[SPARK-18657][SPARK-18668] Make StreamingQuery.id persists across restart and not auto-generate StreamingQuery.name

## What changes were proposed in this pull request?
Here are the major changes in this PR.
- Added the ability to recover `StreamingQuery.id` from checkpoint location, by writing the id to `checkpointLoc/metadata`.
- Added `StreamingQuery.runId` which is unique for every query started and does not persist across restarts. This is to identify each restart of a query separately (same as earlier behavior of `id`).
- Removed auto-generation of `StreamingQuery.name`. The purpose of name was to have the ability to define an identifier across restarts, but since id is precisely that, there is no need for a auto-generated name. This means name becomes purely cosmetic, and is null by default.
- Added `runId` to `StreamingQueryListener` events and `StreamingQueryProgress`.

Implementation details
- Renamed existing `StreamExecutionMetadata` to `OffsetSeqMetadata`, and moved it to the file `OffsetSeq.scala`, because that is what this metadata is tied to. Also did some refactoring to make the code cleaner (got rid of a lot of `.json` and `.getOrElse("{}")`).
- Added the `id` as the new `StreamMetadata`.
- When a StreamingQuery is created it gets or writes the `StreamMetadata` from `checkpointLoc/metadata`.
- All internal logging in `StreamExecution` uses `(name, id, runId)` instead of just `name`

TODO
- [x] Test handling of name=null in json generation of StreamingQueryProgress
- [x] Test handling of name=null in json generation of StreamingQueryListener events
- [x] Test python API of runId

## How was this patch tested?
Updated unit tests and new unit tests

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

Closes #16113 from tdas/SPARK-18657.

1 files changed, 17 insertions, 2 deletions

diff --git a/python/pyspark/sql/streaming.py b/python/pyspark/sql/streaming.py
index 4a7d17ba51..ee7a26d00d 100644
--- a/python/pyspark/sql/streaming.py
+++ b/python/pyspark/sql/streaming.py
@@ -51,14 +51,29 @@ class StreamingQuery(object):
     @property
     @since(2.0)
     def id(self):
-        """The id of the streaming query.
+        """Returns the unique id of this query that persists across restarts from checkpoint data.
+        That is, this id is generated when a query is started for the first time, and
+        will be the same every time it is restarted from checkpoint data.
+        There can only be one query with the same id active in a Spark cluster.
+        Also see, `runId`.
         """
         return self._jsq.id().toString()
 
     @property
+    @since(2.1)
+    def runId(self):
+        """Returns the unique id of this query that does not persist across restarts. That is, every
+        query that is started (or restarted from checkpoint) will have a different runId.
+        """
+        return self._jsq.runId().toString()
+
+    @property
     @since(2.0)
     def name(self):
-        """The name of the streaming query. This name is unique across all active queries.
+        """Returns the user-specified name of the query, or null if not specified.
+        This name can be specified in the `org.apache.spark.sql.streaming.DataStreamWriter`
+        as `dataframe.writeStream.queryName("query").start()`.
+        This name, if set, must be unique across all active queries.
         """
         return self._jsq.name()