Merge pull request #2333 from langchain-ai/nc/apply-writes-order

lib: Enforce write application order in apply_writes

libs/langgraph/langgraph/pregel/__init__.py

@@ -941,7 +941,7 @@ class Pregel(PregelProtocol):
             if not writers:
                 raise InvalidUpdateError(f"Node {as_node} has no writers")
             writes: deque[tuple[str, Any]] = deque()
-            task = PregelTaskWrites(as_node, writes, [INTERRUPT])
+            task = PregelTaskWrites((), as_node, writes, [INTERRUPT])
             task_id = str(uuid5(UUID(checkpoint["id"]), INTERRUPT))
             run = RunnableSequence(*writers) if len(writers) > 1 else writers[0]
             # execute task
@@ -1121,7 +1121,7 @@ class Pregel(PregelProtocol):
             if not writers:
                 raise InvalidUpdateError(f"Node {as_node} has no writers")
             writes: deque[tuple[str, Any]] = deque()
-            task = PregelTaskWrites(as_node, writes, [INTERRUPT])
+            task = PregelTaskWrites((), as_node, writes, [INTERRUPT])
             task_id = str(uuid5(UUID(checkpoint["id"]), INTERRUPT))
             run = RunnableSequence(*writers) if len(writers) > 1 else writers[0]
             # execute task

libs/langgraph/langgraph/pregel/algo.py

@@ -67,6 +67,9 @@ class WritesProtocol(Protocol):
     """Protocol for objects containing writes to be applied to checkpoint.
     Implemented by PregelTaskWrites and PregelExecutableTask."""
 
+    @property
+    def path(self) -> tuple[Union[str, int], ...]: ...
+
     @property
     def name(self) -> str: ...
 
@@ -81,6 +84,7 @@ class PregelTaskWrites(NamedTuple):
     """Simplest implementation of WritesProtocol, for usage with writes that
     don't originate from a runnable task, eg. graph input, update_state, etc."""
 
+    path: tuple[Union[str, int], ...]
     name: str
     writes: Sequence[tuple[str, Any]]
     triggers: Sequence[str]
@@ -190,6 +194,9 @@ def apply_writes(
     """Apply writes from a set of tasks (usually the tasks from a Pregel step)
     to the checkpoint and channels, and return managed values writes to be applied
     externally."""
+    # sort tasks on path
+    tasks = sorted(tasks, key=lambda t: t.path)
+
     # update seen versions
     for task in tasks:
         checkpoint["versions_seen"].setdefault(task.name, {}).update(
@@ -444,7 +451,9 @@ def prepare_single_task(
                                 checkpoint,
                                 channels,
                                 managed,
-                                PregelTaskWrites(packet.node, writes, triggers),
+                                PregelTaskWrites(
+                                    task_path, packet.node, writes, triggers
+                                ),
                                 config,
                             ),
                             CONFIG_KEY_STORE: (
@@ -552,7 +561,7 @@ def prepare_single_task(
                                     checkpoint,
                                     channels,
                                     managed,
-                                    PregelTaskWrites(name, writes, triggers),
+                                    PregelTaskWrites(task_path, name, writes, triggers),
                                     config,
                                 ),
                                 CONFIG_KEY_STORE: (

libs/langgraph/langgraph/pregel/loop.py

@@ -477,7 +477,10 @@ class PregelLoop(LoopProtocol):
             mv_writes = apply_writes(
                 self.checkpoint,
                 self.channels,
-                [*discard_tasks.values(), PregelTaskWrites(INPUT, input_writes, [])],
+                [
+                    *discard_tasks.values(),
+                    PregelTaskWrites((), INPUT, input_writes, []),
+                ],
                 self.checkpointer_get_next_version,
             )
             assert not mv_writes, "Can't write to SharedValues in graph input"

libs/langgraph/tests/test_pregel.py

@@ -1787,11 +1787,11 @@ def test_concurrent_emit_sends() -> None:
         "0",
         "1",
         "1.1",
+        "3.1",
         "2|1",
         "2|2",
         "2|3",
         "2|4",
-        "3.1",
         "3",
     ]
 
@@ -1836,13 +1836,13 @@ def test_send_sequences() -> None:
     assert graph.invoke(["0"]) == [
         "0",
         "1",
+        "3.1",
         "2|Control(send=Send(node='2', arg=3))",
         "2|Control(send=Send(node='2', arg=4))",
-        "3.1",
+        "3",
         "2|3",
         "2|4",
         "3",
-        "3",
     ]
 
 

libs/langgraph/tests/test_pregel_async.py

@@ -2004,11 +2004,11 @@ async def test_concurrent_emit_sends() -> None:
         "0",
         "1",
         "1.1",
+        "3.1",
         "2|1",
         "2|2",
         "2|3",
         "2|4",
-        "3.1",
         "3",
     ]
 
@@ -2053,13 +2053,13 @@ async def test_send_sequences() -> None:
     assert await graph.ainvoke(["0"]) == [
         "0",
         "1",
+        "3.1",
         "2|Control(send=Send(node='2', arg=3))",
         "2|Control(send=Send(node='2', arg=4))",
-        "3.1",
+        "3",
         "2|3",
         "2|4",
         "3",
-        "3",
     ]