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With the advent of deep learning and algorithms like word2vec and doc2vec, vectors-based representations are increasingly being used in search to represent anything from documents to images and products. However, search engines work with documents made of tokens, and not vectors, and are typically not designed for fast vector matching out of the box. In this talk, I will give an overview of how vectors can be derived from documents to produce a semantic representation of a document that can be used to implement semantic / conceptual search without hurting performance. I will then describe a few different techniques for efficiently searching vector-based representations in an inverted index, including LSH, vector quantization and k-means tree, and compare their performance in terms of speed and relevancy. Finally, I will describe how each technique can be implemented efficiently in a lucene-based search engine such as Solr or Elastic Search.

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- 1. Searching with Vectors Simon Hughes Chief Data Scientist, Dice.com Twitter: @hughes_meister
- 2. Who Am I? • Chief Data Scientist at DHI (owns Dice.com) • Key Projects: • Search and Match • Dice Recommender Systems • Dice Job Search • Dice Talent Search 3.0 and 4.0 • Dice Skill Center • Dice Career Advisory Pages • Dice Salary Predictor • Dice Career Paths • PhD Candidate DePaul University • Subject Area – Machine Learning and NLP • Thesis – Extracting Causal Relations from Scientific Essays • Contact Info: • Email: simon.hughes@dhigroupinc.com • Twitter: https://twitter.com/hughes_meister
- 3. Motivation • Dice.com - leading US technology professional job board • Jobs marketplace • We connect technology talent with employers • High quality searching and matching are critical to our value proposition, for both our customers and our clients • Need – high quality content-based recommender engine • Automatically determine how well a job seeker matches a particular position, and vice versa • Requirements: • A semantic matching engine – goes beyond keyword search, to extracting semantic information from job postings and resume • Deployed at scale using existing search infrastructure (Solr and ElasticSearch) • Github Repository for Talk: • https://github.com/DiceTechJobs/VectorsInSearch
- 4. Agenda • Why a Vector Representation? • Learning Vector Representations • Vector Based Search in an Inverted Index
- 5. Understanding Textual Data Key Challenges: • Synonymy – Multiple Words with the Same Meaning • Related – typos, miss-spellings, acronyms, metonyms • E.g. QA, Quality Assurance, Tester • Polysemy – Ambiguity, a word has multiple meanings • E.g. Bank, Book, Ape • Hypernyms/Hyponyms – ‘type of’ relationships • E.g. a dog (hyponym) is a type of animal (hypernym) • Meronyms/Holonyms – ‘part of’ relationships • E.g. finger (meronym) is a ‘part of’ a hand (holonym) • What Words / Phrases are More Important? • Named Entity Extraction (NER), Controlled Vocabularies • Colocation (phrases) detection – e.g. “data scientist” vs “scientist who works with data” • Stop words • Term weighting schemes - e.g. tf.idf
- 6. How to Solve these Problems? • Map documents and queries to a semantic space • “From Strings to Things”? • Google KG marketing • Map words into concepts / semantics • From strings to concepts • How to represent? Java Technologies Big Data Tools Javascript Frameworks
- 7. Representations Java • Local representation • Non distributed • Sparse • E.g. one-hot-vector • One vector component per unique word • Similar items have different representations
- 8. Representations • Distributed Representation • Dense vector • Components of the vector represent learned concepts / latent variables • Similar items have similar representations • Most existing approaches produce dense vectors Java Java • Local representation • Non distributed • Sparse • E.g. one-hot-vector • One vector component per unique word • Similar items have different representations
- 9. Agenda • Why a Vector Representation? • Learning Vector Representations • Vector Based Search in an Inverted Index
