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How we constructed close to real-time “X for you” recommender programs at Bol

On this weblog put up, we current a high-level description of the methodology underpinning these feeds, which we’ve documented in additional element in a paper obtainable on ArXiv.


Given historic and up to date prospects’ interactions, what are essentially the most related objects to show on the house web page of each buyer from a given set of things resembling promotional objects or newly launched objects? To reply this query at scale, there are 4 challenges that we wanted to beat:

  • Buyer illustration problem – Bol has greater than 13 million prospects with various pursuits and interplay habits. How can we develop buyer profiles?
  • Merchandise illustration problem – Bol has greater than 40 million objects on the market, every having its personal wealthy metadata and interplay knowledge. How can we characterize objects?
  • Matching problem – how can we effectively and successfully match interplay knowledge of 13 million prospects with doubtlessly 40 million objects?
  • Rating problem – In what order can we present the highest N objects per buyer from a given set of related merchandise candidates?

On this weblog, we concentrate on addressing the primary three challenges.


To handle the three of the 4 challenges talked about above, we use embeddings. Embeddings are floating level numbers of a sure dimension (e.g. 128). They’re additionally known as representations or (semantic) vectors. Embeddings have semantics. They’re educated in order that related objects have related embeddings, whereas dissimilar objects are educated to have totally different embeddings. Objects might be any kind of knowledge together with textual content, picture, audio, and video. In our case, the objects are merchandise and prospects. As soon as embeddings can be found, they’re used for a number of functions resembling environment friendly similarity matching, clustering, or serving as enter options in machine studying fashions. In our case, we use them for environment friendly similarity matching. See Determine 1 for examples of merchandise embeddings.

Determine 1: Objects in a catalog are represented with embeddings, that are floating numbers of a sure dimension (e.g. 128). Embeddings are educated to be related when objects have widespread traits or serve related capabilities, whereas people who differ are educated to have dissimilar embeddings. Embeddings are generally used for similarity matching. Any kind of knowledge will be embedded. Textual content (language knowledge), tabular knowledge, picture, and audio can all be embedded both individually or collectively.

The widespread strategy to utilizing embeddings for personalization is to depend on a user-item framework (see Determine 2). Within the user-item framework, customers and objects are represented with embeddings in a shared embedding house. Customers have embeddings that replicate their pursuits, derived from their historic searches, clicks and purchases, whereas objects have embeddings that seize the interactions on them and the metadata data obtainable within the catalog. Personalization within the user-item framework works by matching consumer embeddings with the index of merchandise embeddings.

Determine 2: Consumer-to-item framework: Single vectors from the consumer encoder restrict illustration and interpretability as a result of customers have various and altering pursuits. Conserving consumer embeddings contemporary (i.e.capturing most up-to-date pursuits) calls for high-maintenance infrastructure due to the necessity to run the embedding mannequin with most up-to-date interplay knowledge.

We began with the user-item framework and realized that summarizing customers with single vectors has two points:

  1. Single vector illustration bottleneck. Utilizing a single vector to characterize prospects introduces challenges because of the variety and complexity of consumer pursuits, compromising each the capability to precisely characterize customers and the interpretability of the illustration by obscuring which pursuits are represented and which aren’t.
  2. Excessive infrastructure and upkeep prices. Producing and sustaining up-to-date consumer embeddings requires substantial funding by way of infrastructure and upkeep. Every new consumer motion requires executing the consumer encoder to generate contemporary embeddings and the following suggestions. Moreover, the consumer encoder should be massive to successfully mannequin a sequence of interactions, resulting in costly coaching and inference necessities. 

To beat the 2 points, we moved from a user-to-item framework to utilizing an item-to-item framework (additionally known as query-to-item or query-to-target framework). See Determine 3. Within the item-to-item framework, we characterize customers with a set of question objects. In our case, question objects consult with objects that prospects have both seen or bought. Usually, they may additionally embody search queries.

Determine 3: Question-to-item framework: Question embeddings and their similarities are precomputed. Customers are represented by a dynamic set of queries that may be up to date as wanted.

