Billionscale vector search using hybrid HNSWIF
Photo by Graham Holtshausen on Unsplash
The first blog post on billionscale vector search covered methods for compressing realvalued vectors to binary representations and using hamming distance for efficient coarse level search. The second post described approximate nearest neighbor search tradeoffs using Hierarchical Navigable Small World (HNSW), including memory usage, vector indexing performance, and query performance versus accuracy.
This post in this series on billion scale search introduces a costefficient hybrid method
for approximate nearest neighbor (ANN) search combining (HNSW
) with diskbacked inverted file.
We name this hybrid method for ANN search for HNSWIF
.
Introduction
Inmemory algorithms, like HNSW,
for approximate nearest neighbor search, offer fast, high accuracy vector search
but quickly become expensive for massive vector datasets due to memory requirements.
The HNSW
algorithm requires storing the vector data in memory for low latency access during query and indexing.
For example, a billion scale vector dataset using 768 dimensions with float precision requires
close to 3TiB of memory. In addition, the HNSW
graph data structure needs to be inmemory,
which adds 2040% in addition to the vector data.
Given this, indexing a 1B vector dataset using HNSW
will need about 4TiB of memory.
In 2022, many cloud providers offer cloud instance types with large amounts of memory, but these instance types also come with many vCPUs, which drives production deployment costs. These highmemory and highcompute instance types support massive queries per second and might be the optimal instance type for applications needing to support large query throughput with high recall. However, many realworld applications using vector search do not need enormous query throughput but still need to search large billionscale vector datasets with relatively low latency with high accuracy. Therefore, large cloud instance types with thousands of GiB of memory and hundreds of vCPUs are not costefficient for those low query volume use cases.
Due to this, there is an increasing interest in hybrid ANN search solutions using solidstate disks (SSD) to store most of the vector data combined with inmemory graph data structures. SPANN: Highlyefficient Billionscale Approximate Nearest Neighbor Search introduces a simple and effective solution for hybrid ANN search.
Introducing SPANN
SPANN combines the graphbased inmemory method for ANN search with the inverted file using clustering.
SPANN partitions the vector dataset of M
vectors into N
clusters.
The paper explores setting N
to a number between 4% to 20% of M
.
A centroid vector represents each cluster.
The paper describes different algorithms for
clustering the vector dataset into N
clusters and finds that hierarchical balanced clustering (HBC) works best.
See Figure 10 in the paper: Different types of centroid selection.
The cluster centroid vector points to a posting list containing the vectors close to the cluster centroid. Diskbased data structures back the posting lists of noncentroids, and centroids are indexed using an inmemory ANN search algorithm. Unlike quantization techniques for ANN search, all vector distance calculations are performed with the fullprecision vector representation.
SPANN searches for the k
closest centroid vectors of the query vector in the inmemory ANN search
data structure. Then, it reads the k
associated posting lists for the retrieved
centroids and computes the distance between the query vector
and the vector data read from the posting list.
Figure 1 illustrates SPANN.
Figure 1 gives a conceptual overview of SPANN for a small vector dataset of ten vectors. There are two centroid vectors, vectors 4 and 9, referencing a posting list consisting of the vectors close to the cluster the centroid represents. A vector might be close to multiple cluster centroids, for example, vector 5 and vector 8 in the figure above. These are examples of boundary vectors that lay in between multiple centroids.
The offline clustering part of SPANN tries to balance the clusters so
that the posting lists are equal in size to reduce the time it takes to read the posting list from disk.
For example, if the vector has 100 dimensions using int8
precision and int32
for the vector id,
each posting list entry uses 104 bytes. With a 4KiB disk read page size,
one can read 1024 posting entries in a single IO read operation.
Hybrid HNSWIF with Vespa
Inspired by the SPANN paper, we at the Vespa team
implemented a simplified version of SPANN
using Vespa primitives, released
as a Vespa sample application.
We call this hybrid ANN search method for HNSWIF
.
