A web developer with 10 years of experience is generally expected to demonstrate strong technical knowledge, architectural thinking, problem-solving ability, security awareness, and experience handling production systems.

This list contains 50 important web developer interview questions for 10 years of experience, covering frontend, backend, databases, APIs, system design, security, performance, DevOps, and technical leadership.

1. How would you design a scalable web application for millions of users?

What the interviewer expects:
Explain a complete architecture involving load balancers, application servers, caching, databases, queues, CDN, object storage, monitoring, and horizontal scaling.

You should discuss:

• Load balancing

• Horizontal scaling

• Database replication

• Redis or similar caching

• CDN

• Asynchronous queues

• Database indexing

• API optimization

• Monitoring and logging

• Failover and disaster recovery

2. What is the difference between monolithic, modular monolith, and microservices architecture?

What the interviewer expects:
Explain the trade-offs rather than simply saying that microservices are better.

Discuss:

• Deployment complexity

• Scalability

• Service communication

• Database management

• Development team structure

• Monitoring

• Failure handling

• Operational overhead

A modular monolith can be a practical choice when a system does not yet need independent service deployment.

3. How would you identify and fix a slow web application?

What the interviewer expects:
Explain a systematic debugging process.

A good approach is:

1. Measure the problem.

2. Check server response time.

3. Analyze database queries.

4. Check API latency.

5. Inspect frontend performance.

6. Check CPU and memory usage.

7. Analyze logs and traces.

8. Identify bottlenecks.

9. Apply an optimization.

10. Measure again.

Avoid blindly optimizing code without profiling.

4. How do you optimize a slow SQL query?

What the interviewer expects:

Discuss:

• EXPLAIN / execution plans

• Proper indexes

• Avoiding unnecessary columns

• Reducing joins where appropriate

• Query restructuring

• Pagination

• Avoiding N+1 queries

• Database statistics

• Query caching where appropriate

You should also explain that indexes can improve reads but increase storage and write overhead.

5. What is database indexing and how does it improve performance?

What the interviewer expects:
Explain that an index provides a data structure that allows the database to locate rows more efficiently instead of scanning the entire table.

Also discuss:

• Single-column indexes

• Composite indexes

• Unique indexes

• Covering indexes

• Index selectivity

• Index maintenance cost

6. What is the N+1 query problem?

Answer:
The N+1 problem occurs when an application executes one query to retrieve a list and then executes an additional query for each returned record.

For example:

1 query → fetch 100 users
100 queries → fetch orders for each user

Instead, use appropriate joins, eager loading, batching, or carefully designed queries.

7. How would you design an API that can handle high traffic?

What the interviewer expects:

Consider:

• Stateless application servers

• HTTP caching

• CDN

• Rate limiting

• Authentication

• Pagination

• Efficient database queries

• Caching

• Asynchronous processing

• Load balancing

• Monitoring

• API versioning

8. REST vs GraphQL — when would you choose each?

What the interviewer expects:

REST is often straightforward for resource-oriented APIs and HTTP caching.

GraphQL can be useful when clients need flexible data selection and multiple frontend clients have different data requirements.

Discuss:

• Query flexibility

• Caching

• Complexity

• Security

• Over-fetching/under-fetching

• API governance

9. How do you design API versioning?

Common approaches include:

/api/v1/users
/api/v2/users

Other approaches include versioning through headers or media types.

A good answer should discuss backward compatibility, deprecation strategy and migration plans.

10. How do you handle backward compatibility in APIs?

What the interviewer expects:

Explain:

• Avoid breaking existing fields

• Introduce new optional fields

• Maintain old versions during migration

• Deprecate APIs gradually

• Communicate changes

• Monitor usage of older versions

Frontend and Browser

11. Explain the browser rendering process.

A strong answer should cover:

HTML
 ↓
DOM
 ↓
CSS
 ↓
CSSOM
 ↓
Render Tree
 ↓
Layout
 ↓
Paint
 ↓
Composite

Also discuss JavaScript execution, resource loading and how layout/paint operations can affect performance.

