[Docs] Add explanation for error handling when branching (#591)

examples/branching.ipynb

@@ -50,6 +50,7 @@
     "    # The operator.add reducer fn makes this append-only\n",
     "    aggregate: Annotated[list, operator.add]\n",
     "\n",
+    "\n",
     "from typing import Any\n",
     "\n",
     "\n",
@@ -61,6 +62,7 @@
     "        print(f\"Adding {self._value} to {state['aggregate']}\")\n",
     "        return {\"aggregate\": [self._value]}\n",
     "\n",
+    "\n",
     "builder = StateGraph(State)\n",
     "builder.add_node(\"a\", ReturnNodeValue(\"I'm A\"))\n",
     "builder.set_entry_point(\"a\")\n",
@@ -126,7 +128,23 @@
     }
    ],
    "source": [
-    "graph.invoke({\"aggregate\": []})"
+    "graph.invoke({\"aggregate\": []}, {\"configurable\": {\"thread_id\": \"foo\"}})"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "c392b3d2",
+   "metadata": {},
+   "source": [
+    "<details class=\"note\"> <summary>Exception handling?</summary>\n",
+    " <p>LangGraph executes nodes within <a href=\"https://langchain-ai.github.io/langgraph/concepts/#core-design\">\"supersteps\"</a>, meaning that while parallel branches are executed in parallel, the entire superstep is <b>transactional</b>. If any of these branches raises an exception, <b>none</b> of the updates are applied to the state (the entire superstep errors).<br><br>\n",
+    " If you have error-prone (perhaps want to handle flakey API calls), LangGraph provides two ways to address this:<br>\n",
+    " <ol>\n",
+    "  <li>You can write regular python code within your node to catch and handle exceptions.</li>\n",
+    "  <li>You can set a <b><a href=\"https://langchain-ai.github.io/langgraph/reference/graphs/#langgraph.graph.graph.CompiledGraph.retry_policy\">retry_policy</a></b> to direct the graph to retry nodes that raise certain types of exceptions. Only failing branches are retried, so you needn't worry about performing redundant work.</li>\n",
+    "</ol></p>\n",
+    "Together, these let you perform parallel execution and fully control exception handling.\n",
+    "</details>"
    ]
   },
   {
@@ -241,7 +259,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 6,
+   "execution_count": 7,
    "id": "95f5e026",
    "metadata": {},
    "outputs": [],
@@ -290,7 +308,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 7,
+   "execution_count": 8,
    "id": "1d0e6c56",
    "metadata": {},
    "outputs": [
@@ -313,7 +331,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 8,
+   "execution_count": 9,
    "id": "7134f652",
    "metadata": {},
    "outputs": [
@@ -322,8 +340,8 @@
      "output_type": "stream",
      "text": [
       "Adding I'm A to []\n",
-      "Adding I'm C to [\"I'm A\"]\n",
       "Adding I'm B to [\"I'm A\"]\n",
+      "Adding I'm C to [\"I'm A\"]\n",
       "Adding I'm E to [\"I'm A\", \"I'm B\", \"I'm C\"]\n"
      ]
     },
@@ -333,7 +351,7 @@
        "{'aggregate': [\"I'm A\", \"I'm B\", \"I'm C\", \"I'm E\"], 'which': 'bc'}"
       ]
      },
-     "execution_count": 8,
+     "execution_count": 9,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -344,7 +362,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 9,
+   "execution_count": 10,
    "id": "b130e694",
    "metadata": {},
    "outputs": [
@@ -364,7 +382,7 @@
        "{'aggregate': [\"I'm A\", \"I'm C\", \"I'm D\", \"I'm E\"], 'which': 'cd'}"
       ]
      },
-     "execution_count": 9,
+     "execution_count": 10,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -389,7 +407,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 10,
+   "execution_count": 11,
    "id": "836bc12d",
    "metadata": {},
    "outputs": [],
@@ -481,7 +499,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 11,
+   "execution_count": 12,
    "id": "932c497e",
    "metadata": {},
    "outputs": [
@@ -504,7 +522,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 12,
+   "execution_count": 13,
    "id": "933b3afd",
    "metadata": {},
    "outputs": [
@@ -525,7 +543,7 @@
        " 'which': 'bc'}"
       ]
      },
-     "execution_count": 12,
+     "execution_count": 13,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -536,7 +554,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 13,
+   "execution_count": 14,
    "id": "e30531bf",
    "metadata": {},
    "outputs": [
@@ -557,7 +575,7 @@
        " 'which': 'cd'}"
       ]
      },
-     "execution_count": 13,
+     "execution_count": 14,
      "metadata": {},
      "output_type": "execute_result"
     }
@@ -565,14 +583,6 @@
    "source": [
     "graph.invoke({\"aggregate\": [], \"which\": \"cd\"})"
    ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "id": "7c8a7b99",
-   "metadata": {},
-   "outputs": [],
-   "source": []
   }
  ],
  "metadata": {
@@ -591,7 +601,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.11.1"
+   "version": "3.12.2"
   }
  },
  "nbformat": 4,