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From u..@apache.org
Subject [32/51] [partial] flink-web git commit: [hotfix] Manual build of docs
Date Tue, 30 Jun 2015 10:16:02 GMT
http://git-wip-us.apache.org/repos/asf/flink-web/blob/396616d4/content/docs/0.9/internals/types_serialization.html
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+  <div class="col-sm-10 col-sm-offset-1">
+    <h1>Type Extraction and Serialization</h1>
+
+
+
+<p>Flink handles types in a unique way, containing its own type descriptors,
+generic type extraction, and type serialization framework.
+This document describes the concepts and the rationale behind them.</p>
+
+<p>There are fundamental differences in the way that the Scala API and
+the Java API handle type information, so most of the issues described 
+here relate only to one of the to APIs.</p>
+
+<ul id="markdown-toc">
+  <li><a href="#type-handling-in-flink" id="markdown-toc-type-handling-in-flink">Type
handling in Flink</a></li>
+  <li><a href="#flinks-typeinformation-class" id="markdown-toc-flinks-typeinformation-class">Flink’s
TypeInformation class</a></li>
+  <li><a href="#type-information-in-the-scala-api" id="markdown-toc-type-information-in-the-scala-api">Type
Information in the Scala API</a></li>
+  <li><a href="#type-information-in-the-java-api" id="markdown-toc-type-information-in-the-java-api">Type
Information in the Java API</a></li>
+</ul>
+
+<h2 id="type-handling-in-flink">Type handling in Flink</h2>
+
+<p>Flink tries to know as much information about what types enter and leave user functions
as possible.
+This stands in contrast to the approach to just assuming nothing and letting the
+programming language and serialization framework handle all types dynamically.</p>
+
+<ul>
+  <li>
+    <p>To allow using POJOs and grouping/joining them by referring to field names,
Flink needs the type
+information to make checks (for typos and type compatibility) before the job is executed.</p>
+  </li>
+  <li>
+    <p>The more we know, the better serialization and data layout schemes the compiler/optimizer
can develop.
+That is quite important for the memory usage paradigm in Flink (work on serialized data
+inside/outside the heap and make serialization very cheap).</p>
+  </li>
+  <li>
+    <p>For the upcoming logical programs (see roadmap draft) we need this to know the
“schema” of functions.</p>
+  </li>
+  <li>
+    <p>Finally, it also spares users having to worry about serialization frameworks
and having to register
+types at those frameworks.</p>
+  </li>
+</ul>
+
+<h2 id="flinks-typeinformation-class">Flink’s TypeInformation class</h2>
+
+<p>The class <a href="https://github.com/apache/flink/blob/master//flink-core/src/main/java/org/apache/flink/api/common/typeinfo/TypeInformation.java">TypeInformation</a>
+is the base class for all type descriptors. It reveals some basic properties of the type
and can generate serializers
+and, in specializations, comparators for the types.
+(<em>Note that comparators in Flink do much more than defining an order - they are
basically the utility to handle keys</em>)</p>
+
+<p>Internally, Flink makes the following distinctions between types:</p>
+
+<ul>
+  <li>
+    <p>Basic types: All Java primitives and their boxed form, plus <code>void</code>,
<code>String</code>, and <code>Date</code>.</p>
+  </li>
+  <li>
+    <p>Primitive arrays and Object arrays</p>
+  </li>
+  <li>
+    <p>Composite types</p>
+
+    <ul>
+      <li>
+        <p>Flink Java Tuples (part of the Flink Java API)</p>
+      </li>
+      <li>
+        <p>Scala <em>case classes</em> (including Scala tuples)</p>
+      </li>
+      <li>
+        <p>POJOs: classes that follow a certain bean-like pattern</p>
+      </li>
+    </ul>
+  </li>
+  <li>
+    <p>Scala auxiliary types (Option, Either, Lists, Maps, …)</p>
+  </li>
+  <li>
+    <p>Generic types: These will not be serialized by Flink itself, but by Kryo.</p>
+  </li>
+</ul>
+
+<p>POJOs are of particular interest, because they support the creation of complex types
and the use of field
+names in the definition of keys: <code>dataSet.join(another).where("name").equalTo("personName")</code>.
