Github user steveloughran commented on a diff in the pull request:
https://github.com/apache/spark/pull/14731#discussion_r79209668
--- Diff: docs/streaming-programming-guide.md ---
@@ -644,13 +644,44 @@ methods for creating DStreams from files as input
sources.
</div>
</div>
- Spark Streaming will monitor the directory `dataDirectory` and process
any files created in that directory (files written in nested directories not
supported). Note that
-
- + The files must have the same data format.
- + The files must be created in the `dataDirectory` by atomically
*moving* or *renaming* them into
- the data directory.
- + Once moved, the files must not be changed. So if the files are
being continuously appended, the new data will not be read.
-
+ Spark Streaming will monitor the directory `dataDirectory` and process
any files created in that directory.
+
+ + A simple directory can be supplied, such as
`hdfs://namenode:8040/logs/`.
+ All files directly such a path will be processed as they are
discovered.
+ + A regular expression can be supplied instead, such as
+ `hdfs://namenode:8040/logs/2016-*-31`.
+ Here, the DStream will consist of all files directly under those
directories
+ matching the regular expression.
+ That is: it is a pattern of directories, not of files in
directories.
+ + All files must be in the same data format.
+ * A file is considered part of a time period based on its
modification time
+ ânot its creation time.
+ + Files must be created in/moved under the `dataDirectory`
directory/directories by
+ an atomic operation. In HDFS and similar filesystems, this can be
done *renaming* them
+ into the data directory from another part of the same filesystem.
+ * If a wildcard is used to identify directories, such as
`hdfs://namenode:8040/logs/2016*`,
+ renaming an entire directory to match the path will add the
directory to the list of
+ monitored directories. However, unless the modification time of the
directory's files
+ are within that of the current window, they will not be recognized
as new files.
+ + Once processed, changes to a file will not cause the file to be
reread.
+ That is: Updates are ignored.
+ + The more files under a directory/wildcard pattern, the longer it
will take to
+ scan for changes âeven if no files have actually changed.
+
+ Special points for object stores
--- End diff --
For object stores, direct writes to the directory, resulting in a PUT on
close(), will guarantee that a file is picked up immediately. Things are
actually a bit quirky for HDFS; even file length doesn't get updated reliably
during a write-in-progress. I'll add a section there and then ask people who
understand HDFS what is really happening
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