Github user steveloughran commented on a diff in the pull request:
https://github.com/apache/spark/pull/14731#discussion_r94407382
--- Diff: docs/streaming-programming-guide.md ---
@@ -644,17 +644,90 @@ 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
+ Spark Streaming will monitor the directory `dataDirectory` and process
any files created in that directory.
+
+ ++ The files must have the same data format.
+ + A simple directory can be monitored, such as
`hdfs://namenode:8040/logs/`.
+ All files directly such a path will be processed as they are
discovered.
+ + A POSIX glob pattern can be supplied, 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. Only the files in the directory whose
modification time is
+ within the current window will be included in the stream.
+ + Once processed, changes to a file within the current window 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.
+ + Calling `FileSystem.setTimes()` to fix the timestamp is a way to
have the file picked
+ up in a later window, even if its contents have not changed.
- + 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.
For simple text files, there is an easier method
`streamingContext.textFileStream(dataDirectory)`. And file streams do not
require running a receiver, hence does not require allocating cores.
<span class="badge" style="background-color: grey">Python API</span>
`fileStream` is not available in the Python API, only `textFileStream` is
available.
+
+ Special points for HDFS
--- End diff --
done
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