+They are also transparent to the runtime and can be handled very efficiently by Flink.</p>
+
+<p><strong>Rules for POJO types</strong></p>
+
+<p>Flink recognizes a data type as a POJO type (and allows “by-name” field referencing)
if the following
+conditions are fulfilled:</p>
+
+<ul>
+  <li>The class is public and standalone (no non-static inner class)</li>
+  <li>The class has a public no-argument constructor</li>
+  <li>All fields in the class (and all superclasses) are either public or
+or have a public getter and a setter method that follows the Java beans
+naming conventions for getters and setters.</li>
+</ul>
+
+<h2 id="type-information-in-the-scala-api">Type Information in the Scala API</h2>
+
+<p>Scala has very elaborate concepts for runtime type information though <em>type
manifests</em> and <em>class tags</em>. In
+general, types and methods have access to the types of their generic parameters - thus, Scala
programs do
+not suffer from type erasure as Java programs do.</p>
+
+<p>In addition, Scala allows to run custom code in the Scala Compiler through Scala
Macros - that means that some Flink
+code gets executed whenever you compile a Scala program written against Flink’s Scala API.</p>
+
+<p>We use the Macros to look at the parameter types and return types of all user functions
during compilation - that
+is the point in time when certainly all type information is perfectly available. Within the
macro, we create
+a <em>TypeInformation</em> for the function’s return types (or parameter types)
and make it part of the operation.</p>
+
+<h4 id="no-implicit-value-for-evidence-parameter-error">No Implicit Value for Evidence
Parameter Error</h4>
+
+<p>In the case where TypeInformation could not be created, programs fail to compile
with an error
+stating <em>“could not find implicit value for evidence parameter of type TypeInformation”</em>.</p>
+
+<p>A frequent reason if that the code that generates the TypeInformation has not been
imported.
+Make sure to import the entire flink.api.scala package.</p>
+
+<div class="highlight"><pre><code class="language-scala" data-lang="scala"><span
class="k">import</span> <span class="nn">org.apache.flink.api.scala._</span></code></pre></div>
+
+<p>Another common cause are generic methods, which can be fixed as described in the
following section.</p>
+
+<h4 id="generic-methods">Generic Methods</h4>
+
+<p>Consider the following case below:</p>
+
+<div class="highlight"><pre><code class="language-scala" data-lang="scala"><span
class="k">def</span><span class="o">[</span><span class="kt">T</span><span
class="o">]</span> <span class="n">selectFirst</span><span class="o">(</span><span
class="n">input</span><span class="k">:</span> <span class="kt">DataSet</span><span
class="o">[(</span><span class="kt">T</span>, <span class="k">_</span><span
class="o">)])</span> <span class="k">:</span> <span class="kt">DataSet</span><span
class="o">[</span><span class="kt">T</span><span class="o">]</span>
<span class="k">=</span> <span class="o">{</span>
+  <span class="n">input</span><span class="o">.</span><span class="n">map</span>
<span class="o">{</span> <span class="n">v</span> <span class="k">=&gt;</span>
<span class="n">v</span><span class="o">.</span><span class="n">_1</span>
<span class="o">}</span>
+<span class="o">}</span>
+
+<span class="k">val</span> <span class="n">data</span> <span class="k">:</span>
<span class="kt">DataSet</span><span class="o">[(</span><span class="kt">String</span>,
<span class="kt">Long</span><span class="o">)</span> <span class="kt">=</span>
<span class="kt">...</span>
+
+<span class="kt">val</span> <span class="kt">result</span> <span
class="kt">=</span> <span class="kt">selectFirst</span><span class="o">(</span><span
class="kt">data</span><span class="o">)</span></code></pre></div>
+
+<p>For such generic methods, the data types of the function parameters and return type
may not be the same
+for every call and are not known at the site where the method is defined. The code above
will result
+in an error that not enough implicit evidence is available.</p>
+
+<p>In such cases, the type information has to be generated at the invocation site and
passed to the
+method. Scala offers <em>implicit parameters</em> for that.</p>
+
+<p>The following code tells Scala to bring a type information for <em>T</em>
into the function. The type
+information will then be generated at the sites where the method is invoked, rather than
where the
+method is defined.</p>
+
+<div class="highlight"><pre><code class="language-scala" data-lang="scala"><span
class="k">def</span><span class="o">[</span><span class="kt">T</span>
<span class="kt">:</span> <span class="kt">TypeInformation</span><span
class="o">]</span> <span class="n">selectFirst</span><span class="o">(</span><span
class="n">input</span><span class="k">:</span> <span class="kt">DataSet</span><span
class="o">[(</span><span class="kt">T</span>, <span class="k">_</span><span
class="o">)])</span> <span class="k">:</span> <span class="kt">DataSet</span><span
class="o">[</span><span class="kt">T</span><span class="o">]</span>
<span class="k">=</span> <span class="o">{</span>
+  <span class="n">input</span><span class="o">.</span><span class="n">map</span>