- 10. The Importance of Context How do we learn the meaning (semantics) of words? • Distributional Hypothesis • Words occurring in similar contexts have similar meanings • Harris 1954 • “a word is characterized by the company it keeps” • Firth 1957 • Ignores word order, grammar and syntax • Latent Relation Hypothesis • Pairs of words occurring in similar patterns have similar semantic relations • Turney et al, 2003 • Patterns – X cuts Y, X works with Y, etc • Word order and grammatical relations matter • Further reading - Distributional approaches to word meanings
- 11. Learning Meaning from Context Bag of Words Approaches – ignore word order • Latent Models • Context - Documents • LSA • LDA • Semantic Vector Space Model • Word Embeddings • Context – word window • Word2vec • Glove • Simple linear language models • History - http://blog.aylien.com/a-review-of-the-recent-history-of-natural-language-processing/ • For document embeddings • Average or idf weighted average of word vectors • Sentence / Document Embeddings • Context – document + word window • E.g. Doc2vec • Context – surrounding sentences • E.g. skip-thought vectors
- 12. Word2Vec • By Aelu013 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0) ], from Wikimedia Commons
- 13. Limitations of BOW Approaches: • Shallow representation • Word embeddings – limited to the word level • Latent models – document level but doesn’t encode relational information • Synonymy - learn relatedness, not true synonyms • E.g. Antonyms have similar vectors • Polysemy – cannot encode different meanings of same word • Global model not a local model
- 14. Beyond BOW - Deep Language Models • Deep Language Model Embeddings • Derived from the internal state of a deep LM • Learns deep representation of sequences of words in context • Can adjust word vectors based on their current context • “NLP’s imagenet moment” • Achieved state of the art results on many NLP tasks • Consistently out-perform word embedding models • Example models - ELMO, BERT, ULMFit, OpenAI Transformer • Used for encoding sentences not whole documents • Hard to scale
- 15. Deep Language Models p(w1,w2,w3, w4,…,wn) = p(wn|w1,w2,…,wn-1) ….. ….. ….. p(w1) p(w2|W1) p(w3|w1,w2) p(w4|w1,w2,w3) Begin w1 w2 w3 LSTM LSTM LSTM LSTM
- 16. Embedding Models for Search • Word Embedding Approaches • Cluster Word Embeddings • “Representing Documents and Queries as Sets of Word Embedded Vectors for Information Retrieval” • Clustered word2vec vectors using k-means • Documents represented as clusters of word vectors • Query - map query vectors as similarity to cluster centroids • Out performed Jelinek Mercer LM similarity using VSM • Average Word Embeddings • From Chapter 5 of Deep Learning for Search • Author - Tommaso Teofili • Query and document represented as average of word2vec vectors • Computing a weighted average using idf worked best • Outperformed BM25 using cosine similarity • BM25 + word2vec – highest NDCG score
- 17. Embedding Models for Search • Dual Embedding Space Model (DESM) • Research from Microsoft • Extends word2vec • Learns a dual embedding for queries and documents • Paper - https://arxiv.org/pdf/1602.01137.pdf • Evaluation • Compared BM25, LSA and DESM on Bing Query Log Data • Metrics - NDCG@1, NDCG@3, NDCG@5 • Results • LSA and DESM both out-performed BM25 • DESM out-performed LSA • DESM + BM25 out-performed all other approaches
- 18. Agenda • Why a Vector Representation? • Learning Vector Representations • Vector Based Search in an Inverted Index
- 19. Vectors in Search • Dense Embedding Vector: • Dense • D dimensional • D = 50-1000 • Inverted index: • Sparse • Pivoted by term • V = Vocabulary • |V| =100k+ • Fast because sparse [+0.12, -0.34, -0.12, +0.27, +0.63] Term Posting List Java 1,5,100,102 .NET 2,4,600,605,1000 C# 2,88,105,800 SQL 130,433,648,899,1200 Html 1,2,10,30,55,202,252,30,598,
