feat: Persist Variables for Enhanced Debugging Workflow (#20699)

This pull request introduces a feature aimed at improving the debugging experience during workflow editing. With the addition of variable persistence, the system will automatically retain the output variables from previously executed nodes. These persisted variables can then be reused when debugging subsequent nodes, eliminating the need for repetitive manual input.

By streamlining this aspect of the workflow, the feature minimizes user errors and significantly reduces debugging effort, offering a smoother and more efficient experience.

Key highlights of this change:

- Automatic persistence of output variables for executed nodes.
- Reuse of persisted variables to simplify input steps for nodes requiring them (e.g., `code`, `template`, `variable_assigner`).
- Enhanced debugging experience with reduced friction.

Closes #19735.

api/core/workflow/nodes/base/node.py

@@ -1,7 +1,7 @@
 import logging
 from abc import abstractmethod
 from collections.abc import Generator, Mapping, Sequence
-from typing import TYPE_CHECKING, Any, Generic, Optional, TypeVar, Union, cast
+from typing import TYPE_CHECKING, Any, ClassVar, Generic, Optional, TypeVar, Union, cast
 
 from core.workflow.entities.node_entities import NodeRunResult
 from core.workflow.entities.workflow_node_execution import WorkflowNodeExecutionStatus
@@ -23,7 +23,7 @@ GenericNodeData = TypeVar("GenericNodeData", bound=BaseNodeData)
 
 class BaseNode(Generic[GenericNodeData]):
     _node_data_cls: type[GenericNodeData]
-    _node_type: NodeType
+    _node_type: ClassVar[NodeType]
 
     def __init__(
         self,
@@ -90,8 +90,38 @@ class BaseNode(Generic[GenericNodeData]):
         graph_config: Mapping[str, Any],
         config: Mapping[str, Any],
     ) -> Mapping[str, Sequence[str]]:
-        """
-        Extract variable selector to variable mapping
+        """Extracts references variable selectors from node configuration.
+
+        The `config` parameter represents the configuration for a specific node type and corresponds
+        to the `data` field in the node definition object.
+
+        The returned mapping has the following structure:
+
+            {'1747829548239.#1747829667553.result#': ['1747829667553', 'result']}
+
+        For loop and iteration nodes, the mapping may look like this:
+
+            {
+                "1748332301644.input_selector": ["1748332363630", "result"],
+                "1748332325079.1748332325079.#sys.workflow_id#": ["sys", "workflow_id"],
+            }
+
+        where `1748332301644` is the ID of the loop / iteration node,
+        and `1748332325079` is the ID of the node inside the loop or iteration node.
+
+        Here, the key consists of two parts: the current node ID (provided as the `node_id`
+        parameter to `_extract_variable_selector_to_variable_mapping`) and the variable selector,
+        enclosed in `#` symbols. These two parts are separated by a dot (`.`).
+
+        The value is a list of string representing the variable selector, where the first element is the node ID
+        of the referenced variable, and the second element is the variable name within that node.
+
+        The meaning of the above response is:
+
+        The node with ID `1747829548239` references the variable `result` from the node with
+        ID `1747829667553`. For example, if `1747829548239` is a LLM node, its prompt may contain a
+        reference to the `result` output variable of node `1747829667553`.
+
         :param graph_config: graph config
         :param config: node config
         :return:
@@ -101,9 +131,10 @@ class BaseNode(Generic[GenericNodeData]):
             raise ValueError("Node ID is required when extracting variable selector to variable mapping.")
 
         node_data = cls._node_data_cls(**config.get("data", {}))
-        return cls._extract_variable_selector_to_variable_mapping(
+        data = cls._extract_variable_selector_to_variable_mapping(
             graph_config=graph_config, node_id=node_id, node_data=cast(GenericNodeData, node_data)
         )
+        return data
 
     @classmethod
     def _extract_variable_selector_to_variable_mapping(
@@ -139,6 +170,16 @@ class BaseNode(Generic[GenericNodeData]):
         """
         return self._node_type
 
+    @classmethod
+    @abstractmethod
+    def version(cls) -> str:
+        """`node_version` returns the version of current node type."""
+        # NOTE(QuantumGhost): This should be in sync with `NODE_TYPE_CLASSES_MAPPING`.
+        #
+        # If you have introduced a new node type, please add it to `NODE_TYPE_CLASSES_MAPPING`
+        # in `api/core/workflow/nodes/__init__.py`.
+        raise NotImplementedError("subclasses of BaseNode must implement `version` method.")
+
     @property
     def should_continue_on_error(self) -> bool:
         """judge if should continue on error