Added some comments and docstrings

exasol/analytics/query_handler/graph/execution_graph.py

@@ -8,6 +8,11 @@
 
 
 class ExecutionGraph(Generic[T]):
+    """
+    The class is a generic implementation of a directed acyclic graph, with nodes
+    of type T.
+    The graph provides a node traverse order.
+    """
 
     def __init__(self, start_node: T, end_node: T, edges: Set[Tuple[T, T]]):
         self._graph = nx.DiGraph()

exasol/analytics/query_handler/graph/stage/sql/dataset.py

@@ -13,6 +13,9 @@ class Dataset:
 
     The TableLike refers to a database table containing the actual data that
     can be used for instance in training or testing.
+
+    Q. A TableLike is basically a list of columns and a name. Why do we have
+    a separate list of columns here?
     """
     table_like: TableLike
     identifier_columns: List[Column]

exasol/analytics/query_handler/graph/stage/sql/execution/input.py

@@ -10,6 +10,12 @@
 
 @dataclasses.dataclass(frozen=True, eq=True)
 class SQLStageGraphExecutionInput:
+    """
+    The class is an input for the graph query handler (not to be confused with
+    a node query handler provided by a user).
+    It includes the input data for the root node of the graph, the place in the BucketFS
+    where the result should be stored and the graph itself.
+    """
     input: SQLStageInputOutput
     result_bucketfs_location: bfs.path.PathLike
     sql_stage_graph: SQLStageGraph

exasol/analytics/query_handler/graph/stage/sql/execution/query_handler.py

@@ -24,6 +24,11 @@
 class SQLStageGraphExecutionQueryHandler(
     QueryHandler[SQLStageGraphExecutionInput, SQLStageInputOutput]
 ):
+    """
+    Implementation of the execution graph QueryHandler. Each node of the graph contains
+    its own, user provided, query handler.
+    """
+
     def __init__(
         self,
         parameter: SQLStageGraphExecutionInput,
@@ -34,12 +39,20 @@ def __init__(
         self._state = query_handler_state_factory(parameter, query_handler_context)
 
     def start(self) -> Union[Continue, Finish[SQLStageInputOutput]]:
+        # Start traversing the graph from the root node.
         result = self._run_until_continue_or_last_stage_finished()
         return result
 
     def handle_query_result(
         self, query_result: QueryResult
     ) -> Union[Continue, Finish[SQLStageInputOutput]]:
+        """
+        Here we call the handle_query_result() of the query handler at the current node.
+        If this handler returns a Finish object then we proceed to the next node, if
+        there is any, and start traversing the graph from there (see
+        _run_until_continue_or_last_stage_finished()). Otherwise, if the returned object
+        is Continue, we stay on the same node and return the result.
+        """
         result = self._state.get_current_query_handler().handle_query_result(
             query_result
         )
@@ -57,6 +70,13 @@ def handle_query_result(
     def _run_until_continue_or_last_stage_finished(
         self,
     ) -> Union[Continue, Finish[SQLStageInputOutput]]:
+        """
+        We start traversing the graph from the current node by calling the start() on
+        its query handler. If the handler returns a Continue object we will exit the
+        loop and return the result. Otherwise, if the handler returns a Finish object,
+        we will move to the next node and call the start() on its query handler. We will
+        carry on until a Continue object is returned or the whole graph is traversed.
+        """
         while True:
             handler = self._state.get_current_query_handler()
             result = handler.start()

exasol/analytics/query_handler/graph/stage/sql/execution/query_handler_state.py

@@ -17,9 +17,8 @@
 )
 from exasol.analytics.query_handler.graph.stage.sql.sql_stage import SQLStage
 from exasol.analytics.query_handler.graph.stage.sql.sql_stage_query_handler import (
-    SQLStageQueryHandlerInput,
+    SQLStageQueryHandlerInput, SQLStageQueryHandler
 )
-from exasol.analytics.query_handler.query_handler import QueryHandler
 from exasol.analytics.query_handler.result import Continue, Finish
 from exasol.analytics.utils.errors import UninitializedAttributeError
 