Vespa features used to implement HNSWIF
:
 Realtime HNSW vector indexing
 Realtime inverted index data structures
 Disk based vectors using Vespa dense tensor type using paged option
 Phased ranking
 Stateless search and document processors
The following sections outline the differences between the method described in the SPANN
paper and the
Vespa HNSWIF
sample application
implementation using Vespa primitives.
Vector Indexing and Serving architecture

Instead of clustering and computing centroids offline, let vectors from the original dataset represent centroids and use the original vector id as the centroid id. This approach does not waste any distance calculations at query time as the centroids are valid eligible vectors. A subset of the vector dataset (20%) is selected randomly to represent centroids. Random centroid selection only requires one pass through the vector dataset, splitting the dataset into centroids and noncentroids.

The vectors representing centroids are indexed in memory using Vespa’s support for vector indexing using Hierarchical Navigable Small World (HNSW). Searching 200M centroid vectors indexed with
HNSW
typically takes 23 milliseconds, singlethreaded (depending on recall target andHNSW
settings). Both the graph data structure and the vector data are stored in memory. 
During indexing of vectors that are not cluster centroids, search for the
k
closest centroids in theHNSW
graph of centroids and index the closest centroid ids using Vespa’s support for inverted indexes. Later, when the index build is complete, a search for a centroid id efficiently retrieves the closest noncentroid vector id. The inverted index consists of a dictionary of centroid ids pointing to posting lists of noncentroid vector ids. For a given billion scale dataset with 20% centroids, the maximum centroid dictionary vocabulary size is 200M. 
A noncentroid vector might be present in multiple centroid clusters. Instead of storing the vector data in the posting lists, the vector data is stored in a separate Vespa data structure and avoids duplication caused by storing the same vector data in multiple posting lists. Instead, the Vespa posting list entry stores the closeness (inverted distance) of the vector to the centroid, scaled to integer weight. Only the vector ids are duplicated across centroid posting lists, not the vector data itself. Vespa posting lists are compressed using standard techniques for lossless posting list compression.
Querying Vectors
For an input query vector, first search the vectors representing centroids, using HNSW
, for the k
closest centroids.
Next, using the retrieved k
nearest centroids from HNSW
search,
search the inverted index using logical disjunction (OR) of the centroid ids retrieved
by the HNSW
graph search. The actual implementation uses the
Vespa dotProduct multivalued query operator.
Each node involved in the inverted index query ranks the retrieved noncentroid vectors by calculating the distance between the vector and the query vector. Finally, the result of the two searches is merged and returned.
The query serving flow can be optimized by two heuristics:

Cluster centroid dynamic pruning. After retrieving the
k
closest centroids from searching theHNSW
graph, distant centroids (compared to the nearest centroid) can be pruned without significantly impacting recall. This distant centroid pruning heuristic reduces the number of seeks and reads for the inverted index query evaluation. The centroid pruning heuristic is dynamic; a query vector that retrieves many equally close centroids allows little pruning, while a query vector that retrieves centroids with more considerable distance differences might allow pruning many. 
Retrieve using dynamic pruning. This heuristic sorts the retrieved vector ids by the
closeness(q, centroid) * closeness(centroid, v)
transitive closeness score whereq
is the query vector andv
is the document vector. This phase is implemented as a Vespa firstphase ranking phase. Thecloseness(centroid,v)
weight is stored in the posting list, and thecloseness(q, centroid)
is passed as a query term weight with thedotProduct
query operator. This heuristic enables limiting the number of vector pageins by using Vespa’s support for controlling phased ranking. The local per node secondphase ranking calculates the full precision,(closeness(q,v)
, which involves paging the vector data into memory from disk. The maximum reranking depth is a query time hyperparameter enabling easy experimentation.
Realworld considerations
Realworld applications using vector search need both batch and realtime vector indexing:
 Batch indexing: An embedder model (for example, Data2Vec) that maps data to vector representation is trained, and embedding vector representations are produced for all known data items.
