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 <h1 id="apache-yetus-interface-taxonomy-audience-and-stability-classification">Apache Yetus Interface Taxonomy: Audience and Stability Classification</h1>

 <!-- MarkdownTOC levels="1,2,3" autolink="true" indent="  " bullets="*" bracket="round" -->

 <ul>
   <li><a href="#motivation">Motivation</a></li>
   <li><a href="#interface-classification">Interface Classification</a>
     <ul>
       <li><a href="#audience">Audience</a>
         <ul>
           <li><a href="#private">Private</a></li>
           <li><a href="#limited-private">Limited-Private</a></li>
           <li><a href="#public">Public</a></li>
         </ul>
       </li>
       <li><a href="#stability">Stability</a>
         <ul>
           <li><a href="#stable">Stable</a></li>
           <li><a href="#evolving">Evolving</a></li>
           <li><a href="#deprecated">Deprecated</a></li>
         </ul>
       </li>
     </ul>
   </li>
   <li><a href="#how-are-the-classifications-recorded">How are the Classifications Recorded</a></li>
   <li><a href="#faq">FAQ</a></li>
 </ul>

 <!-- /MarkdownTOC -->

 <h1 id="motivation">Motivation</h1>

 <p>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.</p>

 <ul>
   <li>
     <p>Benefits to the user of an interface: Knows which interfaces to use or not use and their stability.</p>
   </li>
   <li>
     <p>Benefits to the developer: to prevent accidental changes of interfaces and<br />
 hence accidental impact on users or other components or system. This is<br />
 particularly useful in large systems with many developers who may not all have a shared state/history of the project.</p>
   </li>
 </ul>

 <h1 id="interface-classification">Interface Classification</h1>

 <p>Yetus provides the following interface classification, derived from the<br />
 <a href="https://web.archive.org/web/20061013114610/http://opensolaris.org/os/community/arc/policies/interface-taxonomy/">OpenSolaris taxonomy</a><br />
 and, to some extent, from taxonomy used inside Yahoo.<br />
 Interfaces have two main attributes: Audience and Stability</p>

 <h2 id="audience">Audience</h2>

 <p>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.</p>

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

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

 <h3 id="private">Private</h3>

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

 <h3 id="limited-private">Limited-Private</h3>

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

 <h3 id="public">Public</h3>

 <p>The interface is for general use by any application.</p>

 <h2 id="stability">Stability</h2>

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

 <h3 id="stable">Stable</h3>

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

 <h3 id="evolving">Evolving</h3>

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

 <h4 id="unstable">Unstable</h4>

 <p>Incompatible changes to Unstable APIs are allowed any time. This usually makes<br />
 sense for only private interfaces.</p>

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

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

 <h3 id="deprecated">Deprecated</h3>

 <p>APIs that could potentially be removed in the future and should not be used.</p>

 <h1 id="how-are-the-classifications-recorded">How are the Classifications Recorded</h1>

 <p>How should the classification be recorded for the annotated APIs?</p>

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

 <h1 id="faq">FAQ</h1>

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

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	<h1 id="apache-yetus-interface-taxonomy-audience-and-stability-classification">Apache Yetus Interface Taxonomy: Audience and Stability Classification</h1>

	<!-- MarkdownTOC levels="1,2,3" autolink="true" indent=" " bullets="*" bracket="round" -->

	<ul>
	<li><a href="#motivation">Motivation</a></li>
	<li><a href="#interface-classification">Interface Classification</a>
	<ul>
	<li><a href="#audience">Audience</a>
	<ul>
	<li><a href="#private">Private</a></li>
	<li><a href="#limited-private">Limited-Private</a></li>
	<li><a href="#public">Public</a></li>
	</ul>
	</li>
	<li><a href="#stability">Stability</a>
	<ul>
	<li><a href="#stable">Stable</a></li>
	<li><a href="#evolving">Evolving</a></li>
	<li><a href="#deprecated">Deprecated</a></li>
	</ul>
	</li>
	</ul>
	</li>
	<li><a href="#how-are-the-classifications-recorded">How are the Classifications Recorded</a></li>
	<li><a href="#faq">FAQ</a></li>
	</ul>

	<!-- /MarkdownTOC -->

	<h1 id="motivation">Motivation</h1>

	<p>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.</p>

	<ul>
	<li>
	<p>Benefits to the user of an interface: Knows which interfaces to use or not use and their stability.</p>
	</li>
	<li>
	<p>Benefits to the developer: to prevent accidental changes of interfaces and<br />
	hence accidental impact on users or other components or system. This is<br />
	particularly useful in large systems with many developers who may not all have a shared state/history of the project.</p>
	</li>
	</ul>

	<h1 id="interface-classification">Interface Classification</h1>

	<p>Yetus provides the following interface classification, derived from the<br />
	<a href="https://web.archive.org/web/20061013114610/http://opensolaris.org/os/community/arc/policies/interface-taxonomy/">OpenSolaris taxonomy</a><br />
	and, to some extent, from taxonomy used inside Yahoo.<br />
	Interfaces have two main attributes: Audience and Stability</p>

	<h2 id="audience">Audience</h2>

	<p>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.</p>

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

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

	<h3 id="private">Private</h3>

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

	<h3 id="limited-private">Limited-Private</h3>

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

	<h3 id="public">Public</h3>

	<p>The interface is for general use by any application.</p>

	<h2 id="stability">Stability</h2>

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

	<h3 id="stable">Stable</h3>

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

	<h3 id="evolving">Evolving</h3>

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

	<h4 id="unstable">Unstable</h4>

	<p>Incompatible changes to Unstable APIs are allowed any time. This usually makes<br />
	sense for only private interfaces.</p>

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

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

	<h3 id="deprecated">Deprecated</h3>

	<p>APIs that could potentially be removed in the future and should not be used.</p>

	<h1 id="how-are-the-classifications-recorded">How are the Classifications Recorded</h1>

	<p>How should the classification be recorded for the annotated APIs?</p>

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

	<h1 id="faq">FAQ</h1>

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

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