# Models hot swap tutorial

This tutorial builds on the news recommendation tutorial. It describes ways to manage a real-world Vespa application, which is updated on a regular basis while users are performing searches (hot swap). It also extends the application to support multiple recommendation models while minimizing data duplication. Lastly, it demonstrates an efficient garbage collection mechanism, removing obsolete data from the application.

In the news recommendation tutorial, we created a Vespa application that recommends news articles to a given user. Recommendations are based on a model consisting of two sets of embeddings - user and news. To obtain the recommendation, we first obtain the given user’s embeddings, and then search for news whose embeddings are closest to the user’s. The closest news articles are the most relevant ones for that particular user.

This series started with an introduction to basic Vespa capabilities, proceeded to describe model-based recommendations, and moved to more advanced models relying on neural networks. This tutorial is unrelated to the model generation and ranking specifications. Hence, for simplicity, we assume that embeddings are available along with some ranking function.

## Real-world requirements

Moving from a demo app to a real-world one introduces additional requirements:

1. Hot swap - models should be updated on a regular basis, as new users and news articles enter the system, and more events (users viewing and clicking news) are collected. Model updates must be atomic from the users’ perspective, namely a ranking procedure must not consider embeddings from different models at the same time. Further, there must be no downtime for model updates.
2. Multiple models - our initial model recommends news articles to users based on other users’ interactions with those articles (as well as categories and content). This is the basic model. Now, let us consider two other (somewhat contrived) models: recent, which only considers events from the last day, and fully-read, which only considers articles in which the user has scrolled all the way down. While all 3 models contain the same users and articles, and all rely on the same model generation procedure, each model is composed of a completely different set of embeddings. Further, we should be able to use all models simultaneously for different queries and feeds.
3. Shared model-agnostic data - news articles carry data and metadata that can be used for filtering - title, author, category, language, and so on. Models, on the other hand, are created based on events reporting access to the document from specific users. We thus distinguish between document data, which depends only on the article and is model agnostic, and event-based model data. Although the document data is needed for filtering (regardless of the model used), there is no need to replicate this data across the per-model documents. Further, while the models rely on user events coming from some tracking system, the model-agnostic fields may come from some database. Decoupling operations relying on each data source can eliminate coordination issues.
4. Cleanup - news articles might be deleted and users might opt out of the system. Our system should not rely on or recommend stale data, hence obsolete entities should be removed. Assuming news articles and users aren’t deleted from Vespa in real time, we need a way to remove the obsolete users and articles from the application.

Luckily, Vespa provides useful building blocks to handle these real-world requirements. While the requirements might seem mostly independent, we shall see how certain Vespa features can be reused in order to address multiple requirements.

## Hot swap

Every new feeding uploads a new model to Vespa. The central requirement for hot swap is atomicity: an ongoing query should always see embeddings belonging to a single model, regardless of whether new documents are being fed to the app. Yet each model consists of multiple records. Since Vespa doesn’t support transactional feeding, it is up to the application to implement the model swap as an atomic transaction. The weapon of choice is versioning, which consists of two parts, a set bit externally defined as part of the document ID, and a version stored internally within the document:

1. External versioning - a set number is appended to each document ID. This separates documents belonging to the current model from the documents belonging to the next (or previous) model, allowing them to coexist without overwriting each other. Sets alternate between 0 and 1. This way, when a new model is being fed, it does not overwrite documents pertaining to the current model but does overwrite documents pertaining to the previous one, which is already obsolete.
2. Internal versioning - a version number is stored inside each document belonging to the model. This model number is increased in every feeding, allowing queries to only target the current model, filtering our documents belonging to the next or previous one.

In principle, to ensure atomic swap, we could have used (full) external versions (instead of just sets) and avoided internal versions. But this would create an abundance of obsolete versions for each document. By using a single set bit in the document ID, we ensure that no more than two versions of a document are stored at any given time. Using the internal version, we can filter out old versions that were not overwritten and hence still exist with a set bit that again became current.

Versioning can be stored in a database or some configuration file, but the simplest place is probably the Vespa app itself, using a dedicated type:

search config {
document config {

field set type int {
indexing: summary | attribute
}
field version type long {
indexing: summary | attribute
}
}
}


Versioning is used as depicted below. The client running recommendation queries starts by obtaining the configuration of the target model. It then adds a filter clause to the following query, limiting it to the current internal version. On the feeding side, the configuration is also read first. Documents are then generated such that they have the next internal version and the alternative set. Once all documents are fed successfully, the configuration document is updated with the new internal version and set. From that point onward, any new search will target the newly fed model.