 Incremental Realtime indexing: A new data item arrives and is encoded with the current version of the embedder model and needs to be indexed.
In addition, data items (with vector representation) need to be updated and deleted. The hybrid method described in this blog post supports all CRUD (Create, Read, Update, Delete) operations using the standard Vespa APIs.

Batch indexing with a new embedder model is handled by adding a model version field to the schema. Serving queries must restrict the search for vectors to the given model id using standard inverted index query evaluation and constrained vector search. See Query Time Constrained Approximate Nearest Neighbor Search and Embedding model hot swap. Having multiple active model versions increases the storagerelated deployment cost linearly with the number of models.

New vectors using an existing embedding model are added as a noncentroid vector. As long as the ratio of centroids is large, one can expect to grow the vector volume significantly without significantly degrading accuracy.
The only thing the application owner needs to consider is that deleting large amounts of centroid vectors will negatively impact recall. For most largescale vector use cases, this is not a real problem. If the use case requires deleting many vector items, one can consider decoupling centroids from real vectors so that centroids are real centroids and not vectors part of the dataset.
Vespa Experimental Setup
The following section describes our experiments with the Vespa HNSWIF
sample application using
Vespa Cloud’s performance environment.
The Vespa Cloud performance environment makes it easy to iteratively develop applications and choose the ideal instance types
for any size vector dataset.
Vespa Cloud Console  sample app deployment in Vespa Cloud perf environment in awsuseast1c region.
Vespa HNSWIF
serving architecture overview.
The Vespa HNSWIF
representation uses the same
Vespa document schema to represent centroid and noncentroid vectors.
They are differentiated using a single field of type bool
.
Using two content clusters with the same document schema enables using different instance types for the two vector types:
 High memory instances with remote storage for the centroid vectors using inmemory
HNSW
.  Inexpensive low memory instances with fast local storage for the noncentroid vectors.
This optimizes the deployment and resource cost  the vectors indexed using HNSW
does not need fast local disks since queries will never page data from disk during queries.
Similarly, for vectors indexed using inverted file, the instances don’t
need an awful amount of memory, but more memory can improve query performance
due to page caching.
The Vespa inverted index posting lists do not contain the vector data. Instead, vector data is stored using Vespa paged tensor attributes, a type of diskbacked memory mapped forwardindex. The downside of not storing the vector data in the postings file is that paging in a vector from disk for distance calculation requires one additional disk seek. However, in our experience, locally attached SSD disks are rarely limited by random seek but by GiB/s throughput bandwidth.
Vector Dataset
For our experiments with HNSWIF
, we use the 1B Microsoft SPACEV1B vector dataset:
Microsoft SPACEV1B is a new web searchrelated dataset released by Microsoft Bing for this competition. It consists of document and query vectors encoded by the Microsoft SpaceV Superior model to capture generic intent representation.
The SPACEV1B vector dataset consists of 1Billion 100dimensional vectors using int8
precision
and 29,3K queries with 100 ground truth (exact neighbors) per query.
The distancemetric used for the dataset
is euclidean
which is the default Vespa nearest neighbor search distancemetric.
The dataset’s ground truth neighbors are used to evaluate the
accuracy (recall) of the hybrid HNSWIF
approach.
Vespa Schema
The sample application uses the following Vespa document schema.
Supported Vespa schema field types
include string
, long
, int
, float
, double
, geo position
, bool
, byte
, and tensor
fields.
Vespa’s firstorder dense tensor fields represent vector fields.
Vespa’s tensor fields support different tensor cell precision types,
including int8
, bfloat16
, float
, and double
for realvalued vectors. The SPACEV1B
vector dataset uses int8
precision.
schema vector { document vector { field id type int {} field vector type tensor<int8>(x[100]) { indexing: attribute  index } field neighbors type weightedset<string> { indexing: summary  index } field disk_vector type tensor<int8>(x[100]) { indexing: attribute attribute: paged } field in_graph type bool {} } }
Vespa vector
document schema. See full version
The random centroid selection is performed outside of Vespa using a
python script that reads the
input vector data file and randomly selects 20% to represent centroids and sets the in_graph
field
of type bool
to true
and populate the vector
field with the vector data.