12. What is the difference between reflow and repaint?

Reflow occurs when the browser recalculates layout.

Repaint occurs when visual appearance changes without necessarily changing layout.

Frequent layout changes can negatively affect performance.

13. How would you improve frontend performance?

Discuss:

• Code splitting

• Lazy loading

• Image optimization

• Browser caching

• CDN

• Compression

• Minification

• Reducing JavaScript

• Reducing render-blocking resources

• Efficient CSS

• Optimizing Core Web Vitals

14. What is event delegation in JavaScript?

Event delegation involves attaching an event listener to a parent element instead of adding individual listeners to every child.

It uses event bubbling to determine which child triggered the event.

This can be useful for dynamically generated elements and reducing the number of event listeners.

15. Explain closures in JavaScript.

A closure occurs when a function retains access to variables from its surrounding lexical scope even after the outer function has finished executing.

Example:

function counter() {
    let count = 0;

    return function () {
        return ++count;
    };
}

const increment = counter();

increment(); // 1
increment(); // 2

The returned function retains access to count.

16. What is the difference between debounce and throttle?

Debounce: Executes after a period of inactivity.

Useful for:

• Search boxes

• Form validation

• Auto-save

Throttle: Limits execution to a fixed frequency.

Useful for:

• Scroll events

• Mouse movement

• Window resizing

17. How would you prevent unnecessary API calls from a search box?

Use techniques such as:

• Debouncing

• Minimum search length

• Request cancellation

• Caching previous results

• Pagination

• Server-side filtering

For example, instead of sending an API request for every keystroke, wait until the user stops typing for a short period.

18. What are Core Web Vitals?

Important metrics include:

• LCP — Largest Contentful Paint

• INP — Interaction to Next Paint

• CLS — Cumulative Layout Shift

They help evaluate loading performance, responsiveness and visual stability.

Backend Development

19. What makes an application stateless?

A stateless application does not depend on server-local session state between requests.

Session information can instead be stored in:

• Database

• Redis

• Secure client-side mechanisms

• Dedicated session storage

Stateless application servers are easier to scale horizontally.

20. Session vs JWT authentication — what are the differences?

Server-side sessions:

• Session state is maintained server-side.

• Easy revocation.

• Requires session storage.

JWT:

• Authentication information is contained in a signed token.

• Useful in distributed systems.

• Token revocation requires additional design considerations.

• Large or long-lived tokens can create security and operational problems.

The correct choice depends on the application architecture.

21. How does caching work?

A typical caching flow is:

Request
   ↓
Check Cache
   ↓
Cache Hit → Return Data
   ↓
Cache Miss
   ↓
Database/API
   ↓
Store Result in Cache
   ↓
Return Response

Common caching layers include:

• Browser cache

• CDN

• Reverse proxy

• Application cache

• Redis

• Database cache

22. What caching problems can occur in production?

Discuss:

• Stale data

• Cache invalidation

• Cache stampede

• Hot keys

• Memory limits

• Incorrect TTL

• Inconsistent cache and database state

One common strategy is to use TTLs combined with explicit invalidation where necessary.

23. How would you handle a cache stampede?

Possible approaches include:

• Request coalescing

• Locking

• Randomized TTLs

• Background refresh

• Stale-while-revalidate

• Pre-warming important cache entries

24. What is asynchronous processing and when would you use it?

Use background jobs when an operation does not need to block the user's request.

Examples:

• Sending emails

• Generating reports

• Image processing

• Notifications

• Data imports

• Large file processing

Architecture:

User Request
     ↓
Application
     ↓
Message Queue
     ↓
Worker
     ↓
Background Processing

25. What is a message queue and why is it useful?

A message queue allows producers and consumers to communicate asynchronously.

Benefits include:

• Decoupling

• Traffic smoothing

• Retry handling

• Background processing

• Better fault tolerance

• Independent scaling of workers

Security

26. Explain SQL Injection and how to prevent it.

SQL Injection occurs when untrusted input is incorporated into SQL statements in an unsafe manner.

Prevent it using:

• Parameterized queries

• Prepared statements

• ORM/query builders

• Input validation

• Least-privilege database accounts

Never rely solely on escaping user input as the primary defense.

