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| <h1 id="getting-help"> |
| Getting Help |
| <a class="anchor" href="#getting-help">#</a> |
| </h1> |
| <h2 id="having-a-question"> |
| Having a Question? |
| <a class="anchor" href="#having-a-question">#</a> |
| </h2> |
| <p>The Apache Flink community answers many user questions every day. You can search for answers and advice in the archives or reach out to the community for help and guidance.</p> |
| <h3 id="user-mailing-list"> |
| User Mailing List |
| <a class="anchor" href="#user-mailing-list">#</a> |
| </h3> |
| <p>Many Flink users, contributors, and committers are subscribed to Flink’s user mailing list. The user mailing list is a very good place to ask for help.</p> |
| <p>Before posting to the mailing list, you can search the mailing list archives for email threads that discuss issues related to yours on the following websites.</p> |
| <ul> |
| <li><a href="https://lists.apache.org/list.html?user@flink.apache.org">Apache Mailing List Archive</a></li> |
| </ul> |
| <p>If you’d like to post to the mailing list, you need to</p> |
| <ol> |
| <li>subscribe to the mailing list by sending an email to <code>user-subscribe@flink.apache.org</code>,</li> |
| <li>confirm the subscription by replying to the confirmation email, and</li> |
| <li>send your email to <code>user@flink.apache.org</code>.</li> |
| </ol> |
| <p>Please note that you won’t receive a response to your mail if you are not subscribed.</p> |
| <h3 id="slack"> |
| Slack |
| <a class="anchor" href="#slack">#</a> |
| </h3> |
| <p>You can join the <a href="https://join.slack.com/t/apache-flink/shared_invite/zt-1t4khgllz-Fm1CnXzdBbUchBz4HzJCAg">Apache Flink community on Slack.</a> |
| After creating an account in Slack, don’t forget to introduce yourself in #introductions. |
| Due to Slack limitations the invite link expires after 100 invites. If it is expired, please reach out to the <a href="/what-is-flink/community/#mailing-lists">Dev mailing list</a>. |
| Any existing Slack member can also invite anyone else to join.</p> |
| <p>There are a couple of community rules:</p> |
| <ul> |
| <li><strong>Be respectful</strong> - This is the most important rule!</li> |
| <li>All important decisions and conclusions <strong>must be reflected back to the mailing lists.</strong> |
| “If it didn’t happen on a mailing list, it didn’t happen.” - <a href="http://theapacheway.com/on-list/">The Apache Mottos</a></li> |
| <li>Use <strong>Slack threads</strong> to keep parallel conversations from overwhelming a channel.</li> |
| <li>Use either <a href="https://apache-flink.slack.com/archives/C03G7LJTS2G">#pyflink</a> (for all Python Flink questions) or <a href="https://apache-flink.slack.com/archives/C03G7LJTS2G">#troubleshooting</a> (for all other Flink questions).</li> |
| <li>Please <strong>do not direct message</strong> people for troubleshooting, Jira assigning and PR review. Doing this can result in removal from Slack.</li> |
| </ul> |
| <h3 id="stack-overflow"> |
| Stack Overflow |
| <a class="anchor" href="#stack-overflow">#</a> |
| </h3> |
| <p>Many members of the Flink community are active on <a href="https://stackoverflow.com">Stack Overflow</a>. You can search for questions and answers or post your questions using the <a href="https://stackoverflow.com/questions/tagged/apache-flink">[apache-flink]</a> tag.</p> |
| <h2 id="found-a-bug"> |
| Found a Bug? |
| <a class="anchor" href="#found-a-bug">#</a> |
| </h2> |
| <p>If you observe an unexpected behavior that might be caused by a bug, you can search for reported bugs or file a bug report in <a href="/what-is-flink/community/#issue-tracker">Flink’s JIRA</a>.</p> |
| <p>If you are unsure whether the unexpected behavior happened due to a bug or not, please post a question to the <a href="/what-is-flink/community/#user-mailing-list">user mailing list</a>.</p> |
| <h2 id="got-an-error-message"> |
| Got an Error Message? |
| <a class="anchor" href="#got-an-error-message">#</a> |
| </h2> |
| <p>Identifying the cause for an error message can be challenging. In the following, we list the most common error messages and explain how to handle them.</p> |
