPLADE's killer feature is zero-GPU-at-query-time while staying within ~2x of BM25 latency.

Document vs Query Models

Some SPLADE releases provide a separate lighter query encoder. Use it at query time when latency matters.

QUERY_MODEL = "naver/efficient-splade-V-large-query"
DOC_MODEL = "naver/efficient-splade-V-large-doc"

Encode queries with the query model, documents with the doc model — they share the same vocabulary space.

Anti-Patterns

Production Checklist

• Query encoder runs on GPU with warm pool (or distilled query model)
• Document encoder batched at ingest; throughput measured
• Sparse index uses native sparse-vector type, not dense-with-zeros
• Nonzero-term distribution monitored (median, p95) for drift
• Hybrid fusion tuned on a labeled set (see rag/hybrid-search)
• Tokenizer version pinned alongside model weights
• Fallback to BM25 if SPLADE encoder pool is unhealthy
• Ingest reprocessing plan when swapping SPLADE checkpoints
• Evaluation against BM25 baseline documented