The feeder feeds the vector data with in_graph
set to true
first, to
populate the graph
content cluster using HNSW
indexing, before feeding the noncentroid vectors.
The neighbors
field is of type
weightedset<string>.
The weightedset<string>
type allows mapping a string key (the centroid id in this case) to an integer weight.
This field is populated by a
custom document processor
which searches the HNSW
graph when feeding noncentroid vectors with in_graph
set to false
.
For example, for vector 8 from figure 1, the field would be populated with
two centroids found from searching the HNSW
graph:
{ "put": "id:spann:vector::8" "fields": { "id": "8", "neighbors": { "4": 45 "9": 100 }, "in_graph": false, "disk_vector": { "values": [12, 8, 1, ..] } } }
This neighbors
field is inverted by the Vespa content process (proton), so that a query for
where neighbors contains "4"
would retrieve vector 8 and expose it to the Vespa ranking framework.
The integer weight represents the closeness of vector 8 to the centroid id.
Closeness is the inverted distance, and a lower distance means higher closeness.
The original float
closeness value returned from the HNSW
search is scaled to integer representation by
multiplying with a constant and rounded to the closest integer.
The document processor clears the incoming vector
field and instead creates
the disk_vector
field, which uses the attribute: paged
option for paging in
the vector data ondemand at ranking time.
The schema also has two rankprofile
’s which determines how vectors are ranked while performing
distance calculations. One profile used for the HNSW
search and one for the Inverted File
search
implementing phased ranking heuristic.
Vespa deployment specification
We use multiple stateless and stateful Vespa clusters in the same Vespa application for our experiments.
Everything related to clusters and resources is configured using Vespa services.xml. The
sample application has two services.xml
:
The Vespa Cloud version specifies:
 A stateless
feed
container cluster handling feed operations running a custom document processor which searches the HNSW graph for noncentroid vectors. This cluster uses default resources which are 2 vCPU, 8GiB of memory, and 50GiB of disk:
<nodes deploy:environment="perf" count="4"/>
 A stateless
default
container cluster handling search queries. This cluster also uses default resources.
<nodes deploy:environment="perf" count="2"/>
 A stateful content cluster
graph
which is used with high memory instance types andHNSW
.
<nodes deploy:environment="perf" count="1" groups="1"> <resources memory="128GB" vcpu="16" disk="200Gb" storagetype="remote"/> </nodes>
 A stateful content cluster
if
used for inverted indexing (inverted file) for noncentroid vectors.
<nodes deploy:environment="perf" count="4" groups="1"> <resources memory="32GB" vcpu="8" disk="300Gb" storagetype="local"/> </nodes>
The Vespa Cloud hourly cost for this perf
environment deployment, supporting 1B vectors comfortably, is $ 8.38 per hour, or
$6,038 per month.
Screenshot from Vespa Cloud console with app’s clusters and allocated resources.
As can be seen from the cluster resource scaling summary, the deployment is slightly overprovisioned and could support larger vector volumes comfortably. Vespa Cloud also allows a wide range of resource combinations (memory, CPU, disk) and the number of nodes per Vespa cluster.
Vespa stateless function components
Custom stateless Vespa functions implement the serving and processing logic. The components are deployed inside the Vespa cluster, where communication is secured, and data transferred with optimized binary protocols. The gist of the custom searcher implementing the search logic is given below:
@Override public Result search(Query query, Execution execution) { Tensor queryVector = query.getTensor("query(q)"); CentroidResult centroidResult = clustering.getCentroids( queryVector, nClusters, hnswExploreHits, execution); List<Centroid> centroids = clustering.prune( centroidResult.getCentroids(), pruneThreshold); DotProductItem dp = new DotProductItem("neighbors"); for (Centroid c : centroids) { dp.addToken(c.getId(), c.getIntCloseness()); } query.getModel().getQueryTree().setRoot(dp); query.getModel().setSources("if"); query.getRanking().setRerankCount(reRankCount); return mergeResult(execution.search(query), centroidResult); }
See the full version.