@@ -53,15 +52,13 @@ def __init__(
             self._current_stage_index
         ]
         self._stage_inputs_map[self._current_stage].append(parameter.input)
-        self._current_query_handler: Optional[
-            QueryHandler[List[SQLStageInputOutput], SQLStageInputOutput]
-        ] = None
+        self._current_query_handler: Optional[SQLStageQueryHandler] = None
         self._current_qh_context: Optional[ScopeQueryHandlerContext] = None
         self._create_current_query_handler()
 
     def get_current_query_handler(
         self,
-    ) -> QueryHandler[List[SQLStageInputOutput], SQLStageInputOutput]:
+    ) -> SQLStageQueryHandler:
         value = self._current_query_handler
         if value is None:
             raise RuntimeError("No current query handler set.")
@@ -86,6 +83,16 @@ def _checked_current_stage(self) -> SQLStage:
     def handle_result(
         self, result: Union[Continue, Finish[SQLStageInputOutput]]
     ) -> ResultHandlerReturnValue:
+        """
+        Here we look at the provided last result obtained at the current node.
+
+        If it is encapsulated in a Finish object we then either save the result so that
+        it can be used as an input at the next node, or take it as the final output in
+        case this node is the last one. In the former case we move to the next node and
+        return CONTINUE_PROCESSING. In the latter case we return RETURN_RESULT.
+
+        If the result is encapsulated in a Continue object we return RETURN_RESULT.
+        """
         # check if current query handler is set
         self.get_current_query_handler()
         if isinstance(result, Finish):
@@ -98,6 +105,11 @@ def handle_result(
     def _handle_finished_result(
         self, result: Finish[SQLStageInputOutput]
     ) -> ResultHandlerReturnValue:
+        """
+        We are done with the current node.
+        If there are successors we need to add the final output from this node to the
+        list of inputs of all the successors.
+        """
         if self._is_not_last_stage():
             self._add_result_to_successors(result.result)
         else:

exasol/analytics/query_handler/graph/stage/sql/input_output.py

@@ -8,16 +8,21 @@
 @dataclass(frozen=True)
 class SQLStageInputOutput:
     """
-    A SQLStageInputOutput is used as input and output between the SQLStageQueryHandler.
-    It contains a dataset and dependencies. The dataset is used to represent train and test data.
-    The dependencies can be used to communicate any data to the subsequently stages.
-    For example, a dependency could be a table which the previous stage computed and
-    the subsequent one uses.
+    This is a type root for a class representing an input/output data for a customer
+    provided node-level query handler - SQLStageQueryHandler. The actual content of
+    the input/output is application specific.
     """
     pass
 
 
 @dataclass(frozen=True)
 class MultiDatasetSQLStageInputOutput(SQLStageInputOutput):
+    """
+    An implementation of SQLStageInputOutput holding a collection of datasets and
+    dependencies. Can be used, for example, for model training. The datasets may be
+    used to represent train and test data. The dependencies can be used to communicate
+    any data to the subsequently stages. For example, a dependency could be a table
+    which the previous stage computed and the subsequent one uses.
+    """
     datasets: Dict[object, Dataset]
     dependencies: Dependencies = field(default_factory=dict)

exasol/analytics/query_handler/graph/stage/sql/sql_stage.py

@@ -9,6 +9,12 @@
 
 
 class SQLStage(Stage):
+    """
+    This is a node of an ExecutionGraph.
+    Essentially, this is a node-level query handler factory. The query handler
+    itself is user-provided and so is this factory.
+    """
+
     @abc.abstractmethod
     def create_query_handler(
         self,

exasol/analytics/query_handler/graph/stage/sql/sql_stage_graph.py

@@ -2,3 +2,6 @@
 from exasol.analytics.query_handler.graph.stage.sql.sql_stage import SQLStage
 
 SQLStageGraph = ExecutionGraph[SQLStage]
+"""
+A directed acyclic graph with nodes of type SQLStage.
+"""

exasol/analytics/query_handler/graph/stage/sql/sql_stage_query_handler.py

@@ -18,15 +18,28 @@ def is_empty(obj: Sized):
 
 @dataclasses.dataclass(eq=True)
 class SQLStageQueryHandlerInput:
+    """
+    A Parameter type for the SQLStageQueryHandler.
+    Contains a list of Input/Output objects and a location in the BucketFS for
+    persisting the data if needed.
+
+    Why is the list? Because the input data for a node can come from multiple
+    predecessors.
+    """
     sql_stage_inputs: List[SQLStageInputOutput]
     result_bucketfs_location: AbstractBucketFSLocation
 
     def __post_init__(self):
         if is_empty(self.sql_stage_inputs):
-            raise AssertionError("Empty sql_stage_inputs not allowed.")
+            raise AssertionError("Empty sql_stage_inputs are not allowed.")
 
 
 class SQLStageQueryHandler(
     QueryHandler[SQLStageQueryHandlerInput, SQLStageInputOutput], ABC
 ):
+    """
+    The base class for a QueryHandler of a single node. The implementation shall be
+    provided by the user. See the UDFQueryHandler and the user_guide.md for details
+    and guidelines on how to build a query handler.
+    """
     pass