API Beauty is in the eye of the Clients: 2.2 Million Maven Dependencies reveal the Spectrum of Client-API Usages

TL;DR

This large-scale empirical study of Maven Central reveals a broad spectrum of API usages, showing most API types are used at least once, but a small core of features is used by the majority of clients, informing API design and maintenance.

Contribution

The paper provides the first large-scale empirical analysis of client-API relationships in Maven Central, highlighting usage patterns and practical implications for API development.

Findings

Most API types are used at least once by clients.

A small set of API features is used by the majority of clients.

API maintainers can focus on essential API parts for better management.

Abstract

Hyrum's law states a common observation in the software industry: "With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody". Meanwhile, recent research results seem to contradict this observation when they state that "for most APIs, there is a small number of features that are actually used". We investigate this seeming paradox between the observations in industry and the research literature, with a large scale empirical study of client API relationships in one single ecosystem: Maven Central. We study the 94 most popular libraries in Maven Central, as well as the 829,410 client artifacts that declare a dependency to these libraries and that are available in Maven Central, summing up to 2.2M dependencies. Our analysis indicates the existence of a wide spectrum of API usages, with enough clients, most API types end up being used at least once. Our second key observation is that, for all libraries, there is a small set of API types that are used by the vast majority of its clients. The practical consequences of this study are two-fold: (i) it is possible for API maintainers to find an essential part of their API on which they can focus their efforts; (ii) API developers should limit the public API elements to the set of features for which they are ready to have users.