- 20. Searching with Word Embeddings Approaches for using word embeddings: • Top N terms • Expand query using top n terms from model • Boost expansions by cosine similarity • Can use as a boost query, a re-rank query or a straight term expansion • Q = “java developer”^10 OR ”java j2ee developer”^0.91 OR “java architect”^0.89 OR “lead java developer”^0.87 OR “j2ee developer”^0.86 OR “java engineer”^0.86 • Term Clustering • Cluster embeddings using a clustering algorithm • E.g. k-means • Compute different sized clusters, k=100,1000,10000 • Map clusters to tokens and index • Different fields for each k • Larger k fields – bigger boost or rely on idf scoring • Query expands to top clusters, boosted by similarity • Q = “java developer”^10 OR cluster_k1000:5894^5 OR cluster_k100:23^2.5 OR cluster_k10:8^1.25 • See https://github.com/DiceTechJobs/ConceptualSearch
- 21. Searching Vectors – k-NN Search • K-NN search • Find the k closest neighbors to query vector according to similarity metric • Usually cosine similarity or Euclidean distance • Definitions • D = number of components in the vector • N = number of documents • Brute Force Search: • O(ND) = linear • What if N AND/OR D is(are) very large? • Vs. Inverted Index • Sublinear - makes uses of sparsity of terms • BTree or Distributed Hash Table lookup for terms, iterate posting list, re-rank matches - O(n log n)
- 22. Optimal Vector Representation In An Inverted Index? What properties would such a representation have? • For Performance • Sparse representation necessary to leverage inverted index • For Relevancy • Distributed representation • Each document should be a collection of tokens • Tokens represent some semantic feature of the space • Similarity is preserved • Similar vectors must also be similar under this new representation • Zipfian distribution of tokens • “We need a Zipfian Distribution” – John Berryman (Co-author of ‘Relevant Search’) • Tokenizing Embedding Spaces
- 23. Zipf’s Law • The frequency of terms in a corpus follow a power law distribution • Small number of tokens are very common - filter out irrelevant docs • A large number of tokens are very rare - discriminate between similar matches • Distribution of last names - By Thekohser [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0 )], from Wikimedia Commons
- 24. Approximate Nearest Neighbor Search • Faster than full k-NN, with some loss in accuracy • Approaches can be either: • Data Dependent • Learns and adjusts from the data • Makes indexing new documents hard • Data Independent • Some Approaches: • KD Tree • LSH • Heuristic Methods • K-Means Tree • Randomized KD Forest • Paper: https://arxiv.org/abs/1603.09596 • HNSW (Hierarchical Navigable Small World Graphs – Top on http://ann-benchmarks.com/ • Paper: https://arxiv.org/pdf/1603.09320.pdf • Vector Thresholding • Choice of similarity metric is important in choosing an algorithm
- 25. KD Trees • Construction • Constructs a binary search tree by partitioning the search space along each vector dimension using the dimensions • Partitions are chosen orthogonal to each dimension • Usually the median • Querying • Described here - https://en.wikipedia.org/wiki/K-d_tree#Complexity • Limitations • How to implement efficiently in an inverted index? • Lucene 6.0 dimensional points • See also - https://www.elastic.co/blog/lucene-points-6.0 • Not exposed in Solr and Elastic Search AFAIK • Tree needs rebalancing on each insertion • Curse of dimensionality • N >> 2d - for N points and D dimensions • Complexity essentially linear for real world vectors (D>= 50) • Approximate KNN Search • Possible with KD tree – limit the number of searched nodes • Typically out-performed by other ANNs approaches
- 26. Locality Sensitive Hashing • LSH hashes items to discrete buckets • More buckets – slower but more accurate • Locality Preserving • Maximizes the probability that similar items occupy the same buckets • Random Projection LSH (sim Hash) • LSH variant for cosine similarity • Generate a random d-dimensional unit vector r, and for each vector v • ℎ𝑎𝑠ℎ 𝑣 = 𝑠𝑖𝑔𝑛(𝑣. 𝑟) • Produces a binary encoding, one bit for each hash function (random vector) • Probability 2 vectors’ hashes match - proportional to cosine similarity • Output of hash function can be indexed and searched using Hamming Distance • Intuition - Van Durme and Lall - http://www.cs.jhu.edu/~vandurme/papers/VanDurmeLallACL10-slides.pdf • Data independent, although data dependent variations exist • However, for real data, it is typically out-performed by heuristic methods like k-means trees, and randomized KD- trees • https://lear.inrialpes.fr/pubs/2011/JDS11/jegou_searching_with_quantization.pdf
