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HADOOP-11559. Add links to RackAwareness and InterfaceClassification to site index (Masatake Iwasaki via aw) 

(cherry picked from commit 7eeca90daa)

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	hadoop-common-project/hadoop-common/CHANGES.txt
	hadoop-project/src/site/site.xml

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Allen Wittenauer 2015-02-10 17:06:03 -08:00 committed by Andrew Wang
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206
hadoop-common-project/hadoop-common/src/site/markdown/InterfaceClassification.md
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@ -20,80 +20,196 @@ Hadoop Interface Taxonomy: Audience and Stability Classification
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.
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.
* 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 [OpenSolaris taxonomy](http://www.opensolaris.org/os/community/arc/policies/interface-taxonomy/#Advice) and, to some extent, from taxonomy used inside Yahoo. Interfaces have two main attributes: Audience and Stability
Hadoop adopts the following interface classification,
this classification was derived from the
[OpenSolaris taxonomy](http://www.opensolaris.org/os/community/arc/policies/interface-taxonomy/#Advice)
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.
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.
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.
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.
#### 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.
### 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.
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.
#### 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.
* 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).
* 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.
* 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.
* 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.
* 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.

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hadoop-common-project/hadoop-common/src/site/markdown/RackAwareness.md
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@ -17,17 +17,49 @@
Rack Awareness
==============
Hadoop components are rack-aware. For example, HDFS block placement will use rack awareness for fault tolerance by placing one block replica on a different rack. This provides data availability in the event of a network switch failure or partition within the cluster.
Hadoop components are rack-aware. For example, HDFS block placement
will use rack awareness for fault tolerance by placing one block
replica on a different rack. This provides data availability in the
event of a network switch failure or partition within the cluster.
Hadoop master daemons obtain the rack id of the cluster slaves by invoking either an external script or java class as specified by configuration files. Using either the java class or external script for topology, output must adhere to the java **org.apache.hadoop.net.DNSToSwitchMapping** interface. The interface expects a one-to-one correspondence to be maintained and the topology information in the format of '/myrack/myhost', where '/' is the topology delimiter, 'myrack' is the rack identifier, and 'myhost' is the individual host. Assuming a single /24 subnet per rack, one could use the format of '/192.168.100.0/192.168.100.5' as a unique rack-host topology mapping.
Hadoop master daemons obtain the rack id of the cluster slaves by
invoking either an external script or java class as specified by
configuration files. Using either the java class or external script
for topology, output must adhere to the java
**org.apache.hadoop.net.DNSToSwitchMapping** interface. The interface
expects a one-to-one correspondence to be maintained and the topology
information in the format of '/myrack/myhost', where '/' is the
topology delimiter, 'myrack' is the rack identifier, and 'myhost' is
the individual host. Assuming a single /24 subnet per rack, one could
use the format of '/192.168.100.0/192.168.100.5' as a unique rack-host
topology mapping.
To use the java class for topology mapping, the class name is specified by the **topology.node.switch.mapping.impl** parameter in the configuration file. An example, NetworkTopology.java, is included with the hadoop distribution and can be customized by the Hadoop administrator. Using a Java class instead of an external script has a performance benefit in that Hadoop doesn't need to fork an external process when a new slave node registers itself.
To use the java class for topology mapping, the class name is
specified by the **net.topology.node.switch.mapping.impl** parameter
in the configuration file. An example, NetworkTopology.java, is
included with the hadoop distribution and can be customized by the
Hadoop administrator. Using a Java class instead of an external script
has a performance benefit in that Hadoop doesn't need to fork an
external process when a new slave node registers itself.
If implementing an external script, it will be specified with the **topology.script.file.name** parameter in the configuration files. Unlike the java class, the external topology script is not included with the Hadoop distribution and is provided by the administrator. Hadoop will send multiple IP addresses to ARGV when forking the topology script. The number of IP addresses sent to the topology script is controlled with **net.topology.script.number.args** and defaults to 100. If **net.topology.script.number.args** was changed to 1, a topology script would get forked for each IP submitted by DataNodes and/or NodeManagers.
If implementing an external script, it will be specified with the
**net.topology.script.file.name** parameter in the configuration
files. Unlike the java class, the external topology script is not
included with the Hadoop distribution and is provided by the
administrator. Hadoop will send multiple IP addresses to ARGV when
forking the topology script. The number of IP addresses sent to the
topology script is controlled with **net.topology.script.number.args**
and defaults to 100. If **net.topology.script.number.args** was
changed to 1, a topology script would get forked for each IP submitted
by DataNodes and/or NodeManagers.
If **topology.script.file.name** or **topology.node.switch.mapping.impl** is not set, the rack id '/default-rack' is returned for any passed IP address. While this behavior appears desirable, it can cause issues with HDFS block replication as default behavior is to write one replicated block off rack and is unable to do so as there is only a single rack named '/default-rack'.
An additional configuration setting is **mapreduce.jobtracker.taskcache.levels** which determines the number of levels (in the network topology) of caches MapReduce will use. So, for example, if it is the default value of 2, two levels of caches will be constructed - one for hosts (host -\> task mapping) and another for racks (rack -\> task mapping). Giving us our one-to-one mapping of '/myrack/myhost'.
If **net.topology.script.file.name** or
**net.topology.node.switch.mapping.impl** is not set, the rack id
'/default-rack' is returned for any passed IP address. While this
behavior appears desirable, it can cause issues with HDFS block
replication as default behavior is to write one replicated block off
rack and is unable to do so as there is only a single rack named
'/default-rack'.
python Example
--------------

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hadoop-project/src/site/site.xml
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@ -52,7 +52,7 @@
<item name="Cluster Setup" href="hadoop-project-dist/hadoop-common/ClusterSetup.html"/>
<item name="Commands Reference" href="hadoop-project-dist/hadoop-common/CommandsManual.html"/>
<item name="FileSystem Shell" href="hadoop-project-dist/hadoop-common/FileSystemShell.html"/>
<item name="Compatibility" href="hadoop-project-dist/hadoop-common/Compatibility.html"/>
<item name="Hadoop Compatibility" href="hadoop-project-dist/hadoop-common/Compatibility.html"/>
<item name="Interface Classification" href="hadoop-project-dist/hadoop-common/InterfaceClassification.html"/>
<item name="FileSystem Specification"
href="hadoop-project-dist/hadoop-common/filesystem/index.html"/>