Representing customers with a set of question objects offers three benefits:

  • Simplification of real-time deployment: Buyer question units can dynamically be up to date as interactions occur. And this may be carried out with out working any mannequin in real-time. That is doable as a result of all objects within the catalog are recognized to be potential view or purchase queries, permitting for the pre-computation of outcomes for all queries.
  • Enhanced interpretability: Any customized merchandise advice will be traced again to an merchandise that’s both seen or bought. 
  • Elevated computational effectivity: The queries which can be used to characterize customers are shared amongst customers. This permits computational effectivity because the question embeddings and their respective similarities will be re-used as soon as computed for any buyer.

Pfeed – A way for producing customized feed

Our methodology for creating customized feed suggestions, which we name Pfeed, includes 4 steps (See Figures 4).

Determine 4: The most important steps concerned in producing close to real-time customized suggestions

Step 1 is about coaching a transformer encoder mannequin to seize the item-to-item relationships proven in Determine 5. Right here, our innovation is that we use three particular tokens to seize the distinct roles that objects play in numerous contexts: view question, purchase question and, goal merchandise.

View queries are objects clicked throughout a session resulting in the acquisition of particular objects, thus creating view-buy relationships. Purchase queries, then again, are objects often bought along with or shortly earlier than different objects, establishing buy-buy relationships.

We consult with the objects that observe view or purchase queries as goal objects. A transformer mannequin is educated to seize the three roles of an merchandise utilizing three distinct embeddings. As a result of our mannequin generates the three embeddings of an merchandise in a single shot, we name it a SIMO mannequin (Single Enter Multi Output Mannequin). See paper for extra particulars relating to the structure and the coaching technique.

Determine 5: Product relationships: most prospects that purchase P_2 additionally purchase P_4, ensuing right into a buy-buy relationship. Most prospects that view product P_2 find yourself shopping for P_5, ensuing right into a view-buy relationship. On this instance, P_2 performs three varieties of roles – view question, purchase question ,and goal merchandise. The intention of coaching an encoder mannequin is to seize these present item-to-item relationships after which generalize this understanding to incorporate new potential connections between objects, thereby increasing the graph with believable new item-to-item relationships.

Step 2 is about utilizing the transformer encoder educated in step 1 and producing embeddings for all objects within the catalog.

Step 3 is about indexing the objects that have to be matched (e.g. objects with promotional labels or objects which can be new releases). The objects which can be listed are then matched towards all potential queries (seen or bought objects). The outcomes of the search are then saved in a lookup desk.

Step 4 is about producing customized feeds per buyer based mostly on buyer interactions and the lookup desk from step 3. The method for producing a ranked checklist of things per consumer contains: 1) choosing queries for every buyer (as much as 100), 2) retrieving as much as 10 potential subsequent items- to-buy for every question, and three) combining these things and making use of rating, variety, and enterprise standards (See Determine 4d). This course of is executed every day for all prospects and each two minutes for these lively within the final two minutes. Suggestions ensuing from latest queries are prioritized over these from historic ones. All these steps are orchestrated with Airflow.

Purposes of Pfeed

We utilized Pfeed to generate numerous customized feeds at Bol, viewable on the app or web site with titles like Prime offers for you, Prime picks for you, and New for you. The feeds differ on at the least one in all two elements: the precise objects focused for personalization and/or the queries chosen to characterize buyer pursuits. There may be additionally one other feed known as Choose Offers for you. On this feed, objects with Choose Offers are customized completely for Choose members, prospects who pay annual charges for sure advantages. Yow will discover Choose Offers for you on empty baskets. 

Usually, Pfeed is designed to generate”X for you” feed by limiting the search index or the search output to include solely objects belonging to class 𝑋 for all potential queries.


We carry out two varieties of analysis – offline and on-line. The offline analysis is used for fast validation of the effectivity and high quality of embeddings. The web analysis is used to evaluate the influence of the embeddings in personalizing prospects’ homepage experiences.

