Repository: hadoop
Updated Branches:
refs/heads/branch-2.8 b2667441e -> 15ff590c3
HADOOP-11559. Add links to RackAwareness and InterfaceClassification to site
index (Masatake Iwasaki via aw)
(cherry picked from commit 7eeca90daabd74934d4c94af6f07fd598abdb4ed)
Conflicts:
hadoop-common-project/hadoop-common/CHANGES.txt
hadoop-project/src/site/site.xml
(cherry picked from commit fbdb23d2afa993f96f073b9c4208282e8a280016)
Project: http://git-wip-us.apache.org/repos/asf/hadoop/repo
Commit: http://git-wip-us.apache.org/repos/asf/hadoop/commit/9d473b8d
Tree: http://git-wip-us.apache.org/repos/asf/hadoop/tree/9d473b8d
Diff: http://git-wip-us.apache.org/repos/asf/hadoop/diff/9d473b8d
Branch: refs/heads/branch-2.8
Commit: 9d473b8ddcea659cf362fa0f9331b10b9b4dfb7d
Parents: b266744
Author: Allen Wittenauer <a...@apache.org>
Authored: Tue Feb 10 17:06:03 2015 -0800
Committer: Andrew Wang <w...@apache.org>
Committed: Mon Oct 17 13:32:51 2016 -0700
----------------------------------------------------------------------
.../site/markdown/InterfaceClassification.md | 204 +++++++++++++++----
.../src/site/markdown/RackAwareness.md | 54 ++++-
hadoop-project/src/site/site.xml | 2 +-
3 files changed, 204 insertions(+), 56 deletions(-)
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http://git-wip-us.apache.org/repos/asf/hadoop/blob/9d473b8d/hadoop-common-project/hadoop-common/src/site/markdown/InterfaceClassification.md
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diff --git
a/hadoop-common-project/hadoop-common/src/site/markdown/InterfaceClassification.md
b/hadoop-common-project/hadoop-common/src/site/markdown/InterfaceClassification.md
index 493b0dd..07abdac 100644
---
a/hadoop-common-project/hadoop-common/src/site/markdown/InterfaceClassification.md
+++
b/hadoop-common-project/hadoop-common/src/site/markdown/InterfaceClassification.md
@@ -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.
+
+#### 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.
-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.
+#### 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)
-* 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.
+#### 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 arenât the java scopes (private, package private and public) good
enough?
- * Javaâs 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? Arenât 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.
+ * Javaâs 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?
+ Arenât 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? Isnât 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 â 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 donât 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.
+* 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? Isnât 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 â 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 donât 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.
http://git-wip-us.apache.org/repos/asf/hadoop/blob/9d473b8d/hadoop-common-project/hadoop-common/src/site/markdown/RackAwareness.md
----------------------------------------------------------------------
diff --git
a/hadoop-common-project/hadoop-common/src/site/markdown/RackAwareness.md
b/hadoop-common-project/hadoop-common/src/site/markdown/RackAwareness.md
index 12c7bf6..ced6c84 100644
--- a/hadoop-common-project/hadoop-common/src/site/markdown/RackAwareness.md
+++ b/hadoop-common-project/hadoop-common/src/site/markdown/RackAwareness.md
@@ -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 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.
-
-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 **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'.
+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.
+
+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
+**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 **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
--------------
http://git-wip-us.apache.org/repos/asf/hadoop/blob/9d473b8d/hadoop-project/src/site/site.xml
----------------------------------------------------------------------
diff --git a/hadoop-project/src/site/site.xml b/hadoop-project/src/site/site.xml
index 0705cae..403048e 100644
--- a/hadoop-project/src/site/site.xml
+++ b/hadoop-project/src/site/site.xml
@@ -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"/>
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