[SPARK-15517][SQL][STREAMING] Add support for complete output mode in Structure Streaming

## What changes were proposed in this pull request?
Currently structured streaming only supports append output mode.  This PR adds the following.

- Added support for Complete output mode in the internal state store, analyzer and planner.
- Added public API in Scala and Python for users to specify output mode
- Added checks for unsupported combinations of output mode and DF operations
  - Plans with no aggregation should support only Append mode
  - Plans with aggregation should support only Update and Complete modes
  - Default output mode is Append mode (**Question: should we change this to automatically set to Complete mode when there is aggregation?**)
- Added support for Complete output mode in Memory Sink. So Memory Sink internally supports append and complete, update. But from public API only Complete and Append output modes are supported.

## How was this patch tested?
Unit tests in various test suites
- StreamingAggregationSuite: tests for complete mode
- MemorySinkSuite: tests for checking behavior in Append and Complete modes.
- UnsupportedOperationSuite: tests for checking unsupported combinations of DF ops and output modes
- DataFrameReaderWriterSuite: tests for checking that output mode cannot be called on static DFs
- Python doc test and existing unit tests modified to call write.outputMode.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #13286 from tdas/complete-mode.

1 files changed, 20 insertions, 0 deletions

diff --git a/python/pyspark/sql/readwriter.py b/python/pyspark/sql/readwriter.py
index 73105f881b..9208a527d2 100644
--- a/python/pyspark/sql/readwriter.py
+++ b/python/pyspark/sql/readwriter.py
@@ -497,6 +497,26 @@ class DataFrameWriter(object):
             self._jwrite = self._jwrite.mode(saveMode)
         return self
 
+    @since(2.0)
+    def outputMode(self, outputMode):
+        """Specifies how data of a streaming DataFrame/Dataset is written to a streaming sink.
+
+        Options include:
+
+        * `append`:Only the new rows in the streaming DataFrame/Dataset will be written to
+           the sink
+        * `complete`:All the rows in the streaming DataFrame/Dataset will be written to the sink
+           every time these is some updates
+
+       .. note:: Experimental.
+
+        >>> writer = sdf.write.outputMode('append')
+        """
+        if not outputMode or type(outputMode) != str or len(outputMode.strip()) == 0:
+            raise ValueError('The output mode must be a non-empty string. Got: %s' % outputMode)
+        self._jwrite = self._jwrite.outputMode(outputMode)
+        return self
+
     @since(1.4)
     def format(self, source):
         """Specifies the underlying output data source.