Offline analysis

We use about two million matching query-target pairs and about a million random objects for coaching, validation and testing within the proportion of 80%, 10%, %10. We randomly choose one million merchandise from the catalog, forming a distractor set, which is then combined with the true targets within the take a look at dataset. The target of analysis is to find out, for recognized matching query-target pairs, the proportion of instances the true targets are among the many high 10 retrieved objects for his or her respective queries inthe embedding house utilizing dot product (Recall@10). The upper the rating, the higher. Desk 1 exhibits that two embedding fashions, known as SIMO-128 and SISO-128, obtain comparable Recall@10 scores. The SIMO-128 mannequin generates three 128 dimensional embeddings in a single shot, whereas the SISO-128 generates the identical three 128-dimensional embeddings however in three separate runs. The effectivity benefit of SIMO-128 implies that we will generate embeddings for your complete catalog a lot quicker with out sacrificing embedding high quality.

Desk 1: Recall@Ok on view-buy and buy-buy datasets. The SIMO-128 mannequin performs comparably to the SISO-128 mannequin whereas being 3 instances extra environment friendly throughout inference.

The efficiency scores in Desk 1 are computed from an encoder mannequin that generates 128-dimensional embeddings. What occurs if we use bigger dimensions? Desk 2 offers the reply to that query. After we improve the dimensionality of embeddings with out altering another facet, bigger dimensional vectors have a tendency to provide increased high quality embeddings, as much as a sure restrict.

Desk 2: Affect of hidden dimension vector dimension on Recall@Ok. Conserving different components of the mannequin the identical and growing solely the hidden dimension results in elevated efficiency till a sure restrict.

One difficult facet in Pfeed is dealing with query-item pairs with advanced relations (1-to-many, many-to-one, and many-to-many). An instance is a diaper buy.

There are fairly a couple of objects which can be equally more likely to be bought together with or shortly earlier than/after the acquisition of diaper objects resembling child garments and toys.

Such advanced query-item relations are more durable to seize with embeddings. Desk 3 exhibits Recall@10 scores for various ranges of relationship complexity. Efficiency on query-to-item with advanced relations is decrease than these with easy relations (1-to-1 relation). 

Desk 3: Retrieval efficiency is increased on take a look at knowledge with easy 1 x 1 relations than with advanced relations (1 x n, m x 1 and m x n relations).

On-line experiment

We ran a web based experiment to judge the enterprise influence of Pfeed. We in contrast a therapy group receiving customized Prime offers for you merchandise lists (generated by Pfeed) towards a management group that acquired a non-personalized Prime offers checklist, curated by promotion specialists.

This experiment was performed over a two-week interval with a good 50- 50 break up between the 2 teams. Customized high offers suggestions result in a 27% improve in engagement (want checklist additions) and a 4.9% uplift in conversion in comparison with expert-curated non-personalized high offers suggestions (See Desk 4).

Desk 4: Customized high offers suggestions result in a 27% improve in engagement (want checklist additions) and a 4.9% uplift in conversion in comparison with expert-curated non-personalized high offers suggestions.

Conclusions and future work

We launched Pfeed, a way deployed at Bol for producing customized product feeds: Prime offers for you, Prime picks for you, New for you, and Choose offers for you. Pfeed makes use of a query-to-item framework, which differs from the dominant user-item framework in customized recommender programs. We highlighted three advantages: 1) Simplified real-time deployment. 2) Improved interpretability. 3) Enhanced computational effectivity.

Future work on Pfeed will concentrate on increasing the mannequin embedding capabilities to deal with advanced query-to-item relations resembling that of diaper objects being co-purchased with various different child objects. Second line of future work can concentrate on dealing with express modelling of generalization and memorization of relations, adaptively selecting both strategy based mostly on frequency. Steadily occurring query-to-item pairs might be memorized and people who contain tail objects (low frequency or newly launched objects) might be modelled based mostly on content material options resembling title and descriptions. At present, Pfeed solely makes use of content material for modelling each head and tail objects.

If this kind of work evokes you or you’re in search of new challenges, take into account checking for obtainable alternatives on bol’s careers web site.


We thank Nick Tinnemeier and Eryk Lewinson for suggestions on this put up.



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