<span class="o">{</span> <span class="n">v</span> <span class="k">=&gt;</span>
<span class="n">v</span><span class="o">.</span><span class="n">_1</span>
<span class="o">}</span>
+<span class="o">}</span></code></pre></div>
+
+<h2 id="type-information-in-the-java-api">Type Information in the Java API</h2>
+
+<p>Java in general erases generic type information. Only for subclasses of generic
classes, the subclass
+stores the type to which the generic type variables bind.</p>
+
+<p>Flink uses reflection on the (anonymous) classes that implement the user functions
to figure out the types of
+the generic parameters of the function. This logic also contains some simple type inference
for cases where
+the return types of functions are dependent on input types, such as in the generic utility
method below:</p>
+
+<div class="highlight"><pre><code class="language-java" data-lang="java"><span
class="kd">public</span> <span class="kd">class</span> <span class="nc">AppendOne</span><span
class="o">&lt;</span><span class="n">T</span><span class="o">&gt;</span>
<span class="kd">extends</span> <span class="n">MapFunction</span><span
class="o">&lt;</span><span class="n">T</span><span class="o">,</span>
<span class="n">Tuple2</span><span class="o">&lt;</span><span
class="n">T</span><span class="o">,</span> <span class="n">Long</span><span
class="o">&gt;&gt;</span> <span class="o">{</span>
+
+    <span class="kd">public</span> <span class="n">Tuple2</span><span
class="o">&lt;</span><span class="n">T</span><span class="o">,</span>
<span class="n">Long</span><span class="o">&gt;</span> <span
class="nf">map</span><span class="o">(</span><span class="n">T</span>
<span class="n">value</span><span class="o">)</span> <span class="o">{</span>
+        <span class="k">return</span> <span class="k">new</span>
<span class="n">Tuple2</span><span class="o">&lt;</span><span
class="n">T</span><span class="o">,</span> <span class="n">Long</span><span
class="o">&gt;(</span><span class="n">value</span><span class="o">,</span>
<span class="mi">1L</span><span class="o">);</span>
+    <span class="o">}</span>
+<span class="o">}</span></code></pre></div>
+
+<p>Not in all cases can Flink figure out the data types of functions reliably in Java.
+Some issues remain with generic lambdas (we are trying to solve this with the Java community,
+see below) and with generic type variables that we cannot infer.</p>
+
+<h4 id="type-hints-in-the-java-api">Type Hints in the Java API</h4>
+
+<p>To help cases where Flink cannot reconstruct the erased generic type information,
the Java API
+offers so called <em>type hints</em> from version 0.9 on. The type hints tell
the system the type of
+the data set produced by a function. The following gives an example:</p>
+
+<div class="highlight"><pre><code class="language-java" data-lang="java"><span
class="n">DataSet</span><span class="o">&lt;</span><span class="n">SomeType</span><span
class="o">&gt;</span> <span class="n">result</span> <span class="o">=</span>
<span class="n">dataSet</span>
+    <span class="o">.</span><span class="na">map</span><span class="o">(</span><span
class="k">new</span> <span class="n">MyGenericNonInferrableFunction</span><span
class="o">&lt;</span><span class="n">Long</span><span class="o">,</span>
<span class="n">SomeType</span><span class="o">&gt;())</span>
+        <span class="o">.</span><span class="na">returns</span><span
class="o">(</span><span class="n">SomeType</span><span class="o">.</span><span
class="na">class</span><span class="o">);</span></code></pre></div>
+
+<p>The <code>returns</code> statement specifies the produced type, in this
case via a class. The hints support
+type definition through</p>
+
+<ul>
+  <li>Classes, for non-parameterized types (no generics)</li>
+  <li>Strings in the form of <code>returns("Tuple2&lt;Integer, my.SomeType&gt;")</code>,
which are parsed and converted
+to a TypeInformation.</li>
+  <li>A TypeInformation directly</li>
+</ul>
+
+<h4 id="type-extraction-for-java-8-lambdas">Type extraction for Java 8 lambdas</h4>
+
+<p>Type extraction for Java 8 lambdas works differently than for non-lambdas, because
lambdas are not associated
+with an implementing class that extends the function interface.</p>
+
+<p>Currently, Flink tries to figure out which method implements the lambda and uses
Java’s generic signatures to
+determine the parameter types and the return type. However, these signatures are not generated
for lambdas
+by all compilers (as of writing this document only reliably by the Eclipse JDT compiler 4.5
from Milestone 2
+onwards)</p>
+
+<p><strong>Improving Type information for Java Lambdas</strong></p>
+
+<p>One of the Flink committers (Timo Walther) has actually become active in the Eclipse
JDT compiler community and
+in the OpenJDK community and submitted patches to the compiler to improve availability of
type information 
+available for Java 8 lambdas.</p>
+
+<p>The Eclipse JDT compiler has added support for this as of version 4.5 M4. Discussion
about the feature in the
+OpenJDK compiler is pending.</p>
+
+<h4 id="serialization-of-pojo-types">Serialization of POJO types</h4>
+
+<p>The PojoTypeInformation is creating serializers for all the fields inside the POJO.
