N+1 Problem in REST API

The N+1 problem is a common issue in data retrieval scenarios, especially when working with relational databases or REST APIs that interact with them. It occurs when a client makes a request to retrieve a list of resources (N), and for each resource, an additional query (1) is made to fetch related data. As a result, for N resources, N+1 queries are executed.

1. N+1 Problem

The N+1 problem is mostly talked about in the context of ORMs. In this kind of problem, the system needs to load N children of a one-parent entity where only the parent entity was requested. By default, ORMs are configured with lazy-loading disabled, so one query issued for the parent entity causes N more queries, i.e. one each for N child entities.

This N+1 problem is often considered a significant performance bottleneck, and so shall be solved at the design level of the application.

2. N+1 Problem in REST

Though mostly directly associated, the N+1 problem is not specific to ORMs only. This problem can be related to the context of web APIs as well, e.g. REST APIs.

In the case of web APIs, the N+1 problem is a situation where client applications are required to call the server N+1 times to fetch one collection resource + N child resources.

This is mostly because the collection resource did not provide enough information about the child resources to help the client application to build its user interface altogether.

For example, a REST API returns a collection of books as a resource.

<books uri="/books" size="100">
	<book uri="/books/1" id="1">
		<isbn>3434253561</isbn>
	</book>
	<book uri="/books/2" id="2">
		<isbn>3423423534</isbn>
	</book>
	<book uri="/books/3" id="3">
		<isbn>5352342344</isbn>
	</book>
	...
	...
</books>

Here /books resource return list of books with information including only it’s id and isbn. This information is not enough to build a client application UI, which will want to typically show the books name in UI rather than ISBN.

In some situations, the clients may want to show other information such as the author’s name and the publication year as well.

In the above scenario, the client application MUST make N more requests for each individual book resource at /books/{id}. So in total client will end up invoking REST APIs N+1 times.

HTTP GET /books/1
HTTP GET /books/2
HTTP GET /books/3
HTTP GET /books/4
...
...
N Times

The above scenario is only for example. The idea is that insufficient information in collection resources may lead to the N+1 problem in REST APIs.

3. How to Solve N+1 Problem

The good thing about the previously discussed problem is that we know what exactly is the issue. And this makes the solution pretty easy.

When designing APIs we can ensure that endpoints allow clients to specify the depth of related data they require. This prevents over-fetching and mitigates the N+1 problem.

To solve N+1 problem in REST, include enough information in single resources inside collection resources.

We may be required to consult with API consumers, do market research for similar applications and their user interfaces, or simply put ourselves in the client’s shoes.

Moreover, we may evolve our APIs over time as our understanding of client requirements improves. This is possible using API versioning.

4. Summary

The N+1 problem is a performance concern that can impact the efficiency and scalability of REST APIs. In the context of REST APIs, sometimes the resources have relationships, and fetching related sub-resources can trigger the N+1 problem if not managed properly.

By understanding its causes and the impacts it can have, developers can take proactive steps to design and optimize their APIs to prevent or mitigate the N+1 problem and build an efficient API design that leads to more responsive and resource-efficient APIs

Happy Learning !!