## Supporting multiple models

As said, we’d like our app to support multiple recommendation models - basic, recent news and only fully read articles, for instance. To separate those models, who all share the same news article and user schemas, we can use a model code, stored in two places: (1) in the document ID, to avoid documents from different models from overriding each other, and (2) inside the document, to allow us to filter out documents from other models in our queries.

Since we’re already storing an internal version in each model, it sounds reasonable to combine the version with the model code. However, as we’ll soon see, we also use the internal version for garbage collection, so keeping the version and model apart simplifies other tasks.

The following fields will thus be added to the news and user schemas (defined in part 2):

schema news {
document news {

# ... fields described in part 2 ...

field model type string {
indexing: index | attribute
}
field version type long {
indexing: attribute
attribute: fast-search
}
}
}


Each model is generated using a different dataset, hence it makes sense to separate the feeding procedures of each model. Consequently, each model should also have a separate configuration holding (possibly) different inner versions and sets.

## Managing model-agnostic data using parent documents

In our example with three models, each document has 6 instances at any given moment - 3 different models * 2 sets. Most of the news article data is model agnostic, hence a single instance of that data can be used by all models. Further, the events used to create the prediction models (tuples like <user ID, news ID, date, fully_read>) come from a different source than the news article data, hence processing and feeding them separately is advantageous.

In order to have all document instances share the model-agnostic data, we use Vespa’s parent-child relations, also described in part 7 in the series. Namely, we extract model-related fields into a new schema called news_model, leave all other fields in news, and add a reference from the model docs to the shared docs. This reference allows us to import fields from the parent (shared) doc to the child (model) doc, making them searchable and retrievable as if they were part of the model doc. The news_model schema should look as follows:

schema news_model {
document news_model {

field embedding type tensor<float>(d0[51]) {
indexing: attribute
attribute {
distance-metric: euclidean
}
}
field model type string {
indexing: index | attribute
}
field version type long {
indexing: attribute
attribute: fast-search
}

field news_ref type reference<news> {
indexing: attribute
}
}
import field news_ref.title as title {}
import field news_ref.language as language {}
}


The shared news schema neither holds the model vector nor has a model field. However, it still has a version field, obtained from a dedicated news configuration. This version is used only for garbage collection of removed documents, as described below.

## Garbage collection

After adding all the machinery above, it’s time to make sure that we keep our app clean and tidy. Recall that we keep two sets per model, such that version 1 goes to set 1, version 2 goes to set 0, version 3 goes to set 1, and so on. Now, consider a news article that has been deleted by its author before version 3 was created. After feeding version 3, set 1 will include up-to-date articles with version 3, but will also still include the deleted article with version 1. While our queries filter by version hence ignore the deleted article, we still need to remove it from the app, otherwise we’ll gather a lot of obsolete data over time.

Vespa provides a built-in garbage collection mechanism, which relies on a filter that determines which documents should be included in the app, and consequently which documents should be removed from it. In our case, we detect up-to-date documents based on their internal version. Unfortunately, the filter expression is specified in the application settings, while the actual versions of interest change with each feeding. How can we filter out obsolete documents then?

To solve this problem, we leverage parent-child relations again. To this end, each document forms this relation with its model configuration document and imports the version field, renaming it to latest_version for instance. Each document then has two version fields: the one with which it was created, and the one imported from the configuration. While the former is fixed and will never change, the latter will change any time the configuration is updated (imported fields are references, they don’t hold the actual data). The relevant portions of the user and news schemas will now look as follows:

        field version type long {
indexing: attribute
attribute: fast-search
}

field config_ref type reference<config> {
indexing: attribute
}

import field config_ref.version as latest_version {}


Now, we can distinguish between up-to-date and obsolete documents by comparing the version and latest_version fields: if they are equal the document is part of the latest feed, otherwise it isn’t. Note that a difference may be briefly acceptable during a hot swap, where on-going queries should access documents from the set that became old. We can therefore allow some slack in our filter definition, defined in services.xml under the <content> tag:

    <documents garbage-collection="true" garbage-collection-interval="21600">
<document mode='index' type='config' global='true'/>
<document mode='index' type='news' global='true'
selection='(news.latest_version == null) or (news.version >= news.latest_version - 2)'/>
<document mode='index' type='news_model'
selection='(news_model.latest_version == null) or (news_model.version >= news_model.latest_version - 2)'/>
<document mode='index' type='user'
selection='(user.latest_version == null) or (user.version >= user.latest_version - 2)'/>
</documents>


Note that the filter not only compares the internal and the latest versions but also checks whether the latest version is null. This is required as the filter is also applied during feeding. At that point, the reference to the configuration document isn't active yet, hence all imported fields contain null values. Without the null check, all our documents would be filtered out during feeding.

## Dealing with feeding failures

Consider the following scenario: your automated feeding workflow starts by obtaining the target version for the next feeding from the configuration, then feeds the vespa app, but then some failure happens before the hot swap is completed. Since the configuration wasn’t updated, no query will consider the documents that were just fed. No harm done, right? Well, the next day, the feeding workflow starts again, with the next day’s model. During the day that passed, some users and articles were removed from the system, hence aren’t included in the new model. The feeding completes successfully this time, including the hot swap. The current model now has today’s model, along with the articles and users that were removed yesterday, all sharing the same version and all accessible to queries. This can be a problem.

To solve this problem, unsuccessful feeds must have distinct versions as well, separating them from following unsuccessful feeds and successful ones. This cannot be achieved using a single version, as several unsuccessful attempts might take place before one goes well, and during that time queries should keep using the latest successful version.

The solution is to separate read and write versions: at the beginning of every feed, the write version is incremented, and the result is stored back in the configuration and used in all the model documents. Once feeding completes successfully, the configuration is updated again, setting the read version to the write version. This way, any leftovers from unsuccessful feedings will have smaller versions, hence will be ignored by queries. Importantly, the active set should only be modified after a successful feed, otherwise we’ll break the hot swap by overwriting the currently active set.

The new configuration schema can look as follows, using the new next_version field to store the write version and the previously defined version field (imported for GC) for the read version:

search config {
document config {

field set type int {
indexing: summary | attribute
}
field version type long {
indexing: summary | attribute
}
field next_version type long {
indexing: summary | attribute
}
}
}


## Dealing with multi-region apps

A Vespa app can be deployed to multiple regions, ensuring availability in case one region cannot be reached. On the search side, a single global endpoint can be used, automatically redirecting calls to one of the regions. On the feeding side, however, each region must be fed separately. Now, consider one of the regions becomes temporarily unavailable and the daily feeding fails, while another region is successfully fed as usual. The discrepancy between the regions can lead to two issues:

1. Version mismatch - since each region had a different number of successful feedings, each will have a different current version. Given the global endpoint can go either way, a user that gets the version from a configuration in one region might be directed to a different region when performing the recommendation query. This will lead to using an obsolete model at best, and a non-existing one at worst.
2. Model mismatch - even if the versions somehow converge, the models themselves might differ when one region isn’t fed (possibly due to a feeding issue). Recall that recommendations are based on two steps: first, the user’s vector is obtained, and then a dot product with each news article is performed. If the user vector is obtained from one region, ranking is performed on another, and the models don’t match, the result might be meaningless.

The solution to this problem is ensuring all Vespa queries required for a recommendation - getting the versions, getting the user vector, and performing the ranking - are done on the same region. One way to achieve this is by implementing the sequence of searches in the application’s Searcher class. In this case, a single Vespa call will be required. If the sequence of searches is better implemented at the client side (e.g., in order to integrate some business logic), the client must use the per-region endpoint. While it’d be advised to use the global endpoint at least for the configuration query (considering load balancing, location, availability, etc.), the global endpoint provides no way to know to which particular region should the following queries be sent to. Luckily, we can achieve the desired “stickiness” by extending the configuration as follows:

1. Add a region endpoint field to the configuration schema.
2. In each region’s configuration document, store the region’s endpoint (each region has a separate configuration document, which can hold different values).
3. When retrieving the version using the global endpoint, also obtain the region endpoint.
4. Use the region endpoint in follow-up queries instead of the global endpoint.

## Putting it all together

The mechanisms described above introduce new types, fields, and relations to the application:

While these changes increase the complexity of the application, they enable features that are a must in any application that is highly available, updated often, lean, and efficient.