Standard types such as 
+int, long, String etc. are handled by serializers we ship with Flink.
+For all other types, we fall back to Kryo.</p>
+
+<p>If Kryo is not able to handle the type, you can ask the PojoTypeInfo to serialize
the POJO using Avro.
+To do so, you have to call</p>
+
+<div class="highlight"><pre><code class="language-java" data-lang="java"><span
class="kd">final</span> <span class="n">ExecutionEnvironment</span> <span
class="n">env</span> <span class="o">=</span> <span class="n">ExecutionEnvironment</span><span
class="o">.</span><span class="na">getExecutionEnvironment</span><span
class="o">();</span>
+<span class="n">env</span><span class="o">.</span><span class="na">getConfig</span><span
class="o">().</span><span class="na">enableForceAvro</span><span class="o">();</span></code></pre></div>
+
+<p>Note that Flink is automatically serializing POJOs generated by Avro with the Avro
serializer.</p>
+
+<p>If you want your <strong>entire</strong> POJO Type to be treated by
the Kryo serializer, set</p>
+
+<div class="highlight"><pre><code class="language-java" data-lang="java"><span
class="kd">final</span> <span class="n">ExecutionEnvironment</span> <span
class="n">env</span> <span class="o">=</span> <span class="n">ExecutionEnvironment</span><span
class="o">.</span><span class="na">getExecutionEnvironment</span><span
class="o">();</span>
+<span class="n">env</span><span class="o">.</span><span class="na">getConfig</span><span
class="o">().</span><span class="na">enableForceKryo</span><span class="o">();</span></code></pre></div>
+
+<p>If Kryo is not able to serialize your POJO, you can add a custom serializer to Kryo,
using</p>
+
+<div class="highlight"><pre><code class="language-java" data-lang="java"><span
class="n">env</span><span class="o">.</span><span class="na">getConfig</span><span
class="o">().</span><span class="na">addDefaultKryoSerializer</span><span
class="o">(</span><span class="n">Class</span><span class="o">&lt;?&gt;</span>
<span class="n">type</span><span class="o">,</span> <span class="n">Class</span><span
class="o">&lt;?</span> <span class="kd">extends</span> <span class="n">Serializer</span><span
class="o">&lt;?&gt;&gt;</span> <span class="n">serializerClass</span><span
class="o">)</span></code></pre></div>
+
+<p>There are different variants of these methods available.</p>
+
+
+  </div>
+
+  <div class="col-sm-10 col-sm-offset-1">
+    <!-- Disqus thread and some vertical offset -->
+    <div style="margin-top: 75px; margin-bottom: 50px" id="disqus_thread"></div>
+  </div>
+</div>
+
+    </div><!-- /.container -->
+
+    <!-- jQuery (necessary for Bootstrap's JavaScript plugins) -->
+    <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
+    <!-- Include all compiled plugins (below), or include individual files as needed -->
+    <script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.4/js/bootstrap.min.js"></script>
+    <script src="http://flink.apache.org/docs/0.9/page/js/codetabs.js"></script>
+
+    <!-- Google Analytics -->
+    <script>
+      (function(i,s,o,g,r,a,m){i['GoogleAnalyticsObject']=r;i[r]=i[r]||function(){
+      (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o),
+      m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
+      })(window,document,'script','//www.google-analytics.com/analytics.js','ga');
+
+      ga('create', 'UA-52545728-1', 'auto');
+      ga('send', 'pageview');
+    </script>
+
+    <!-- Disqus -->
+    <script type="text/javascript">
+    var disqus_shortname = 'stratosphere-eu';
+    (function() {
+        var dsq = document.createElement('script'); dsq.type = 'text/javascript'; dsq.async
= true;
+        dsq.src = '//' + disqus_shortname + '.disqus.com/embed.js';
+        (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq);
+    })();
+</script>
+  </body>
+</html>

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+All image files in the folder and its subfolders are
+licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements.  See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership.  The ASF licenses this file
+to you 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.
\ No newline at end of file

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