- 27. Encoding LSH Hash into the Index • Hash into Bits • Store hash fingerprint as a single token • Store each bit as a token using it’s position and value • Use mm parameter to speed up search • Or store shingles of the binary tokens • This is not sparse! [+0.08, -0.16, -0.12, +0.27, +0.63, -0.01, +0.16, -0.48] [1, 0, 1, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1] [“10110110100101”] ["00_1","01_0","02_1","04_1","04_0","05_1","06_1","07_0","08_1","09_0","10_0","11_1","12_0","13_1”] OR
- 28. Hamming Similarity Class • Custom similarity class • Computes the number of matching tokens
- 29. K-Means Tree • Hierarchical Clustering Algorithm • Recursively partitions vector space using k-Means clustering • Fast - k-means runs in linear time using Lloyd’s heuristic • Most other clustering algorithms run in quadratic time or worse • Tree Construction • For some branching factor b create b clusters • Create b nodes, store centroid for each node • For each new cluster, cluster its members into b smaller clusters • These form child nodes of their parent clusters, forming a tree structure • Continue until < b members per cluster • Paper • "Scalable Nearest Neighbor Algorithms for High Dimensional Data" - Marius Muja, 2014 – implemented in the FLANN library
- 30. K-Means Tree Second Layer (Leaf Nodes) Root Node First Layer …. ….….. …. Documents • Depth 3 K-Means Tree
- 31. Lucene Implementation Details • Pre-train a k-means tree on a representative subset of the index • Indexing: • Convert all nodes from tree into unique tokens • For each vector, find the closest matching leaf node • Index vector with tokens for that leaf node, and all parent nodes • Querying • Find top n matching nodes from tree • Convert nodes into a query, boosted by similarity to query vector • 'q': 'clusters:(“121”^0.9 “909”^0.88 ”523”^0.91)’ • Create a re-rank query to brute force re-rank the top matching documents • 'rq’: '{!rerank reRankQuery=$rqq reRankDocs=1000 reRankWeight=99}’ • 'rqq': '{!payloadEdismax v=$vq}’ • ‘vq’: vector:(”0”^-0.0136 ”1”^0.05387 ”2”^0.070476 ”3”^0.14529 …) • Uses a special payload query parser (payload_score is insufficient) • See https://github.com/DiceTechJobs/VectorsInSearch • *Better approach – use doc values field or Lucene dimensional points • Trade speed for accuracy depending on depth of tree search, and how many vectors are re-ranked • Tree nodes follow a Zipfian distribution
- 32. Lucene Implementation Details • Cluster Field – stores cluster tokens • Turn off all norms, tf and idf weighting, custom hamming similarity class • Vector Field – stores vectors for re-ranking • Stores components plus payloads, custom similarity class using payloads • Similarity classes: https://github.com/DiceTechJobs/SolrPlugins
- 33. Lucene Implementation Details Vector field analysis chain: Cluster fields:
- 34. Other Heuristic Methods • Randomized KD Forest • Constructs a number of KD trees choosing axis to split on randomly • Searches all trees in parallel to a fixed number of leaf nodes • KD Trees are very deep • How to implement efficiently in an inverted index? • Hierarchical Navigable Small World Graphs • Hierarchical graph based model - https://arxiv.org/pdf/1603.09320.pdf • Consistently out-performs other ANNs methods on the ANNs benchmarks page - http://ann-benchmarks.com/
- 35. Distribution of Vector Components • Distribution of components from our vectors is Gaussian • Mean is 0 • This means that most vector components are very small • These components will have minimal impact on cosine score Histogram of components taken from 350k vectors Mean = 0.0
- 36. Vector Thresholding with Tokenization [+0.08, -0.16, -0.12, +0.27, +0.63, -0.01, +0.16, -0.48] [ 0, 0, 0, 0, +0.63, 0, 0, -0.48] • Drop all but the largest components [“04i+0.6”, “07i-0.5”] • Round weight to lower precision • Encode position and weight as a single token • Paper: “Semantic Vector Encoding and Similarity Search Using Fulltext Search Engines”
