# News Recommendation Tutorial - parent child and tensor ranking

## Introduction

This is the seventh part of the tutorial series for setting up a Vespa application for personalized news recommendations. The parts are:

1. Getting started
2. A basic news search application - application packages, feeding, query
3. News search - sorting, grouping, and ranking
4. Generating embeddings for users and news articles
5. News recommendation - partial updates (news embeddings), ANNs, filtering
6. News recommendation with searchers - custom searchers, doc processors
7. News recommendation with parent-child - parent-child, tensor ranking
8. Advanced news recommendation - intermission - training a ranking model
9. Advanced news recommendation - ML models

In this part of the series, we’ll introduce a new ranking signal: category click-through rate (CTR). The idea is that we can recommend popular content for users that don’t have a click history yet. Rather than just recommending based on articles, we recommend based on categories. However, these global CTR values can often change continuously, so we need an efficient way to update this value for all documents. We’ll do that by introducing parent-child relationships between documents in Vespa. We will also use sparse tensors directly in ranking.

For reference, the final state of this tutorial can be found in the app-7-parent-child sub-directory of the news sample application.

## Parent-child relationships in Vespa

Recall that most features come from either attributes in the document or parameters passed with the query when ranking a document. Parent-child relationships introduce the option of using attributes found in other documents. Parent-child relationships work as a form of scalable document joins.

For instance, assume we have a global CTR value for the sports category of 0.2. If we want to use this value during ranking, we could have a field in each news article holding this value. However, when we need to update this value, we need to issue a partial update to all documents, which seems wasteful.

Another way would be to take inspiration from our UserProfileSearcher, where we retrieved the tensor embedding for a user in a search before passing that with the news article query. We could have a single document holding all global values and retrieve that with each query. However, that isn’t particularly efficient.

For these cases, Vespa introduced the parent-child relationship. Parents are global documents, which are automatically distributed to all content nodes. Other documents can reference these parents and “import” values for use in ranking. The benefit is that the global category CTR values only need to be written to one place: the global document.

## Setting up a global category CTR document

So, let’s set this up for our application. First we need to add a new document type to hold the CTR values. We introduce the category_ctr document type, which we add in schemas/category_ctr.sd:

schema category_ctr {
document category_ctr {
field ctrs type tensor<float>(category{}) {
indexing: attribute
attribute: fast-search
}
}
}


This document holds a single field: a tensor of type tensor<float>(category{}). This is a tensor with a single sparse dimension, which is slightly different from the tensors we have seen so far. Sparse tensors have strings as dimension addresses rather than a numeric index. More concretely, an example of such a tensor is (using the tensor literal form):

{
{category: entertainment}: 0.2 },
{category: news}: 0.3 },
{category: sports}: 0.5 },
{category: travel}: 0.4 },
{category: finance}: 0.1 },
...
}


This tensor holds all the CTR scores for all the categories. When updating this tensor, we can update individual cells if we don’t need to update the whole tensor. This is called tensor modify and can be helpful when you have large tensors.

To use this document, we need to add it to services.xml:

  <content id='mind' version='1.0'>
...
<documents>
<document type='news' mode="index"/>
<document type='user' mode="index"/>
<document type='category_ctr' mode="index" global="true"/>
</documents>
...
</content>


Notice that we’ve set global="true", which instructs Vespa to keep a copy of these documents on all content nodes. This is required for using it in a parent-child relationship.

## Importing parent values in child documents

To use the category_ctr tensor when ranking news documents, we need to “import” the tensor. There are two things we need to set up: the reference to the parent document, and which fields to import. We modify our schemas/news.sd:

schema news {
document news {
...
field category_ctr_ref type reference<category_ctr> {
indexing: attribute
}
...
}
import field category_ctr_ref.ctrs as global_category_ctrs {}
}


The field category_ctr_ref is a field of type reference of a category_ctr document type. When feeding this field, Vespa expects the fully qualified document id. For instance, if our global CTR document has the id id:category_ctr:category_ctr::global, that is what this field must be set to. Usually, there are many parent documents that children can reference, but our application will only hold one.

You can think of the reference field as holding a foreign key to the parent document, and the import as performing a real-time join between the child and parent document using this foreign key. The imported values are useable as if they were stored with the child.

The import statement defines that we should import the ctrs field from the document referenced in the category_ctr_ref field. We name this as global_category_ctrs, and we can reference this as attribute(global_category_ctrs) during ranking.

## Tensor expressions in ranking

Up until this point, we’ve only used tensors as storage. We used tensors to hold news and user embeddings, and Vespa used these tensors to calculate the euclidean distances in our nearest-neighbor search.

However, Vespa has a rich language to perform calculations with tensors. We’ll exploit that by looking up the news article’s category in the global CTR tensor and using that as a feature in ranking.

