diff --git a/permchain/pubsub.py b/permchain/pubsub.py
index 0d9408283..b9ce6efbe 100644
--- a/permchain/pubsub.py
+++ b/permchain/pubsub.py
@@ -1,9 +1,9 @@
 from __future__ import annotations
 
 from abc import ABC
+from collections import defaultdict
 from concurrent.futures import CancelledError, Future
 from functools import partial
-from itertools import groupby
 from typing import Any, Iterator, List, Optional, Sequence, Set, TypeVar
 
 from langchain.callbacks.manager import CallbackManagerForChainRun
@@ -39,10 +39,6 @@ class PubSub(Runnable[Any, Any], ABC):
         self.processes = processes
         self.connection = connection
 
-    class Config:
-        arbitrary_types_allowed = True
-        revalidate_instances = False
-
     def with_retry(self, **kwargs: Any) -> Runnable[Any, Any]:
         return self.__class__(
             processes=[p.with_retry(**kwargs) for p in self.processes],
@@ -55,13 +51,13 @@ class PubSub(Runnable[Any, Any], ABC):
         run_manager: CallbackManagerForChainRun,
         config: RunnableConfig,
     ) -> Iterator[Any]:
-        subscribers: list[RunnableSubscriber[Any]] = []
-        reducers: list[RunnableReducer[Any]] = []
+        subscribers: defaultdict[str, list[RunnableSubscriber[Any]]] = defaultdict(list)
+        reducers: defaultdict[str, list[RunnableReducer[Any]]] = defaultdict(list)
         for process in self.processes:
             if isinstance(process, RunnableReducer):
-                reducers.append(process)
+                reducers[process.topic.name].append(process)
             elif isinstance(process, RunnableSubscriber):
-                subscribers.append(process)
+                subscribers[process.topic.name].append(process)
             else:
                 raise ValueError(f"Unknown process type: {process}")
 
@@ -74,7 +70,7 @@ class PubSub(Runnable[Any, Any], ABC):
                 input_value += chunk
 
         with get_executor_for_config(config) as executor:
-            # Namespace topics for each run, default to run_id, ie. isolate runs
+            # Namespace topics for each run, default to run_id, ie. isolated
             topic_prefix = str(config.get("state_id") or run_manager.run_id)
             # Track inflight futures
             inflight: Set[Future] = set()
@@ -82,17 +78,19 @@ class PubSub(Runnable[Any, Any], ABC):
             exceptions: List[Exception] = []
 
             def on_idle():
+                """Called when all subscribed topics are empty.
+                It first runs any topic reducers. Then, if all subscribed topics
+                still empty, it closes the computation.
+                """
                 if reducers:
-                    reducers_by_topic = groupby(
-                        sorted(reducers, key=lambda p: p.topic.name),
-                        lambda p: p.topic.name,
-                    )
-                    for topic_name, processes in reducers_by_topic:
+                    for topic_name, processes in reducers.items():
+                        # Collect all pending messages for each topic
                         messages = list(
                             self.connection.iterate(
                                 topic_prefix, topic_name, wait=False
                             )
                         )
+                        # Run each reducer once with the collected messages
                         if messages:
                             for process in processes:
                                 run_once(process, messages)
@@ -100,28 +98,28 @@ class PubSub(Runnable[Any, Any], ABC):
                 if not inflight:
                     self.connection.disconnect(topic_prefix)
 
+            def check_if_idle(fut: Future) -> None:
+                """Cleanup after a process runs."""
+                inflight.discard(fut)
+
+                try:
+                    exc = fut.exception()
+                except CancelledError:
+                    exc = None
+                except Exception as e:
+                    exc = e
+                if exc is not None:
+                    exceptions.append(exc)
+
+                # Close output iterator if
+                # - all processes are done, or
+                # - an exception occurred
+                if not inflight or exc is not None:
+                    on_idle()
+
             def run_once(process: RunnableSubscriber[Any], value: Any) -> None:
                 """Run a process once."""
 
-                def cleanup_run(fut: Future) -> None:
-                    """Cleanup after a process runs."""
-                    inflight.discard(fut)
-
-                    try:
-                        exc = fut.exception()
-                    except CancelledError:
-                        exc = None
-                    except Exception as e:
-                        exc = e
-                    if exc is not None:
-                        exceptions.append(exc)
-
-                    # Close output iterator if
-                    # - all processes are done, or
-                    # - an exception occurred
-                    if not inflight or exc is not None:
-                        on_idle()
-
                 def get(topic_name: str) -> Any:
                     if topic_name == INPUT_TOPIC:
                         return input_value
@@ -149,14 +147,10 @@ class PubSub(Runnable[Any, Any], ABC):
 
                 # Add callback to cleanup
                 inflight.add(fut)
-                fut.add_done_callback(cleanup_run)
+                fut.add_done_callback(check_if_idle)
 
             # Listen on all subscribed topics
-            processes_by_topic = groupby(
-                sorted(subscribers, key=lambda p: p.topic.name),
-                lambda p: p.topic.name,
-            )
-            for topic_name, processes in processes_by_topic:
+            for topic_name, processes in subscribers.items():
                 self.connection.listen(
                     topic_prefix,
                     topic_name,
diff --git a/tests/test_invoke.py b/tests/test_invoke.py
index 967f90284..9aaa67ba4 100644
--- a/tests/test_invoke.py
+++ b/tests/test_invoke.py
@@ -171,7 +171,7 @@ def test_invoke_two_processes_two_in_two_out(mocker: MockerFixture):
 
 def test_invoke_two_processes_two_in_join_two_out(mocker: MockerFixture):
     add_one = mocker.Mock(side_effect=lambda x: x + 1)
-    add_10_each = mocker.Mock(side_effect=lambda x: [y + 10 for y in x])
+    add_10_each = mocker.Mock(side_effect=lambda x: sorted(y + 10 for y in x))
     topic_one = Topic("one")
     topic_two = Topic("two")
     chain_one = Topic.IN.subscribe() | add_one | topic_one.publish()