Chains (sequential, router) in Prompt Engineering / GenAI - Full Explanation

Introduction

Imagine you want to complete a task that needs several steps done one after another or choose the right path based on what happens at each step. Chains help organize these steps so the task flows smoothly and correctly.

Explanation

Sequential Chains

Sequential chains connect multiple steps in a fixed order. Each step uses the result from the previous one to continue the process. This way, the output flows through the chain until the final result is ready.

Sequential chains run steps one after another, passing results along.

Router Chains

Router chains decide which path to take based on the input or results from earlier steps. They act like a traffic controller, sending the process down different routes depending on conditions or choices.

Router chains choose the next step based on conditions, directing flow dynamically.

Real World Analogy

Think of making a sandwich where you first lay the bread, then add fillings in order (sequential). But if you want a vegetarian or meat sandwich, you pick the right ingredients based on your choice (router).

Sequential Chains → Following a recipe step-by-step to make a sandwich

Router Chains → Choosing sandwich ingredients based on dietary preference

Diagram

Diagram

┌─────────────┐     ┌─────────────┐     ┌─────────────┐
│ Step 1      │ → → │ Step 2      │ → → │ Step 3      │
└─────────────┘     └─────────────┘     └─────────────┘
        ↓
   ┌─────────────┐
   │ Router      │
   ├─────────────┤
   │ Path A      │ → → ┌─────────────┐
   │ Path B      │ → → │ Alternative │
   └─────────────┘     └─────────────┘

Diagram shows sequential steps flowing in order and a router deciding between two paths.

Key Facts

Sequential Chain → A process where steps run one after another, each using the previous step's output.

Router Chain → A process that selects the next step based on input conditions or results.

Chain → A series of connected steps that work together to complete a task.

Common Confusions

Thinking router chains run all paths at once.

Thinking router chains run all paths at once. Router chains select only one path to follow based on conditions, not all paths simultaneously.

Believing sequential chains can skip steps.

Believing sequential chains can skip steps. Sequential chains always run steps in order without skipping any.

Summary

Chains organize multiple steps to complete tasks smoothly.

Sequential chains run steps one after another, passing results along.

Router chains choose the next step based on conditions, directing flow dynamically.

Practice

1. What is the main purpose of a sequential chain in GenAI?

easy

A. To run all AI steps at the same time

B. To randomly select one AI step to run

C. To run multiple AI steps one after another in order

D. To stop the AI process after the first step

Solution

1. Step 1: Understand sequential chain behavior

   A sequential chain connects AI steps so they run one after another, passing output from one to the next.

2. Step 2: Compare options to definition

   Only To run multiple AI steps one after another in order describes running steps in order, matching the sequential chain purpose.

3. Final Answer:

   To run multiple AI steps one after another in order -> Option C

4. Quick Check:

   Sequential chain = run steps in order [OK]

Hint: Sequential means steps run one after another [OK]

Common Mistakes:

• Thinking sequential means random step selection
• Confusing sequential with parallel execution
• Assuming sequential chains stop early

2. Which of the following is the correct way to define a router chain in GenAI?

easy

A. router = SequentialChain(steps=[step1, step2])

B. router = RouterChain(steps=[step1, step2], router_function=choose_step)

C. router = RouterChain(steps=step1, step2)

D. router = ChainRouter(steps=[step1, step2])

Solution

1. Step 1: Recall router chain syntax

   A router chain requires a list of steps and a router function to decide which step to run.

2. Step 2: Check each option's syntax

   router = RouterChain(steps=[step1, step2], router_function=choose_step) correctly uses RouterChain with steps list and router_function parameter. Others have wrong class names or syntax.

3. Final Answer:

   router = RouterChain(steps=[step1, step2], router_function=choose_step) -> Option B

4. Quick Check:

   RouterChain needs steps list and router_function [OK]

Hint: RouterChain needs steps list and router_function param [OK]

Common Mistakes:

• Using SequentialChain instead of RouterChain
• Passing steps without brackets as list
• Using wrong class names like ChainRouter

3. Given the code below, what will be the output?

def router_func(input_text):
    if 'weather' in input_text.lower():
        return 'weather_step'
    else:
        return 'default_step'

steps = {
    'weather_step': lambda x: 'It is sunny',
    'default_step': lambda x: 'I do not understand'
}

router_chain = RouterChain(steps=steps, router_function=router_func)

result = router_chain.run('What is the weather today?')

medium

A. 'It is sunny'

B. 'I do not understand'

C. Error: router_function missing

D. 'What is the weather today?'

Solution

1. Step 1: Analyze router function behavior

   The router_func checks if 'weather' is in the input text (case-insensitive). Input contains 'weather', so it returns 'weather_step'.

2. Step 2: Determine which step runs

   The router_chain uses 'weather_step' key to run the lambda returning 'It is sunny'.

3. Final Answer:

   'It is sunny' -> Option A

4. Quick Check:

   Input contains 'weather' -> weather_step -> 'It is sunny' [OK]

Hint: Router picks step by input keyword match [OK]

Common Mistakes:

• Ignoring case in input text check
• Confusing step keys with output strings
• Assuming default_step runs always

4. Identify the error in this router chain code snippet:

steps = {
    'step1': lambda x: 'Hello',
    'step2': lambda x: 'Bye'
}

def router_func(input_text):
    if 'hello' in input_text:
        return 'step1'
    else:
        return 'step3'

router_chain = RouterChain(steps=steps, router_function=router_func)
result = router_chain.run('hello there')

medium

A. Lambda functions require two arguments

B. steps dictionary keys are not strings

C. router_function is missing in RouterChain

D. router_func returns a step key not in steps dictionary

Solution

1. Step 1: Check router_func return values

   router_func returns 'step1' if 'hello' in input, else 'step3'. Input contains 'hello', so returns 'step1'.

2. Step 2: Verify steps dictionary keys

   Steps dictionary has keys 'step1' and 'step2', but no 'step3'. Returning 'step3' would cause error if input changed.

3. Final Answer:

   router_func returns a step key not in steps dictionary -> Option D

4. Quick Check:

   Router returns unknown step key 'step3' [OK]

Hint: Router must return keys present in steps dict [OK]

Common Mistakes:

• Ignoring missing step keys in router return
• Assuming lambda needs multiple args
• Forgetting to pass router_function parameter

5. You want to build a GenAI system that first summarizes a text, then translates it to French, but only if the text is longer than 100 words. Which chain setup is best?

hard

A. Use a sequential chain with a router function that skips translation if text is short

B. Use a router chain that chooses between summarization or translation only

C. Use two separate sequential chains running independently

D. Use a sequential chain that always runs summarization then translation

Solution

1. Step 1: Understand task requirements

   The system must summarize first, then translate only if text is longer than 100 words.

2. Step 2: Choose chain type matching conditional flow

   A sequential chain with a router function can run summarization step first, then decide to run translation step based on text length.

3. Step 3: Evaluate other options

   Router chain alone can't enforce sequential order; two separate chains won't coordinate; always running translation ignores condition.

4. Final Answer:

   Use a sequential chain with a router function that skips translation if text is short -> Option A

5. Quick Check:

   Sequential + router for conditional step flow [OK]

Hint: Combine sequential steps with router for conditional logic [OK]

Common Mistakes:

• Using router chain alone without sequence
• Running translation always ignoring condition
• Splitting steps into independent chains