Our news document has a field currently that holds the category as a string. Unfortunately, tensor expressions only work on tensors, so we need to add a new field to hold the category tensor:

    field category_tensor type tensor<float>(category{}) {
indexing: attribute
}


Using a tensor in this way also enables a document to have multiple categories, but our dataset only has a single category per article. For instance, we can represent the finance category of a news article like this:

    {category: finance}: 1.0 }


Since this is a sparse tensor, we don’t need to mention the other categories. Now, we can use this tensor to calculate the global CTR score for an article’s category:

    attribute(category_tensor) * attribute(global_category_ctrs)


Given the global category CTR example above, this would result in the value 0.1. How did we arrive at this? Recall that the value for the cell finance in the category dimension of the example above had a value of 0.1. The multiplication of these two tensors is conceptually an “inner join”, so you can take the matching cells and multiply them together. Due to the sparseness of the tensor, only the finance cell matches, and that value is multiplied by the 1.0 in this document. So in this case, this would effectively work as a lookup.

Let’s add a new ranking profile to perform this calculation:

rank-profile recommendation_with_global_category_ctr inherits recommendation {
function category_ctr() {
expression: sum(attribute(category_tensor) * attribute(global_category_ctrs))
}
function nearest_neighbor() {
expression: closeness(field, embedding)
}
first-phase {
expression: nearest_neighbor * category_ctr
}
summary-features {
attribute(category_tensor)
attribute(global_category_ctrs)
category_ctr
nearest_neighbor
}
}


Here, we’ve added a first phase ranking expression that multiplies the nearest-neighbor score with the category CTR score, implemented with the functions nearest_neighbor and category_ctr, respectively. We’ve added a sum function around the category_ctr expression - this is simply to unbox the single-value tensor to a double value suitable for use in the first phase expression.

Note that, as a first attempt, we just multiply the nearest-neighbor with the category CTR score. This is not necessarily the correct way to combine these values, but we’ll get back to that in a bit.

We’ve added a section for summary features. This is simply a list of features that will be returned with the hit when using this rank profile. Recall that we can specify which features should be returned in the summary with the indexing: summary statement with each field. The summary-features can also include the result of functions as well. This is a helpful debugging tool, and we’ll see how this looks after feeding some data.

## Feeding parent and child updates

After deploying the application, we are ready to feed a global CTR document. For convenience, we’ve added a script that reads the MIND content and impression data to calculate CTR scores for each category:

\$ ./src/python/create_category_ctrs.py mind


This produces two files in the mind directory:

1. mind/global_category_ctr.json - a feed file for the global CTR document containing CTR score for each category.
2. mind/news_category_ctr_update.json - a feed file for partially updating the news articles with the reference to the global CTR document as well as the category tensor.

These files can now be fed to Vespa, but note that the mind/global_category_ctr.json need to be fed first because the global document needs to exist before the child documents can reference it.

## Testing the application

After feeding the above files, we can now test the application with a query:

curl -s "http://localhost:8080/search/?user_id=U33527&ranking.profile=recommendation_with_global_category_ctr" | \
python -m json.tool


Note that we specify the ranking profile to use. The first result of this query is something like the following:

...
"fields": {
"title": "Matthew Stafford's status vs. Bears uncertain, Sam Martin will play",
"abstract": "abstract": "Stafford's start streak could be in jeopardy, according to Ian Rapoport.",
"category": "sports",
...
"summary-features": {
"attribute(category_tensor)": {"cells": [{"address": { "category": "sports"}, "value":1.0 }]},
"attribute(global_category_ctrs)": {"cells": [
...
{ "address": { "category": "sports" }, "value": 0.05611187964677811 },
...
]},
"rankingExpression(category_ctr)": 0.05611187964677811,
"rankingExpression(nearest_neighbor)": 0.14914761220236453,
}
...
"relevance": 0.008368952865503413,
}


This is clearly a sports article. The global CTR document is also listed here, and the CTR score for the sports category is 0.0561. Thus, the result of the category_ctr function is 0.0561 as intended. The nearest_neighbor score is 0.149, and the resulting relevance score is 0.00836. So, this worked as expected.

If we were to feed another value to the global CTR document, this updated value is immediately available. As such, the system responds quickly to changes in the global parameters.

Now, a simple multiplication between these features might not give us what we want. For instance, these features have different average values different standard deviations. Particularly, if we add multiple additional features, just multiplying them together will probably not give a great user experience.

To remedy this, we will try to learn a suitable weighting between features. We’ll address this in the next part of this series.

## Conclusion

This tutorial introduced parent-child relationships and demonstrated it through a global CTR feature we used in ranking. As this feature was based on tensors, we also introduced ranking with tensor expressions. In the next part of the tutorial, we will start using a machine-learned model in ranking.