Sequential chains in LangChain - Step-by-Step Execution

Concept Flow - Sequential chains

Start: Input data

↓

Chain 1: Process input

↓

Output 1: Result from Chain 1

↓

Chain 2: Use Output 1 as input

↓

Output 2: Final result

↓

End

Sequential chains take input, process it step-by-step through multiple chains, passing output from one to the next.

Execution Sample

LangChain

from langchain.chains import SequentialChain

chain1 = ... # first chain
chain2 = ... # second chain
seq_chain = SequentialChain(chains=[chain1, chain2], input_variables=["input"], output_variables=["output2"])
result = seq_chain.run("Hello")

This code runs two chains in order, passing the output of the first as input to the second, starting with 'Hello'.

Execution Table

Step	Action	Input	Chain Output	Next Input
1	Start with input	{"input": "Hello"}	N/A	{"input": "Hello"}
2	Run Chain 1	{"input": "Hello"}	{"output1": "Hello processed"}	{"output1": "Hello processed"}
3	Run Chain 2	{"output1": "Hello processed"}	{"output2": "Final result"}	N/A
4	Return final output	N/A	{"output2": "Final result"}	N/A

💡 All chains executed sequentially; final output returned.

Variable Tracker

Variable	Start	After Chain 1	After Chain 2	Final
input	"Hello"	"Hello"	"Hello"	"Hello"
output1	N/A	"Hello processed"	"Hello processed"	"Hello processed"
output2	N/A	N/A	"Final result"	"Final result"

Key Moments - 2 Insights

Why does the output of Chain 1 become the input of Chain 2?

Because SequentialChain passes the output variables of one chain as input variables to the next, as shown in execution_table step 3.

What happens if the input variable names don't match between chains?

The chain will fail to pass data correctly, causing errors or missing inputs, since SequentialChain relies on matching variable names to connect chains.

Visual Quiz - 3 Questions

Look at the execution table, what is the input to Chain 2 at step 3?

A {"input": "Hello"}

B {"input": "Hello processed"}

C {"output1": "Hello processed"}

D {"output2": "Final result"}

💡 Hint

Check the 'Next Input' column at step 2 and the 'Input' column at step 3.

At which step does the final output become available?

A Step 4

B Step 3

C Step 2

D Step 1

💡 Hint

Look at the 'Chain Output' and 'Action' columns for when final output is returned.

If Chain 1 output variable was named differently and not passed correctly, what would happen?

A Chain 2 would receive the correct input anyway.

B Chain 2 would get no input and likely fail.

C The final output would be from Chain 1 only.

D The SequentialChain would skip Chain 2.

💡 Hint

Refer to key_moments about variable name matching and execution_table flow.

Concept Snapshot

Sequential chains run multiple chains one after another.
Output of one chain becomes input to the next.
Input variables must match for data flow.
Use SequentialChain with chains list and input/output variables.
Final output is from the last chain in sequence.

Full Transcript

Sequential chains in Langchain let you connect multiple chains so that the output of one becomes the input of the next. You start with an initial input, run the first chain, get its output, then feed that output as input to the second chain, and so on. This continues until all chains have run, and the final output is returned. It is important that the variable names used for outputs and inputs match between chains to ensure smooth data flow. The example code shows creating two chains and combining them with SequentialChain. The execution table traces each step: starting input, Chain 1 processing, Chain 2 processing, and final output. The variable tracker shows how variables change after each chain runs. Common confusions include why outputs become inputs and what happens if variable names don't match. The visual quiz tests understanding of input/output flow and error cases. Overall, sequential chains help build complex workflows by linking simple chains in order.

Practice

1. What is the main purpose of a SequentialChain in Langchain?

easy

A. To create a single chain that never passes outputs

B. To run multiple chains all at the same time independently

C. To run multiple chains one after another, passing outputs as inputs

D. To stop chains from running automatically

Solution

1. Step 1: Understand SequentialChain behavior

   A SequentialChain runs chains in order, passing output from one to the next.

2. Step 2: Compare options to this behavior

   Only To run multiple chains one after another, passing outputs as inputs describes this step-by-step passing of outputs between chains.

