Error handling in chains in LangChain - Step-by-Step Execution

Concept Flow - Error handling in chains

Start Chain Execution

↓

Run First Chain Step

↓

Error Occurs?

No→Run Next Chain Step

↓

Chain Complete

↓

Handle Error

↓

Decide: Retry / Skip / Stop

↓

Retry Step

↓

Skip Step

↓

Stop Chain

→Chain Ends with Error

The chain runs each step in order. If an error happens, it handles it by retrying, skipping, or stopping the chain.

Execution Sample

LangChain

from langchain.chains import SimpleSequentialChain

chain = SimpleSequentialChain(chains=[step1, step2], verbose=True)
try:
    result = chain.run(input_data)
except Exception as e:
    handle_error(e)

This code runs two chain steps in order and catches errors to handle them.

Execution Table

Step	Action	Error Occurred?	Error Handling	Next Step	Output
1	Run step1 with input_data	No	None	Proceed to step2	step1 output
2	Run step2 with step1 output	Yes	Retry step2 once	Retry step2	retry output
3	Run step2 retry	No	None	Chain complete	final output
Exit	Chain finished successfully	No	None	End	final output

💡 Chain ends after all steps run without error or after error is handled.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Retry	Final
input_data	initial input	initial input	step1 output	step1 output	step1 output
step1 output	None	produced	produced	produced	produced
step2 output	None	None	error	retry success	final output
error	None	None	raised	cleared	None

Key Moments - 2 Insights

Why does the chain retry step2 instead of stopping immediately when an error occurs?

Because the error handling logic in the execution_table row 2 shows a retry action, allowing the chain to attempt the step again before stopping.

What happens if the error is not resolved after retrying?

The chain would stop with an error, but in this example, the retry succeeds as shown in row 3, so the chain completes.

Visual Quiz - 3 Questions

Look at the execution table, what is the output after the first step completes?

A step1 output

B final output

C None

D error

💡 Hint

Check the Output column at Step 1 in the execution_table.

At which step does the chain handle an error by retrying?

A Step 1

B Exit

C Step 2

D No retry occurs

💡 Hint

Look at the Error Handling column in the execution_table rows.

If the error in step2 was not resolved after retry, what would happen?

A Chain would skip step2 and continue

B Chain would stop with error

C Chain would restart from step1

D Chain would ignore the error and output final output

💡 Hint

Refer to the concept_flow where stopping the chain is an option after error handling.

Concept Snapshot

Error handling in chains:
- Chains run steps sequentially.
- If a step errors, handle by retry, skip, or stop.
- Use try-except to catch errors.
- Retry allows fixing transient errors.
- Skipping continues chain without step output.
- Stopping ends chain with error.

Full Transcript

This visual execution shows how error handling works in Langchain chains. The chain runs each step in order. If an error occurs, it can retry the step, skip it, or stop the chain. The example code runs two steps and catches errors with try-except. The execution table traces each step: step1 runs fine, step2 errors once, then retries successfully. Variables track inputs, outputs, and error state. Key moments clarify why retry happens and what if retry fails. The quiz tests understanding of outputs and error handling decisions. This helps beginners see how chains manage errors step-by-step.