Similarly, a custom document processor implements
the search in the HNSW
graph, and annotates the incoming vector with the nearest centroids.
See AssignNeighborsDocProc.
Vespa HNSWIF Experiments
The following experiments use these fixed indexing side hyperparameters:
 Inmemory centroid indexing using the following HNSW settings:
maxlinkspernode: 18
andneighborstoexploreatinsert: 100
.  For any given noncentroid vector, the 12 closest centroid vector ids are indexed using the inverted index.
Batch indexing performance
With the mentioned allocated resources in Vespa Cloud perf, indexing the SPACEV1B dataset takes approximately 28 hours.
The 200M centroid vectors are fed first and indexed at around 9000 puts/s into the HNSW graph content cluster.
The remaining 800M noncentroid vectors are indexed with similar puts/s, and also search the HNSW
index at the same rate.
Vectors are read from the dataset and converted to Vespa JSON feed format
by a python script. The resulting two JSON files are fed using Vespa feed client
for ultimate batch feed performance using http/2
with mTls
to secure the vector data in transit. Vespa Cloud
also stores all data using encryption (encryption at rest).
Vespa Cloud console screenshot, taken during indexing of noncentroid vectors.
Vespa Cloud metrics dashboard.
The Vespa Cloud metrics provide insight into resource utilization which can help choose the right instance resource specification.
HNSWIF accuracy
Any approximate vector search use case needs to quantify the accuracy impact of using approximate search instead of
exact search. Using the ground truth neighbors for the 29,3K SPACEV1B query vectors,
we can quantify recall@10
with the hybrid HNSWIF
solution:
Recall@10 for 29,3K queries
The above figure is produced by running all 29,3K queries with an increasing number of k
centroids, ranging
from 1 centroid to 256 centroids (1, 2, 4, 8 , 16, 32, 64, 128, 256). The distance prune threshold was set to 0.6,
and the maximum reranking depth was 4K.
Example run using recall.py
with 32 centroid clusters k=32
, reranking at most 4K vectors.
$ python3 recall.py endpoint https://spann.samples.awsuseast1c.perf.z.vespaapp.cloud/search/ \ query_file query.i8bin \ query_gt_file public_query_gt100.bin \ clusters 32 \ distance_prune_threshold 0.6 \ rank_count 4000 \ certificate dataplanepubliccert.pem \ key dataplaneprivatekey.pem
With k=128
centroids, we reach 90% recall@10 at just below 50 ms endtoend.
50 ms is one order of magnitude larger than what inmemory algorithms support at the same recall level,
but for many vector search use cases, 50ms is perfectly acceptable, especially considering the high recall.
To put the number in context, 9 out of 10 queries return the same top10 result as the expensive nearest neighbor search, over 1B vectors!
Summary
This blog post introduced a costeffective hybrid method for billionscale vector search, enabling many new realworld applications using AIpowered vector representations. Today, You can get started using the readytodeploy billionscalevectorsearch.
Also try other Vespa sample applications built using Vespa’s approximate
nearest neighbor search support using HNSW
:

Stateoftheart text ranking: Vector search with AIpowered representations built on NLP Transformer models for candidate retrieval. The application has multivector representations for reranking, using Vespa’s phased retrieval and ranking pipelines. Furthermore, the application shows how embedding models, which map the text data to vector representation, can be deployed to Vespa for runtime inference during document and query processing.

Stateoftheart image search: AIpowered multimodal vector representations to retrieve images for a text query.
These are examples of applications built using AIpowered vector representations and where realworld deployments need querytime constrained nearest neighbor search.
Vespa is available as a cloud service; see Vespa Cloud  getting started, or selfserve Vespa  getting started.