Apache Yetus Interface Taxonomy: Audience and Stability Classification


The interface taxonomy classification provided by Apache Yetus annotations is for guidance to developers and users of interfaces. The classification guides a developer to declare the targeted audience or users of an interface and also its stability.

Interface Classification

Yetus provides the following interface classification, derived from the OpenSolaris taxonomy and, to some extent, from taxonomy used inside Yahoo. Interfaces have two main attributes: Audience and Stability


Audience denotes the potential consumers of the interface. While many interfaces are internal/private to the implementation, others are public/external interfaces and are meant for wider consumption by applications and/or clients. For example, POSIX definitions in libc are external, while large parts of the kernel are internal or private interfaces. Also, some interfaces are targeted towards other specific subsystems.

Identifying the audience of an interface helps define the impact of breaking it. For instance, it might be okay to break the compatibility of an interface whose audience is a small number of specific subsystems. On the other hand, it is probably not okay to break a protocol interfaces that millions of Internet users depend on.

Yetus uses the following kinds of audience in order of increasing/wider visibility:


The interface is for internal use within a project(such as Apache Hadoop) and should not be used by applications or by other projects. It is subject to change at anytime without notice. Most interfaces of a project are Private (also referred to as project-private).


The interface is used by a specified set of projects or systems (typically closely related projects). Other projects or systems should not use the interface. Changes to the interface will be communicated/ negotiated with the specified projects. For example, in the Apache Hadoop project, some interfaces are LimitedPrivate{HDFS, MapReduce} in that they are private to the HDFS and MapReduce subprojects.


The interface is for general use by any application.


Stability denotes how stable an interface is, as in when incompatible changes to the interface are allowed. Yetus provides the following levels of stability.


Can evolve while retaining compatibility for minor release boundaries; in other words, incompatible changes to APIs marked Stable are generally only allowed at major releases (i.e. at m.0).


Evolving, but incompatible changes are allowed at minor release (i.e. m .x)


Incompatible changes to Unstable APIs are allowed any time. This usually makes sense for only private interfaces.

However one may call this out for a supposedly public interface to highlight that it should not be used as an interface; for public interfaces, labeling it as Not-an-interface is probably more appropriate than Unstable.

Examples of publicly visible interfaces that are unstable (i.e. not-an-interface): GUI, CLIs whose output format will change


APIs that could potentially be removed in the future and should not be used.

How are the Classifications Recorded

How should the classification be recorded for the annotated APIs?