- 37. Vector Thresholding with Payloads [+0.08, -0.16, -0.12, +0.27, +0.63, -0.01, +0.16, -0.48] [ 0, 0, 0, 0, +0.63, 0, 0, -0.48] • Drop all but the largest components • I modified the previous idea, using payload score queries • Indexing: Store remaining (non zero) tokens in index with payloads • Querying: Uses custom payload query parser + similarity class • See Github repo, and solr config in Kmeans tree section Q=vector:(”3”^-0.0136 ”14”^0.05387 ”56”^-0.070476 ”71”^0.14529 …) &defType=payloadEdismax
- 38. Performance Comparison - Initial Results • Hardware - Mac Book Pro, 2.6Ghz i7 CPU, 16G Ram, SSD • Search Engine: • Solr 7.5, single shard • Index: 700k documents • 1000 sample vector queries, requests were single threaded • Metric – precision @10 compared to brute force • Updated results – check https://github.com/DiceTechJobs/VectorsInSearch
- 39. Performance Comparison - Initial Results • Each algorithm was ran over a range of different parameter values, to show recall – speed trade off
- 40. Performance Comparison - Initial Results Algorithm Precision@10 Queries Per Sec (Mean Qry Time) LSH (Hamming Similarity) 0.69 1.3 qps (757 ms) Kmeans Tree (trained on index) 0.88 9.2 qps (170 ms) Kmean Tree (trained on sample) 0.85 9.5 qps (105 ms) Vector Thresholding with Tokenization (top 40% of components) 0.85 3.5 qps (312 ms) Vector Threshold with Payloads (top 40% of components) 0.94 1.8 qps (547 ms)
- 41. The Ultimate Solution - Sparse Coding? • Also called ‘Dictionary Learning’ • Learns a sparse ‘overcomplete’ representation of a vector • Example Algorithms: • Sparse Auto-Encoder • K-SVD • Encoding needs to preserve the Metric Space • Similar items need to remain similar after encoding Other Relevant Approaches • Word2bits - learns binary quantized word vectors • https://github.com/agnusmaximus/Word2Bits
- 42. Block Max WAND • https://www.elastic.co/blog/faster-retrieval-of-top-hits-in-elasticsearch-with- block-max-wand • ‘Weak AND’ algorithm to be integrated into Lucene 8.0 and ES 7.0 • Speeds up large OR queries by pruning clauses that won’t occur in top N matches • Speed up can be 40% to 13x • Can help address performance of these larger OR queries
- 43. Thank you! Github Repository: https://github.com/DiceTechJobs/VectorsInSearch Simon Hughes Chief Data Scientist, Dice.com @hughes_meister

- Metrics – recall often used for measuring synonymy and related problems, while precision and traditional IR metrics are better at measuring the efficacy at disambiguating a user’s intent
- Context – bag of words Global - learn semantic representations of terms Address synonymy (word level) Learn colocations (phrases) Local – can be used to disambiguate ambiguous terms Address polysemy
- By Aelu013 [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)], from Wikimedia Commons. For LSA illustration, and an excellent explanation, see here - http://iv.slis.indiana.edu/sw/lsa.html
- Word vectors - don’t learn true synonyms – don’t truly solve synonymy problem, and don’t handle polsemy as the same vector is used for a word regardless of it’s context. Deep LM’s capture the meaning of a a sequence of words in context – not just individual words in isolation. Context – bag of words Global - learn semantic representations of terms Address synonymy (word level) Learn colocations (phrases) Local – can be used to disambiguate ambiguous terms Address polysemy
- Word vectors - don’t learn true synonyms – don’t truly solve synonymy problem, and don’t handle polysemy as the same vector is used for a word regardless of it’s context. Deep LM’s capture the meaning of a a sequence of words in context – not just individual words in isolation. Context – bag of words Global - learn semantic representations of terms Address synonymy (word level) Learn colocations (phrases) Local – can be used to disambiguate ambiguous terms Address polysemy
- How do we represent dense vectors in a form that works inside an inverted index? Dense
- Note – important to do colocation (phrase detection) before building an embedding model. Embeddings work better when phrases are passed as single tokens.
- Excellent explanation of the simHash- Dan Durme and Lall presentation, slide 15