27. What is XSS and how do you prevent it?

Cross-Site Scripting occurs when malicious scripts are injected into content that is rendered in a user's browser.

Protection includes:

• Context-aware output encoding

• Sanitization where HTML is intentionally allowed

• Content Security Policy

• Secure framework defaults

• Avoiding unsafe DOM APIs

28. What is CSRF?

Cross-Site Request Forgery tricks an authenticated browser into sending an unwanted request to a website.

Common protection mechanisms include:

• CSRF tokens

• SameSite cookies

• Origin/Referer validation where appropriate

For state-changing requests, applications should use appropriate CSRF protections.

29. What security headers should a web application consider?

Examples include:

• Content-Security-Policy

• Strict-Transport-Security

• X-Content-Type-Options

• Referrer-Policy

• Permissions-Policy

• Frame-ancestors through CSP

Security headers should be configured according to the application's actual requirements.

30. How should passwords be stored?

Passwords should never be stored in plaintext.

Use a password hashing algorithm designed for passwords, such as:

• Argon2id

• bcrypt

• scrypt

A unique salt should be handled by the password-hashing function.

31. How would you secure an authentication system?

Discuss:

• Strong password hashing

• Secure session management

• HTTPS

• MFA where appropriate

• Login rate limiting

• Account lockout or progressive delays

• Secure cookies

• CSRF protection

• Password reset security

• Audit logging

• Session invalidation

32. How do you prevent brute-force attacks?

Possible measures:

• Rate limiting

• Progressive delays

• IP/device risk controls

• CAPTCHA where appropriate

• MFA

• Monitoring failed authentication attempts

• Credential-stuffing detection

Avoid relying on IP blocking alone.

System Design

33. Design a URL shortener.

A possible architecture:

Client
  ↓
Load Balancer
  ↓
API Servers
  ↓
Cache
  ↓
Database

The system needs:

• Unique short IDs

• Redirect handling

• Database storage

• Caching

• Expiration support

• Rate limiting

• Analytics

• Horizontal scalability

34. Design a notification system.

A scalable design could be:

Application
     ↓
Message Queue
     ↓
Notification Workers
     ↓
 ┌────────┬─────────┬──────────┐
Email     SMS       Push

Discuss retries, idempotency, delivery status and dead-letter queues.

35. Design a file upload system for large files.

Consider:

• Object storage

• Multipart uploads

• Chunked uploads

• Upload validation

• Virus/malware scanning

• Access control

• Signed URLs

• CDN

• Background processing

The application server should not necessarily handle the entire file stream when direct-to-object-storage uploads are practical.

36. How would you design a high-traffic e-commerce website?

Discuss:

• CDN

• Load balancing

• Product caching

• Search service

• Database optimization

• Cart management

• Inventory consistency

• Payment integration

• Order processing

• Message queues

• Monitoring

• Failure recovery

37. How would you prevent duplicate payment or order processing?

Use idempotency.

For example:

Idempotency-Key: abc123

The server stores the result associated with the key and ensures retries do not create duplicate operations.

Also discuss database transactions and unique constraints.

38. What is database replication?

Replication copies data from one database server to another.

Common uses include:

• Read scaling

• High availability

• Disaster recovery

• Geographic distribution

You should also discuss replication lag and consistency considerations.

39. What is database sharding?

Sharding distributes data across multiple database partitions or servers.

For example:

Users 1–1M       → Shard A
Users 1M–2M      → Shard B
Users 2M–3M      → Shard C

Challenges include:

• Choosing a shard key

• Cross-shard queries

• Rebalancing

• Hot shards

• Transactions across shards

40. How do you handle database migrations in production?

A safe migration strategy should consider:

1. Backward-compatible schema changes.

2. Deploy application changes that support both versions.

3. Migrate or backfill data.

4. Switch application behavior.

5. Remove obsolete columns/code later.

Avoid destructive schema changes during the same deployment that still requires the old schema.

DevOps and Production

41. What is CI/CD?

CI — Continuous Integration

Code is regularly integrated and automatically tested.

