Why security must be designed in in HLD - Scalability Evidence

Scalability Analysis - Why security must be designed in

Growth Table: Security Impact at Different Scales

Users	Security Challenges	Impact on System	Required Security Measures
100 users	Basic authentication and access control needed	Low risk, manual monitoring possible	Implement strong passwords, HTTPS
10,000 users	Increased attack surface, data privacy concerns	Moderate risk, manual checks insufficient	Use encryption at rest and in transit, role-based access control
1 million users	High risk of data breaches, automated attacks	High risk, system performance affected by security checks	Automated threat detection, multi-factor authentication, secure APIs
100 million users	Massive attack surface, regulatory compliance critical	Very high risk, security can bottleneck system performance	Security by design, zero trust architecture, continuous monitoring, incident response

First Bottleneck: Security as a System Bottleneck

As user count grows, security checks (authentication, authorization, encryption) add processing overhead.

Without early design, security can slow down response times and increase resource use.

Security flaws also cause system failures through breaches or downtime.

Therefore, security mechanisms become the first bottleneck if not designed in from the start.

Scaling Solutions for Security

• Design security early: Integrate security in architecture, not as an afterthought.
• Use scalable authentication: OAuth, JWT tokens reduce load on servers.
• Implement caching: Cache security tokens to reduce repeated checks.
• Distribute security checks: Use edge servers or CDNs for initial filtering.
• Automate monitoring: Use AI/ML for threat detection to handle scale.
• Adopt zero trust: Continuous verification reduces risk at scale.

Back-of-Envelope Cost Analysis

At 1 million users, assuming 10 requests per user per day:

• Requests per second: ~115 (1,000,000 users * 10 req/day / 86400 seconds)
• Security processing overhead: ~20-30% CPU increase due to encryption and auth checks
• Storage for logs and audit trails: several TBs per month depending on logging detail
• Bandwidth impact: Encryption adds ~10-20% overhead on data size

Ignoring security design leads to higher costs from breaches and downtime.

Interview Tip: Structuring Security Scalability Discussion

Start by explaining why security is critical at all scales.

Discuss how security impacts system performance and user experience.

Identify bottlenecks caused by security checks as user base grows.

Propose concrete scaling solutions like token caching, distributed checks, and zero trust.

Highlight cost trade-offs and importance of early design.

Self-Check Question

Your database handles 1000 QPS. Traffic grows 10x. What do you do first?

Answer: Since security checks add overhead, first optimize or scale authentication mechanisms (e.g., add token caching, use stateless tokens) to reduce load before scaling the database. Also consider adding read replicas and caching for database queries.

Key Result

Security mechanisms become a performance bottleneck as user scale grows; designing security early with scalable methods like token caching and distributed checks prevents system slowdowns and costly breaches.

Practice

1. Why is it important to include security in the system design from the beginning?

easy

A. Because security slows down the system and should be avoided

B. Because security can be added easily after development

C. Because users do not care about security

D. Because fixing security issues later is more expensive and risky

Solution

1. Step 1: Understand the cost of late security fixes

   Fixing security problems after the system is built often requires rework, causing delays and higher costs.

2. Step 2: Recognize the risk of ignoring early security

   Ignoring security early can lead to data breaches and loss of user trust, which are costly and damaging.

3. Final Answer:

   Because fixing security issues later is more expensive and risky -> Option D

4. Quick Check:

   Early security design = Cost saving and risk reduction [OK]

Hint: Security early saves cost and risk later [OK]

Common Mistakes:

• Thinking security can be added easily after development
• Assuming users don't care about security
• Believing security always slows down systems

2. Which of the following is a correct security design practice to include from the start?

easy

A. Encrypting sensitive data during storage and transmission

B. Ignoring authentication to speed up development

C. Using default passwords for all users

D. Allowing open access to all system components

Solution

1. Step 1: Identify secure data handling methods

   Encrypting data protects it from unauthorized access both when stored and during communication.

2. Step 2: Eliminate insecure options

   Ignoring authentication, using default passwords, or open access are insecure and expose the system to attacks.

3. Final Answer:

   Encrypting sensitive data during storage and transmission -> Option A

4. Quick Check:

   Encryption = Secure data handling [OK]

Hint: Encrypt data to protect it always [OK]

Common Mistakes:

• Skipping authentication for speed
• Using default or weak passwords
• Allowing unrestricted access

3. Consider a system designed without early security measures. What is the likely outcome when a data breach occurs?

medium

A. The breach causes loss of user trust and costly fixes

B. The breach improves system performance

C. The breach has no effect if data is public

D. The system can quickly fix the breach without impact

Solution

1. Step 1: Understand consequences of data breaches

   A data breach exposes sensitive information, damaging user trust and requiring expensive fixes.

2. Step 2: Reject incorrect outcomes

   Quick fixes without impact are unrealistic; breaches never improve performance; public data assumption is unsafe.

3. Final Answer:

   The breach causes loss of user trust and costly fixes -> Option A

4. Quick Check:

   Data breach = Trust loss + Costly fixes [OK]

Hint: Breaches cost trust and money [OK]

Common Mistakes:

• Assuming breaches have no impact
• Thinking breaches improve performance
• Believing fixes are always quick and cheap

4. A system was built without authentication and encryption. What is the main security flaw to fix?

medium

A. Disable logging to improve speed

B. Add authentication and encrypt data communication

C. Remove all user access controls

D. Use default passwords for all users

Solution

1. Step 1: Identify missing security features

   Authentication verifies users; encryption protects data in transit. Both are essential for security.

2. Step 2: Exclude harmful fixes

   Removing access controls, disabling logging, or using default passwords worsen security risks.

3. Final Answer:

   Add authentication and encrypt data communication -> Option B

4. Quick Check:

   Authentication + Encryption = Secure system [OK]

Hint: Add auth and encryption to fix security [OK]

Common Mistakes:

• Removing access controls
• Disabling logs to speed up
• Using default passwords

5. You are designing a new online service. Which approach best ensures security is built in from the start?

hard

A. Focus on features first; security can be patched later

B. Add security features only after the system is fully developed

C. Plan authentication, encryption, and secure communication before coding

D. Use default settings and rely on network firewalls only

Solution

1. Step 1: Recognize best security design practice

   Planning authentication, encryption, and secure communication early integrates security deeply and reduces risks.

2. Step 2: Understand risks of delayed security

   Adding security late or relying only on firewalls leaves many vulnerabilities and increases costs.

3. Final Answer:

   Plan authentication, encryption, and secure communication before coding -> Option C

4. Quick Check:

   Early security planning = Strong protection [OK]

Hint: Design security features before coding [OK]

Common Mistakes:

• Delaying security until after development
• Relying only on network firewalls
• Ignoring security to speed up features