Conditional routing in graphs in LangChain - Step-by-Step Execution

Concept Flow - Conditional routing in graphs

Start at Entry Node

↓

Evaluate Condition on Node

↓

Follow Yes Edge

↓

Next Node

↓

End Node

The graph starts at an entry node, evaluates a condition, and routes to the next node based on Yes/No outcomes until reaching the end.

Execution Sample

LangChain

graph = {
  'start': lambda x: 'node1' if x > 0 else 'node2',
  'node1': lambda x: 'end',
  'node2': lambda x: 'end'
}

current = 'start'
while current != 'end':
  current = graph[current](5)

This code routes through a graph starting at 'start', choosing next nodes based on a condition until reaching 'end'.

Execution Table

Step	Current Node	Input Value	Condition Evaluated	Next Node	Action Taken
1	start	5	5 > 0 is True	node1	Follow 'Yes' edge to node1
2	node1	5	No condition (direct to end)	end	Move to end node
3	end	-	Reached end node	-	Stop routing

💡 Reached 'end' node, routing stops.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
current	start	node1	end	end
input_value	5	5	5	5

Key Moments - 3 Insights

Why does the routing stop when current node is 'end'?

Because the execution_table row 3 shows 'end' node reached, which is the termination condition for routing.

What happens if the condition at 'start' node is False?

The routing follows the 'No' edge to 'node2' instead of 'node1', as shown by the condition evaluation in execution_table row 1.

Why do we evaluate conditions at each node?

To decide which path to take next, enabling dynamic routing based on input, as shown in execution_table steps 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'current' after step 1?

A node1

B node2

C start

D end

💡 Hint

Check the 'current node' column in execution_table row 2.

At which step does the routing reach the 'end' node?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look at the 'Current Node' column in execution_table to find when 'end' appears.

If the input value was -1, which node would be visited after 'start'?

A node1

B node2

C end

D start

💡 Hint

Refer to the condition 'x > 0' in execution_table row 1 and think about the False branch.

Concept Snapshot

Conditional routing in graphs:
- Start at an entry node
- Evaluate a condition at each node
- Follow edges based on condition outcome (Yes/No)
- Continue until reaching an end node
- Enables dynamic path selection based on input

Full Transcript

Conditional routing in graphs means starting at a specific node and deciding which path to take next by checking a condition at that node. If the condition is true, you follow one path; if false, another. This continues until you reach an end node, where routing stops. For example, starting at 'start', if the input is greater than zero, you go to 'node1'; otherwise, you go to 'node2'. Both 'node1' and 'node2' lead to 'end', which stops the routing. This method helps in making decisions step-by-step in a graph based on changing inputs.