| <h3 id="i-have-a-notserializableexception"> |
| I have a NotSerializableException. |
| <a class="anchor" href="#i-have-a-notserializableexception">#</a> |
| </h3> |
| <p>Flink uses Java serialization to distribute copies of the application logic (the functions and operations you implement, |
| as well as the program configuration, etc.) to the parallel worker processes. |
| Because of that, all functions that you pass to the API must be serializable, as defined by |
| <a href="http://docs.oracle.com/javase/8/docs/api/java/io/Serializable.html">java.io.Serializable</a>.</p> |
| <p>If your function is an anonymous inner class, consider the following:</p> |
| <ul> |
| <li>make the function a standalone class, or a static inner class.</li> |
| <li>use a Java 8 lambda function.</li> |
| </ul> |
| <p>Is your function is already a static class, check the fields that you assign when you create |
| an instance of the class. One of the fields most likely holds a non-serializable type.</p> |
| <ul> |
| <li>In Java, use a <code>RichFunction</code> and initialize the problematic fields in the <code>open()</code> method.</li> |
| <li>In Scala, you can often simply use “lazy val” to defer initialization until the distributed execution happens. This may come at a minor performance cost. You can naturally also use a <code>RichFunction</code> in Scala.</li> |
| </ul> |
| <h3 id="using-the-scala-api-i-get-an-error-about-implicit-values-and-evidence-parameters"> |
| Using the Scala API, I get an error about implicit values and evidence parameters. |
| <a class="anchor" href="#using-the-scala-api-i-get-an-error-about-implicit-values-and-evidence-parameters">#</a> |
| </h3> |
| <p>This error means that the implicit value for the type information could not be provided. |
| Make sure that you have an <code>import org.apache.flink.streaming.api.scala._</code> (DataStream API) or an |
| <code>import org.apache.flink.api.scala._</code> (DataSet API) statement in your code.</p> |
| <p>If you are using Flink operations inside functions or classes that take |
| generic parameters, then a TypeInformation must be available for that parameter. |
| This can be achieved by using a context bound:</p> |
| <div class="highlight"><pre tabindex="0" class="chroma"><code class="language-scala" data-lang="scala"><span class="line"><span class="cl"><span class="k">def</span> <span class="n">myFunction</span><span class="o">[</span><span class="kt">T:</span> <span class="kt">TypeInformation</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="o">])</span><span class="k">:</span> <span class="kt">DataSet</span><span class="o">[</span><span class="kt">Seq</span><span class="o">[</span><span class="kt">T</span><span class="o">]]</span> <span class="k">=</span> <span class="o">{</span> |
| </span></span><span class="line"><span class="cl"> <span class="n">input</span><span class="o">.</span><span class="n">reduceGroup</span><span class="o">(</span> <span class="n">i</span> <span class="k">=></span> <span class="n">i</span><span class="o">.</span><span class="n">toSeq</span> <span class="o">)</span> |
| </span></span><span class="line"><span class="cl"><span class="o">}</span> |
| </span></span></code></pre></div><p>See <a href="//nightlies.apache.org/flink//dev/types_serialization.html">Type Extraction and Serialization</a> for |
| an in-depth discussion of how Flink handles types.</p> |
| <h3 id="i-see-a-classcastexception-x-cannot-be-cast-to-x"> |
| I see a ClassCastException: X cannot be cast to X. |
| <a class="anchor" href="#i-see-a-classcastexception-x-cannot-be-cast-to-x">#</a> |
| </h3> |
| <p>When you see an exception in the style <code>com.foo.X</code> cannot be cast to <code>com.foo.X</code> (or cannot be assigned to <code>com.foo.X</code>), it means that |
| multiple versions of the class <code>com.foo.X</code> have been loaded by different class loaders, and types of that class are attempted to be assigned to each other.</p> |
| <p>The reason for that can be:</p> |
| <ul> |
| <li> |
| <p>Class duplication through <code>child-first</code> classloading. That is an intended mechanism to allow users to use different versions of the same |
