What is a chain in LangChain - Visual Explanation

Concept Flow - What is a chain in LangChain

Start Chain

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Input Received

↓

Process Step 1: Call LLM or Tool

↓

Process Step 2: Use Output as Input

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... Repeat for all steps ...

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Final Output Produced

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Chain Ends

A chain in LangChain takes an input, processes it step-by-step through linked components, and produces a final output.

Execution Sample

LangChain

from langchain.chains import SimpleSequentialChain
from langchain.llms import OpenAI

llm = OpenAI()
chain = SimpleSequentialChain(llm=llm)
result = chain.run("Hello")

This code creates a simple chain that sends input to an LLM and returns the output.

Execution Table

Step	Action	Input	Process	Output
1	Start Chain	Hello	Receive input	Hello
2	Call LLM	Hello	LLM generates response	Hi! How can I help you?
3	Chain Output	Hi! How can I help you?	Return final output	Hi! How can I help you?
4	End Chain	-	-	-

💡 Chain ends after producing the final output from the LLM.

Variable Tracker

Variable	Start	After Step 1	After Step 2	Final
input_text	-	Hello	Hello	Hello
llm_response	-	-	Hi! How can I help you?	Hi! How can I help you?
chain_output	-	-	-	Hi! How can I help you?

Key Moments - 2 Insights

Why does the chain pass the output of one step as input to the next?

Because chains link multiple steps, each step uses the previous output as input to continue processing, as shown in execution_table row 2.

Is the chain just a single function call?

No, a chain can have multiple linked steps, but SimpleSequentialChain runs them one after another, as seen in the concept_flow.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after Step 2?

A "Chain Ends"

B "Hello"

C "Hi! How can I help you?"

D "Start Chain"

💡 Hint

Check the 'Output' column in execution_table row 2.

At which step does the chain produce the final output?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the 'Action' and 'Output' columns in execution_table row 3.

If the input to the chain changes, which variable in variable_tracker changes first?

A llm_response

B input_text

C chain_output

D None

💡 Hint

See variable_tracker row for 'input_text' and its values after Step 1.

Concept Snapshot

LangChain chains link multiple steps to process input.
Each step uses the previous step's output as input.
Chains can call LLMs, tools, or other logic.
SimpleSequentialChain runs steps one after another.
Final output is returned after all steps complete.

Full Transcript

A chain in LangChain is like a sequence of steps that take an input and process it step-by-step. First, the chain receives the input. Then it sends this input to a language model or tool. The output from that step becomes the input for the next step. This continues until all steps finish, and the chain returns the final output. For example, a SimpleSequentialChain sends the input to an LLM and returns the response. Variables like input_text and llm_response change as the chain runs. Understanding this flow helps beginners see how LangChain organizes complex tasks into simple linked steps.