3. Final Answer:

   To run multiple chains one after another, passing outputs as inputs -> Option C

4. Quick Check:

   SequentialChain = run chains sequentially with output passing [OK]

Hint: Sequential means one after another with output passing [OK]

Common Mistakes:

• Thinking chains run in parallel
• Believing outputs are not passed
• Confusing SequentialChain with single chain

2. Which of the following is the correct way to create a SequentialChain with two chains named chain1 and chain2?

easy

A. SequentialChain([chain1, chain2])

B. SequentialChain(chains=[chain1, chain2], input_variables=["input"], output_variables=["output"])

C. SequentialChain(chain1, chain2)

D. SequentialChain(chains=chain1 + chain2)

Solution

1. Step 1: Recall SequentialChain constructor

   It requires a list of chains and lists of input and output variable names.

2. Step 2: Check each option's syntax

   Only SequentialChain(chains=[chain1, chain2], input_variables=["input"], output_variables=["output"]) correctly uses named parameters with lists for chains and variables.

3. Final Answer:

   SequentialChain(chains=[chain1, chain2], input_variables=["input"], output_variables=["output"]) -> Option B

4. Quick Check:

   Correct constructor syntax = SequentialChain(chains=[chain1, chain2], input_variables=["input"], output_variables=["output"]) [OK]

Hint: Look for named parameters and list brackets [OK]

Common Mistakes:

• Passing chains without list brackets
• Missing input/output variable lists
• Using plus operator to combine chains

3. Given the following code snippet, what will be the final output printed?

from langchain.chains import SequentialChain

chain1 = SomeChain()  # outputs {'intermediate': 'hello'}
chain2 = SomeChain()  # expects input 'intermediate' and outputs {'final': 'hello world'}

seq_chain = SequentialChain(chains=[chain1, chain2], input_variables=['input'], output_variables=['final'])

result = seq_chain.run({'input': 'start'})
print(result)

medium

A. Error: missing input for chain2

B. 'hello world'

C. 'start'

D. {'final': 'hello world'}

Solution

1. Step 1: Understand chain outputs and inputs

   chain1 outputs {'intermediate': 'hello'}, chain2 uses 'intermediate' input and outputs {'final': 'hello world'}.

2. Step 2: SequentialChain runs chain1 then chain2, passing outputs

   Final result is {'final': 'hello world'}, printed as "{'final': 'hello world'}".

3. Final Answer:

   {'final': 'hello world'} -> Option D

4. Quick Check:

   Output of SequentialChain = {'final': 'hello world'} [OK]

Hint: Final output is dict of output_variables [OK]

Common Mistakes:

• Expecting string instead of dict repr
• Confusing input and output keys
• Assuming error due to input passing

4. What is the error in this code snippet that tries to create a SequentialChain?

seq_chain = SequentialChain(chains=[chain1, chain2], input_variables=['input'])
result = seq_chain.run({'input': 'data'})

medium

A. Missing output_variables parameter causes an error

B. Chains list should be a tuple, not a list

C. Input dictionary keys do not match input_variables

D. run() method requires no arguments

Solution

1. Step 1: Check required parameters for SequentialChain

   Both input_variables and output_variables are required parameters.

2. Step 2: Identify missing parameter

   The code misses output_variables, so it will raise an error.

3. Final Answer:

   Missing output_variables parameter causes an error -> Option A

4. Quick Check:

   output_variables missing = error [OK]

Hint: Always provide input and output variable lists [OK]

Common Mistakes:

• Forgetting output_variables
• Using wrong data type for chains
• Passing arguments incorrectly to run()

5. You want to build a SequentialChain that first extracts keywords from text, then summarizes those keywords. Which approach correctly sets up this workflow?

hard

A. Create two chains: keyword_extractor outputs 'keywords'; summary_chain takes 'keywords' as input; combine with SequentialChain passing these variables

B. Create one chain that does both extraction and summary in one step

C. Run keyword_extractor and summary_chain separately without chaining outputs

D. Use SequentialChain but ignore passing variables between chains

Solution

1. Step 1: Identify the workflow steps

   First extract keywords, then summarize them, so output of first is input of second.

2. Step 2: Use SequentialChain with proper variable passing

   Set keyword_extractor to output 'keywords', summary_chain to input 'keywords', then chain them sequentially.

3. Final Answer:

   Create two chains: keyword_extractor outputs 'keywords'; summary_chain takes 'keywords' as input; combine with SequentialChain passing these variables -> Option A

4. Quick Check:

   SequentialChain passes outputs as inputs [OK]

Hint: Chain outputs must match next chain inputs [OK]

Common Mistakes:

• Trying to do both steps in one chain
• Not passing outputs to next chain
• Ignoring variable names in chaining