hadoop-common-project/hadoop-common/src/site/apt/InterfaceClassification.apt.vm

~~ Licensed under the Apache License, Version 2.0 (the "License");
~~ you may not use this file except in compliance with the License.
~~ You may obtain a copy of the License at
~~
~~   http://www.apache.org/licenses/LICENSE-2.0
~~
~~ Unless required by applicable law or agreed to in writing, software
~~ distributed under the License is distributed on an "AS IS" BASIS,
~~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
~~ See the License for the specific language governing permissions and
~~ limitations under the License. See accompanying LICENSE file.

  ---
  Hadoop Interface Taxonomy: Audience and Stability Classification
  ---
  ---
  ${maven.build.timestamp}

Hadoop Interface Taxonomy: Audience and Stability Classification

%{toc|section=1|fromDepth=0}

* Motivation

  The interface taxonomy classification provided here 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

  Hadoop adopts the following interface classification,
  this classification was derived from the
  {{{http://www.opensolaris.org/os/community/arc/policies/interface-taxonomy/#Advice}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,
   other are public/external interfaces are meant for wider consumption by
   applications and/or clients. For example, in posix, libc is an external or
   public interface, 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.

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

   * Private:

     * The interface is for internal use within the project (such as HDFS or
     MapReduce) 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 Hadoop project,
     some interfaces are LimitedPrivate\{HDFS, MapReduce\} in that they
     are private to the HDFS and MapReduce projects.

   * Public

     * The interface is for general use by any application.

   Hadoop doesn't have a Company-Private classification,
   which is meant for APIs which are intended to be used by other projects
   within the company, since it doesn't apply to opensource projects. Also,
   certain APIs are annotated as @VisibleForTesting (from com.google.common
   .annotations.VisibleForTesting) - these are meant to be used strictly for
   unit tests and should be treated as "Private" APIs.

** Stability

   Stability denotes how stable an interface is, as in when incompatible
   changes to the interface are allowed. Hadoop APIs have 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 allowed
     only 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 removed in the future and should not be
     used.

* How are the Classifications Recorded?

  How will the classification be recorded for Hadoop APIs?

  * Each interface or class will have the audience and stability recorded
  using annotations in org.apache.hadoop.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 do so. 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, This is
    fairly low-level interface that we are willing to expose to MR and perhaps
    HBase. We are likely to change it down  the road and at that time we will
    have 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
    Hadoop-Private for the Hadoop family.

  * Lets treat all private interfaces as Hadoop-private. What is the harm in
  projects in the Hadoop family have 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 code that
    we have been cleaning up over the last few years. We don’t want such
    layer violations to creep back in by no separating 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 attampt 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 distruption
    because the stability is controlled by the standards body. Hence it is appropriate to
    label the interface as unstable.