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title: FAQ
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<h2>Spark FAQ</h2>
<p class="question">How does Spark relate to Hadoop?</p>
<p class="answer">
Spark is a fast and general processing engine compatible with Hadoop data. It can run in Hadoop clusters through YARN or Spark's standalone mode, and it can process data in HDFS, HBase, Cassandra, Hive, and any Hadoop InputFormat. It is designed to perform both batch processing (similar to MapReduce) and new workloads like streaming, interactive queries, and machine learning.
</p>
<p class="question">Which languages does Spark support?</p>
<p class="answer">Spark supports Scala, Java and Python.</p>
<p class="question">What is the largest data size Spark can scale to?</p>
<p class="answer">Spark has been shown to work well from megabytes of data to petabytes. It has been used to sort 100 TB of data 3X faster than Hadoop MapReduce on 1/10th of the machines, <a href="http://databricks.com/blog/2014/11/05/spark-officially-sets-a-new-record-in-large-scale-sorting.html">winning the 2014 Daytona GraySort Benchmark</a>. It has also been used to <a href="http://databricks.com/blog/2014/10/10/spark-petabyte-sort.html">sort 1 PB of data</a>. There are also production workloads that <a href="http://databricks.com/blog/2014/08/14/mining-graph-data-with-spark-at-alibaba-taobao.html">use Spark to do ETL and data analysis on PBs of data</a>.
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<p class="question">How large a cluster can Spark scale to?</p>
<p class="answer">Many organizations run Spark on clusters with thousands of nodes. The largest cluster we know has over 8000 nodes.</p>
<p class="question">What happens if my dataset does not fit in memory?</p>
<p class="answer">Often each partition of data is small and does fit in memory, and these partitions are processed a few at a time. For very large partitions that do not fit in memory, Spark's built-in operators perform external operations on datasets.</p>
<p class="question">What happens when a cached dataset does not fit in memory?</p>
<p class="answer">Spark can either spill it to disk or recompute the partitions that don't fit in RAM each time they are requested. By default, it uses recomputation, but you can set a dataset's <a href="{{site.url}}docs/latest/scala-programming-guide.html#rdd-persistence">storage level</a> to <code>MEMORY_AND_DISK</code> to avoid this. </p>
<p class="question">How can I run Spark on a cluster?</p>
<p class="answer">You can use either the <a href="{{site.url}}docs/latest/spark-standalone.html">standalone deploy mode</a>, which only needs Java to be installed on each node, or the <a href="{{site.url}}docs/latest/running-on-mesos.html">Mesos</a> and <a href="{{site.url}}docs/latest/running-on-yarn.html">YARN</a> cluster managers. If you'd like to run on Amazon EC2, Spark provides <a href="{{site.url}}docs/latest/ec2-scripts.html}}">EC2 scripts</a> to automatically launch a cluster.</p>
<p>Note that you can also run Spark locally (possibly on multiple cores) without any special setup by just passing <code>local[N]</code> as the master URL, where <code>N</code> is the number of parallel threads you want.</p>
<p class="question">Do I need Hadoop to run Spark?</p>
<p class="answer">No, but if you run on a cluster, you will need some form of shared file system (for example, NFS mounted at the same path on each node). If you have this type of filesystem, you can just deploy Spark in standalone mode.</p>
<p class="question">How can I access data in S3?</p>
<p class="answer">Use the <code>s3n://</code> URI scheme (<code>s3n://bucket/path</code>). You will also need to set your Amazon security credentials, either by setting the environment variables <code>AWS_ACCESS_KEY_ID</code> and <code>AWS_SECRET_ACCESS_KEY</code> before your program runs, or by setting <code>fs.s3.awsAccessKeyId</code> and <code>fs.s3.awsSecretAccessKey</code> in <code>SparkContext.hadoopConfiguration</code>.</p>
<p class="question">Does Spark require modified versions of Scala or Python?</p>
<p class="answer">No. Spark requires no changes to Scala or compiler plugins. The Python API uses the standard CPython implementation, and can call into existing C libraries for Python such as NumPy.</p>
<p class="question">What are good resources for learning Scala?</p>
<p class="answer">Check out <a href="http://www.artima.com/scalazine/articles/steps.html">First Steps to Scala</a> for a quick introduction, the <a href="http://www.scala-lang.org/docu/files/ScalaTutorial.pdf">Scala tutorial for Java programmers</a>, or the free online book <a href="http://www.artima.com/pins1ed/">Programming in Scala</a>. Scala is easy to transition to if you have Java experience or experience in a similarly high-level language (e.g. Ruby).</p>
<p>In addition, Spark also has <a href="{{site.url}}docs/latest/java-programming-guide.html">Java</a> and <a href="{{site.url}}docs/latest/python-programming-guide.html">Python</a> APIs.</p>
<p class="question">What license is Spark under?</p>
<p class="answer">Starting in version 0.8, Spark is under the <a href="http://www.apache.org/licenses/LICENSE-2.0.html">Apache 2.0 license</a>. Previous versions used the <a href="https://github.com/mesos/spark/blob/branch-0.7/LICENSE">BSD license</a>.</p>
<p class="question">How can I contribute to Spark?</p>
<p class="answer">Contact the <a href="{{site.url}}community.html">mailing list</a> or send us a pull request on <a href="https://github.com/apache/spark">GitHub</a> (instructions <a href="https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark">here</a>). We're glad to hear about your experience using Spark and to accept patches.</p>
<p>If you would like to report an issue, post it to the <a href="https://issues.apache.org/jira/browse/SPARK">Spark issue tracker</a>.</p>
<p class="question">Where can I get more help?</p>
<p class="answer">Please post on the <a href="http://apache-spark-user-list.1001560.n3.nabble.com">Spark Users</a> mailing list. We'll be glad to help!</p>
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