The impact of API evolution on API consumers and how this can be affected by API producers and language designers

The practice of software engineering involves the combination of existing software components with new functionality to create new software. This is where an Application Programming Interface (API) comes in, an API is a definition of a set of functionality that can be reused by a developer to incorporate certain functionality in their codebase. Using an API can be challenging. For example, adopting a new API and correctly using the functionality can be challenging. One of the biggest issues with using an API, is that the API can evolve, with new features being added or existing features being modified or removed. Dealing with this challenge has led to an entire line of research on API evolution.In this thesis, we seek to understand to what extent API evolution more specifically API deprecation affects API consumers and how API consumers deal with the changing API. API producers can impact consumer behavior by adopting specific deprecation policies, to uncover the nature of this relationship, we investigate how and why the API producer deprecates the API and how this impacts the consumer. Deprecation is a language feature, i.e. one that language designers implement. Its implementation can vary across languages and thus the information that is conveyed by the deprecation mechanism can vary as well. The specific design decisions taken by the language designers can have a direct impact on consumer behavior when it comes to dealing with deprecation. We investigate the language designer perspective on deprecation and the impact of the design of a deprecation mechanism on the consumer. In this thesis, we investigate the relationship between API consumers, API producers, and language designers to understand how each has a role to play in reducing the burden of dealing with API evolution.Our findings show that out of the projects that are affected by deprecation of API elements, only a minority react to the deprecation of an API element. Furthermore, out of this minority, an even smaller proportion reacts by replacing the deprecated element with the recommended replacement. A larger proportion of the projects prefer to rollback the version of the API that they use so that they are not affected by deprecation, another faction of projects is more willing to replace the API with the deprecated element with another API. API producers have a direct impact on this behavior with the deprecation policy of the API having a direct impact on the consumer's decision to react to deprecation. If the API producer is more likely to clean up their code i.e. remove the deprecated element, then the consumers are likely to react to the deprecation of the element. This shows us that even for non-web-based APIs, the API producers can impact consumer behavior. We also, observe that the nature and content of the deprecation message can have an impact on consumer behavior. Consumers prefer to know when a deprecated feature is going to go away, what its replacement is and the reason behind the deprecation (informing them of the immediacy of reacting to the deprecation). The design of the deprecation mechanism needs to reflect these needs as the deprecation mechanism is the only direct way in which API producers can communicate with the consumer.Software Engineerin