Github user pwendell commented on a diff in the pull request:
https://github.com/apache/spark/pull/3419#discussion_r20773446
--- Diff:
streaming/src/main/scala/org/apache/spark/streaming/dstream/FileInputDStream.scala
---
@@ -17,18 +17,55 @@
package org.apache.spark.streaming.dstream
-import java.io.{ObjectInputStream, IOException}
-import scala.collection.mutable.{HashSet, HashMap}
+import java.io.{IOException, ObjectInputStream}
+
+import scala.collection.mutable
import scala.reflect.ClassTag
+
import org.apache.hadoop.fs.{FileSystem, Path, PathFilter}
-import org.apache.hadoop.conf.Configuration
import org.apache.hadoop.mapreduce.{InputFormat => NewInputFormat}
-import org.apache.spark.rdd.RDD
-import org.apache.spark.rdd.UnionRDD
-import org.apache.spark.streaming.{StreamingContext, Time}
-import org.apache.spark.util.{TimeStampedHashMap, Utils}
+import org.apache.spark.rdd.{RDD, UnionRDD}
+import org.apache.spark.streaming._
+import org.apache.spark.util.{TimeStampedHashMap, Utils}
+/**
+ * This class represents an input stream that monitors a Hadoop-compatible
filesystem for new
+ * files and creates a stream out of them. The way it works as follows.
+ *
+ * At each batch interval, the file system is queries for files in the
given directory and
+ * detected new files are selected for that batch. In this case "new"
means files that
+ * became visible to readers during that time period. Some extra care is
needed to deal
+ * with the fact that files may become visible after they are created. For
this purpose, this
+ * class remembers the information about the files selected in past
batches for
+ * a certain duration (say, "remember window") as shown in the figure
below.
+ *
+ * |<----- remember window ----->|
+ * ignore threshold --->| |<--- current batch
time
+ * |____.____.____.____.____.____|
+ * | | | | | | |
+ *
---------------------|----|----|----|----|----|----|----------------------->
Time
+ * |____|____|____|____|____|____|
+ * remembered batches
+ *
+ * The trailing end of the window is the "ignore threshold" and all files
whose mod times
+ * are less than this threshold are assumed to have already been selected
and are therefore
+ * ignored. Files whose mode times are within the "remember window" are
checked against files
+ * that have already been selected. At a high level, this is how new files
are identified in
+ * each batch - files whose mod times are greater than the ignore
threshold and
+ * have not been considered within the remember window. See the
documentation on the method
+ * `isNewFile` for more details.
+ *
+ * This makes some assumptions from the underlying file system that the
system is monitoring.
+ * - The clock of the file system is assumed to synchronized with the
clock of the machine running
+ * the streaming app.
+ * - If a file is to be visible in the directory listings, it must be
visible within a certain
+ * duration of the mod time of the file. This duration is the "remember
window", which is set to
+ * 1 minute (see `FileInputDStream.REMEMBER_DURATION`). Otherwise, the
file will not be
+ * selected as the mod time will be less than the ignore threshold when
it become visible.
--- End diff --
to make it a bit more clear maybe say "the fill will never be selected"
rather than "will not be selected"
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