CD — Continuous Delivery/Deployment

Validated code is automatically prepared or deployed to environments.

A mature pipeline may include:

Git
 ↓
Build
 ↓
Unit Tests
 ↓
Static Analysis
 ↓
Security Scanning
 ↓
Integration Tests
 ↓
Deploy
 ↓
Monitoring

42. What is Docker and why is it useful?

Docker packages an application and its dependencies into a container image.

Benefits include:

• Environment consistency

• Reproducible deployments

• Isolation

• Easier scaling

• Simplified development environments

43. Docker vs Virtual Machine — what is the difference?

A virtual machine virtualizes an entire operating system environment.

Containers generally share the host kernel while isolating applications and their dependencies.

Containers are typically lighter and start faster, while VMs provide stronger OS-level isolation boundaries.

44. How would you deploy a web application with zero or minimal downtime?

Possible approaches include:

• Rolling deployment

• Blue-green deployment

• Canary deployment

• Health checks

• Load balancer draining

• Backward-compatible database migrations

• Automated rollback

45. What is blue-green deployment?

Two production environments are maintained:

Blue  → Current Production
Green → New Version

After validating Green, traffic is switched from Blue to Green.

If a serious problem occurs, traffic can be switched back.

46. How do you monitor a production application?

Monitor at multiple levels:

Infrastructure

• CPU

• Memory

• Disk

• Network

Application

• Error rate

• Response time

• Throughput

• Slow requests

Database

• Query latency

• Connections

• Locks

• Replication lag

Business

• Login failures

• Orders

• Payments

• Conversion rates

Logs, metrics and distributed traces should be correlated where possible.

Architecture and Leadership

47. How do you review code written by a junior developer?

A senior developer should review more than syntax.

Check:

• Correctness

• Security

• Performance

• Maintainability

• Error handling

• Test coverage

• Naming

• Architecture

• Database impact

• Edge cases

Feedback should explain why a change is needed rather than simply rejecting the code.

48. How do you decide whether technical debt should be fixed?

Consider:

• Security risk

• Production impact

• Development slowdown

• Maintenance cost

• Frequency of change

• Business importance

• Probability of failure

Technical debt should be prioritized based on measurable risk and impact rather than simply because the code is old.

49. Tell me about a major production issue you solved.

What the interviewer expects:

Use the STAR structure:

Situation: What happened?

Task: What were you responsible for?

Action: How did you investigate and fix it?

Result: What improved?

A strong answer should include measurable results where possible, such as reduced response time, fewer errors or reduced infrastructure cost.

50. How would you lead the technical design of a large web application?

A strong senior-level answer should cover:

• Requirements gathering

• Architecture decisions

• API design

• Database design

• Security

• Scalability

• Performance

• Observability

• CI/CD

• Testing strategy

• Disaster recovery

• Documentation

• Code review

• Team collaboration

• Technical debt management

At 10 years of experience, the interviewer is usually interested not only in how you write code, but also in how you make engineering decisions and take responsibility for a production system.

Bonus: Topics to Prepare for a 10-Year Web Developer Interview

In addition to these 50 questions, prepare practical discussions around:

• PHP / Laravel / CodeIgniter

• JavaScript / TypeScript

• React / Angular / Vue

• REST APIs

• MySQL / PostgreSQL

• Redis

• Docker

• Linux

• Git

• AWS / Azure / GCP

• Nginx / Apache

• CI/CD

• System Design

• OWASP Top 10

• Performance optimization

• Database optimization

• Microservices

• Message queues

• Monitoring and logging

• Technical leadership

Final Preparation Tip

For a 10-year experience interview, avoid answering only with definitions. Try to explain real production examples, trade-offs, failure scenarios, and why you selected one solution over another.

For example, instead of saying:

“Redis is used for caching.”

Explain:

“We used Redis to cache frequently accessed data and reduce database load. We defined appropriate TTLs and invalidated specific keys when important data changed. We also monitored cache hit ratio and memory usage.”

That style demonstrates real-world engineering experience, which is generally more valuable at a senior level than memorized definitions.