Back-of-the-envelope estimation in HLD - Architecture Diagram

System Overview - Back-of-the-envelope estimation

This system helps quickly estimate the capacity and resources needed for a new software service. It provides rough calculations for traffic, storage, and compute requirements to guide early design decisions.

Architecture Diagram

User
  |
  v
Estimation Interface
  |
  v
Estimation Engine
  |
  +-------------------+
  |                   |
  v                   v
Traffic Model      Resource Calculator
  |                   |
  +---------+---------+
            |
            v
       Estimation Output

Components

User

user

Person requesting quick capacity estimates

Estimation Interface

frontend

Collects input parameters from user

Estimation Engine

service

Processes inputs and runs estimation calculations

Traffic Model

module

Estimates expected user traffic and request rates

Resource Calculator

module

Calculates storage, compute, and network needs based on traffic

Estimation Output

output

Displays rough capacity and resource estimates to user

Request Flow - 8 Hops

User → Estimation Interface

Estimation Interface → Estimation Engine

Estimation Engine → Traffic Model

Traffic Model → Estimation Engine

Estimation Engine → Resource Calculator

Resource Calculator → Estimation Engine

Estimation Engine → Estimation Output

Estimation Output → User

Failure Scenario

Component Fails:Estimation Engine

Impact:No capacity estimates can be generated; user receives error or no response

Mitigation:Retry mechanism or fallback to cached previous estimates; alert system admins

Architecture Quiz - 3 Questions

Which component is responsible for calculating storage and compute needs?

A Estimation Interface

B Traffic Model

C Resource Calculator

D Estimation Output

Design Principle

This design shows modular separation of concerns, where traffic estimation and resource calculation are distinct. It enables quick rough estimates by breaking down complex calculations into manageable parts, supporting fast feedback in early design phases.

Practice

1. What is the main purpose of back-of-the-envelope estimation in system design?

easy

A. To quickly estimate system requirements using simple calculations

B. To write detailed code for the system

C. To create a full architecture diagram

D. To perform in-depth performance testing

Solution

1. Step 1: Understand the goal of back-of-the-envelope estimation

   This method is used to quickly guess system needs without detailed work.

2. Step 2: Compare options to the goal

   Only To quickly estimate system requirements using simple calculations matches the quick estimation purpose; others are detailed tasks.

3. Final Answer:

   To quickly estimate system requirements using simple calculations -> Option A

4. Quick Check:

   Quick estimation = To quickly estimate system requirements using simple calculations [OK]

Hint: Focus on quick, simple calculations for early guesses [OK]

Common Mistakes:

• Confusing estimation with detailed design
• Thinking it involves coding or testing
• Assuming it requires full diagrams

2. Which of the following is the correct way to start a back-of-the-envelope estimation for a web service's daily traffic?

easy

A. Calculate exact user behavior with detailed logs

B. Estimate average users per hour and multiply by 24

C. Design the database schema first

D. Write the full API specification

Solution

1. Step 1: Identify the starting point for traffic estimation

   Back-of-the-envelope estimation uses simple averages, not detailed data.

2. Step 2: Evaluate options for simplicity and relevance

   Estimate average users per hour and multiply by 24 uses average users per hour times 24 hours, a simple and effective start.

3. Final Answer:

   Estimate average users per hour and multiply by 24 -> Option B

4. Quick Check:

   Simple average x time = Estimate average users per hour and multiply by 24 [OK]

Hint: Use simple averages and multiply by time units [OK]

Common Mistakes:

• Starting with detailed logs instead of rough estimates
• Jumping to design before estimating traffic
• Confusing estimation with specification writing

3. You estimate a system will have 10,000 users daily, each making 5 requests on average. What is the total number of requests per day?

medium

A. 100,000 requests

B. 2,000 requests

C. 15,000 requests

D. 50,000 requests

Solution

1. Step 1: Multiply users by requests per user

   10,000 users x 5 requests each = 50,000 requests.

2. Step 2: Confirm total requests per day

   The total is simply the product of users and requests per user.

3. Final Answer:

   50,000 requests -> Option D

4. Quick Check:

   Users x requests = 50,000 [OK]

Hint: Multiply users by average requests per user [OK]

Common Mistakes:

• Adding instead of multiplying
• Mixing up users and requests
• Choosing an option too large or too small

4. A designer estimated 1 million requests per day but forgot to consider peak hour traffic. What is the likely problem with this estimation?

medium

A. Ignoring average request size affecting storage needs

B. Overestimating total daily traffic causing wasted resources

C. Underestimating peak load leading to potential system overload

D. Not including user authentication time

Solution

1. Step 1: Understand the impact of ignoring peak traffic

   Peak traffic is often much higher than average, so ignoring it underestimates load.

2. Step 2: Identify the consequence of underestimation

   System may overload during peak times if capacity is based on average only.

3. Final Answer:

   Underestimating peak load leading to potential system overload -> Option C

4. Quick Check:

   Ignoring peak = underestimation risk [OK]

Hint: Always consider peak, not just average, traffic [OK]

Common Mistakes:

• Assuming daily average covers peak needs
• Confusing storage with traffic load
• Focusing on unrelated factors like authentication time

5. You need to estimate the storage required for a photo-sharing app. Each photo is 3MB, and you expect 500,000 uploads per month. What is the approximate monthly storage needed?

hard

A. 1.5 TB

B. 150 GB

C. 15 GB

D. 1.5 GB

Solution

1. Step 1: Calculate total storage in MB

   500,000 photos x 3 MB each = 1,500,000 MB.

2. Step 2: Convert MB to TB

   1,500,000 MB ÷ 1024 ≈ 1465 GB ÷ 1024 ≈ 1.43 TB, approximately 1.5 TB.

3. Final Answer:

   1.5 TB -> Option A

4. Quick Check:

   Uploads x size ≈ 1.5 TB [OK]

Hint: Multiply count by size, then convert units [OK]

Common Mistakes:

• Forgetting to convert MB to GB
• Mixing up MB and GB units
• Using wrong multiplication factors