= Data Locality / Placement
Spark relies on data locality, aka data placement or proximity to data source, that makes Spark jobs sensitive to where the data is located. It is therefore important to have yarn/README.md[Spark running on Hadoop YARN cluster] if the data comes from HDFS.
In yarn/README.md[Spark on YARN] Spark tries to place tasks alongside HDFS blocks.
With HDFS the Spark driver contacts NameNode about the DataNodes (ideally local) containing the various blocks of a file or directory as well as their locations (represented as
InputSplits), and then schedules the work to the SparkWorkers.
Spark's compute nodes / workers should be running on storage nodes.
Concept of locality-aware scheduling.
Spark tries to execute tasks as close to the data as possible to minimize data transfer (over the wire).
.Locality Level in the Spark UI image::sparkui-stages-locality-level.png
There are the following task localities (consult https://spark.apache.org/docs/latest/api/scala/index.html#org.apache.spark.scheduler.TaskLocality$[org.apache.spark.scheduler.TaskLocality] object):
Task location can either be a host or a pair of a host and an executor.