Best Practices for Accepting Lists and Complex Filters in REST API Parameters

Introduction

In developing a RESTful API, one common challenge is designing endpoints that accept lists of parameter values or complex filtering criteria. This often arises when you need to allow users to query multiple resources at once or apply filters across several attributes. Ensuring these inputs are structured efficiently and intuitively can significantly impact the usability and maintainability of your API.

Understanding RESTful Principles

Before diving into specific practices, it’s essential to revisit some core principles of REST architecture:

Resources: Everything in a RESTful system is treated as a resource, uniquely identified by its URI.
Uniform Interface: HTTP methods (GET, POST, PUT, DELETE) and URIs should work uniformly across all resources.
Stateless Interactions: Each client-server interaction must contain all the information needed to understand the request.

Designing for Simplicity

Handling Lists of IDs

When dealing with lists of resource identifiers, simplicity is key:

Standard Approach: Use repeated parameters. For example:
```
http://our.api.com/Product?id=101404&id=7267261
```
This approach leverages the standard way HTTP handles URL parameters and is widely understood by developers.
Delimited Values: Another method involves using a delimiter to separate values, such as commas:
```
http://our.api.com/Product?id=101404,7267261
```
While this can be more concise, it may require additional parsing logic on the server side and might not align with typical expectations for HTTP parameter handling.

Complex Filters

Complex filtering often involves multiple criteria across various fields:

Hierarchical Resource Structure: Instead of using complex query strings that mix filters, consider a hierarchical URI structure:
```
http://our.api.com/Product?term=pumas&productType=Clothing&color=Black
```
This approach maintains clarity and aligns with REST’s resource-based architecture.
Using HATEOAS: Hypermedia As The Engine Of Application State (HATEOAS) can guide clients through available operations. For example:
- First, a client could retrieve all possible product types for "pumas" and then select specific ones like Clothing and Bags.
- This approach makes the API self-descriptive, allowing users to navigate resources dynamically.

Advanced Techniques

URI Templates (RFC 6570)

For more dynamic URL construction, consider using URI templates as outlined in RFC 6570. These templates allow for variable substitution within URIs, enabling flexible yet standardized query parameter handling:

Example:
```
http://our.api.com/Product/{id}
```
This template could be expanded to handle multiple IDs by substituting {id} with each identifier.

Two-Step Querying

For complex queries, consider a two-step process:

Create a Search Criteria: Use a POST request to submit search criteria and receive an ID.
```
POST /SearchCriteria
```
Execute the Search: Use a GET request with the received ID to execute the search.
```
GET /Search?criteriaId=12345
```

This method allows for more complex logic without overloading URL parameters and maintains statelessness by encapsulating query details in server-managed resources.

Conclusion

Designing RESTful APIs that accept lists of parameter values or complex filters requires balancing simplicity, usability, and adherence to REST principles. By using repeated parameters, hierarchical URIs, HATEOAS, and URI templates, you can create a robust API that is both user-friendly and maintainable. Always prioritize clarity and consistency in your design choices to ensure your API remains intuitive for developers.