# Apache Yetus Interface Taxonomy: Audience and Stability Classification

- Motivation
- Interface Classification
  - Audience
    - Private
    - Limited-Private
    - Public
  - Stability
    - Stable
    - Evolving
    - Deprecated
- How are the Classifications Recorded
- FAQ

# Motivation

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.

- Benefits to the user of an interface: Knows which interfaces to use or not use and their stability.

- Benefits to the developer: to prevent accidental changes of interfaces and
  hence accidental impact on users or other components or system. This is
  particularly useful in large systems with many developers who may not all have a shared state/history of the project.

# 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

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:

### Private

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).

### Limited-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.

### Public

The interface is for general use by any application.

## Stability

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

### Stable

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

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

#### Unstable

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

### Deprecated

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?

- Each interface or class will have the audience and stability recorded using
  annotations in org.apache.yetus.classification package.
- The javadoc generated by the maven target javadoc:javadoc lists only the public API.
- One can derive the audience of Java classes and Java interfaces by the
  audience of the package in which they are contained. Hence it is useful to
  declare the audience of each Java package as public or private (along with the private audience variations).

# FAQ

- Why aren't the Java scopes (private, package private and public) good enough?
  - Java's scoping is not very complete. One is often forced to make a class public in order for other internal components to use it. It does not have friends or sub-package-private like C++.
- But I can easily access a private implementation interface if it is Java public. Where is the protection and control?
  - The purpose of this is not providing absolute access control. Its purpose
    is to communicate to users and developers. One can access private
    implementation functions in libc; however if they change the internal
    implementation details, your application will break and you will have
    little sympathy from the folks who are supplying libc. If you use a
    non-public interface you understand the risks.
- Why bother declaring the stability of a private interface?
  Aren't private interfaces always unstable?
  - Private interfaces are not always unstable. In the cases where they are
    stable they capture internal properties of the system and can communicate
    these properties to its internal users and to developers of the interface.
    - e.g. In HDFS, NN-DN protocol is private but stable and can help
      implement rolling upgrades. It communicates that this interface should
      not be changed in incompatible ways even though it is private.
    - e.g. In HDFS, FSImage stability can help provide more flexible roll backs.
- What is the harm in applications using a private interface that is stable? How is it different than a public stable interface?
  - While a private interface marked as stable is targeted to change only at
    major releases, it may break at other times if the providers of that
    interface are willing to changes the internal users of that
    interface. Further, a public stable interface is less likely to break even
    at major releases (even though it is allowed to break compatibility)
    because the impact of the change is larger. If you use a private interface
    (regardless of its stability) you run the risk of incompatibility.
- Why bother with Limited-private? Isn't it giving special treatment to some projects? That is not fair.
  - First, most interfaces should be public or private; actually let us state
    it even stronger: make it private unless you really want to expose it to
    public for general use.
  - Limited-private is for interfaces that are not intended for general
    use. They are exposed to related projects that need special hooks. Such a
    classification has a cost to both the supplier and consumer of the limited
    interface. Both will have to work together if ever there is a need to
    break the interface in the future; for example the supplier and the
    consumers will have to work together to get coordinated releases of their
    respective projects. This should not be taken lightly - if you can get
    away with private then do so; if the interface is really for general use
    for all applications then you should consider making it public. But remember
    that making an interface public has huge responsibility. Sometimes
    Limited-private is just right.
  - A good example of a limited-private interface is BlockLocations in the Apache
    Hadoop Project, This is fairly low-level interface that they are willing to
    expose to MR and perhaps HBase. They are likely to change it down the road
    and at that time they will have to get a coordinated effort with the MR
    team to release matching releases. While MR and HDFS are always released
    in sync today, they may change down the road.
  - If you have a limited-private interface with many projects listed then you are fooling yourself. It is practically public.
  - It might be worth declaring a special audience classification called
    {YourProjectName}-Private for your closely related projects.
- Can't a private interface be treated as project-private also? For example what is the harm in projects in the Apache Hadoop extended ecosystem, having access to private classes?
  - Do we want MR accessing class files that are implementation details inside
    HDFS? There used to be many such layer violations in the Apache Hadoop
    project that they have been cleaning up over the last few years. It is highly
    undesirable for such layer violations to creep back in by no separation
    between the major components like HDFS and MR.
- Aren't all public interfaces stable?
  - One may mark a public interface as evolving in its early days. Here one is
    promising to make an effort to make compatible changes but may need to
    break it at minor releases.
  - One example of a public interface that is unstable is where one is
    providing an implementation of a standards-body based interface that is
    still under development. For example, many companies, in an attempt to be
    first to market, have provided implementations of a new NFS protocol even
    when the protocol was not fully completed by IETF. The implementor cannot
    evolve the interface in a fashion that causes least disruption because
    the stability is controlled by the standards body. Hence it is appropriate
    to label the interface as unstable.