| dependencies that Flink uses. However, if different copies of these classes move between Flink’s core and the user application code, such an exception |
| can occur. To verify that this is the reason, try setting <code>classloader.resolve-order: parent-first</code> in the configuration. |
| If that makes the error disappear, please write to the mailing list to check if that may be a bug.</p> |
| </li> |
| <li> |
| <p>Caching of classes from different execution attempts, for example by utilities like Guava’s Interners, or Avro’s Schema cache. |
| Try to not use interners, or reduce the scope of the interner/cache to make sure a new cache is created whenever a new task |
| execution is started.</p> |
| </li> |
| </ul> |
| <h3 id="i-have-an-abstractmethoderror-or-nosuchfielderror"> |
| I have an AbstractMethodError or NoSuchFieldError. |
| <a class="anchor" href="#i-have-an-abstractmethoderror-or-nosuchfielderror">#</a> |
| </h3> |
| <p>Such errors typically indicate a mix-up in some dependency version. That means a different version of a dependency (a library) |
| is loaded during the execution compared to the version that code was compiled against.</p> |
| <p>From Flink 1.4.0 on, dependencies in your application JAR file may have different versions compared to dependencies used |
| by Flink’s core, or other dependencies in the classpath (for example from Hadoop). That requires <code>child-first</code> classloading |
| to be activated, which is the default.</p> |
| <p>If you see these problems in Flink 1.4+, one of the following may be true:</p> |
| <ul> |
| <li>You have a dependency version conflict within your application code. Make sure all your dependency versions are consistent.</li> |
| <li>You are conflicting with a library that Flink cannot support via <code>child-first</code> classloading. Currently these are the |
| Scala standard library classes, as well as Flink’s own classes, logging APIs, and any Hadoop core classes.</li> |
| </ul> |
| <h3 id="my-datastream-application-produces-no-output-even-though-events-are-going-in"> |
| My DataStream application produces no output, even though events are going in. |
| <a class="anchor" href="#my-datastream-application-produces-no-output-even-though-events-are-going-in">#</a> |
| </h3> |
| <p>If your DataStream application uses <em>Event Time</em>, check that your watermarks get updated. If no watermarks are produced, |
| event time windows might never trigger, and the application would produce no results.</p> |
| <p>You can check in Flink’s web UI (watermarks section) whether watermarks are making progress.</p> |
| <h3 id="i-see-an-exception-reporting-insufficient-number-of-network-buffers"> |
| I see an exception reporting “Insufficient number of network buffers”. |
| <a class="anchor" href="#i-see-an-exception-reporting-insufficient-number-of-network-buffers">#</a> |
| </h3> |
| <p>If you run Flink with a very high parallelism, you may need to increase the number of network buffers.</p> |
| <p>By default, Flink takes 10% of the JVM heap size for network buffers, with a minimum of 64MB and a maximum of 1GB. |
| You can adjust all these values via <code>taskmanager.network.memory.fraction</code>, <code>taskmanager.network.memory.min</code>, and |
| <code>taskmanager.network.memory.max</code>.</p> |
| <p>Please refer to the <a href="https://nightlies.apache.org/flink/flink-docs-stable/docs/deployment/">Configuration Reference</a> for details.</p> |
| <h3 id="my-job-fails-with-various-exceptions-from-the-hdfshadoop-code-what-can-i-do"> |
| My job fails with various exceptions from the HDFS/Hadoop code. What can I do? |
| <a class="anchor" href="#my-job-fails-with-various-exceptions-from-the-hdfshadoop-code-what-can-i-do">#</a> |
| </h3> |
| <p>The most common cause for that is that the Hadoop version in Flink’s classpath is different than the |
| Hadoop version of the cluster you want to connect to (HDFS / YARN).</p> |
| <p>The easiest way to fix that is to pick a Hadoop-free Flink version and simply export the Hadoop path and |
| classpath from the cluster.</p> |
| </article> |
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