Building the Foundation: A Backend Interview Guide

If the frontend is what users see, the backend is the powerful, invisible engine that makes everything work. It’s the central nervous system of any application, handling business logic, data management, and security. A backend development interview is designed to test your ability to build this foundation—to create systems that are not just functional, but also scalable, efficient, and secure. This guide will demystify the process, covering the essential concepts, common questions, and pro tips you need to succeed.

Key Concepts to Understand

A great backend developer has a firm grasp of the architectural principles that govern server-side applications.

API Paradigms (REST vs. GraphQL): An Application Programming Interface (API) is the contract that allows the frontend and backend (or any two services) to communicate. Interviewers will expect you to know the difference between REST, a traditional approach based on accessing resources via different URLs, and GraphQL, a more modern approach that allows clients to request exactly the data they need from a single endpoint.

Database Knowledge: At its core, the backend manages data. You must be comfortable with database interactions, from designing a relational schema to writing efficient queries. Understanding the trade-offs between SQL (structured, reliable) and NoSQL (flexible, scalable) databases is essential, as is knowing how to prevent common performance bottlenecks. This goes hand-in-hand with the rise of smart, autonomous databases.

Authentication & Authorization: These two concepts are the cornerstones of application security. Authentication is the process of verifying a user’s identity (proving you are who you say you are). Authorization is the process of determining what an authenticated user is allowed to do (checking your permissions).

Common Interview Questions & Answers

Let’s look at how these concepts are tested in real interview questions.

Question 1: Compare and contrast REST and GraphQL.

What the Interviewer is Looking For:

This question assesses your high-level architectural awareness. They want to know if you understand the pros and cons of different API design philosophies and when you might choose one over the other.

Sample Answer:

REST (Representational State Transfer) is an architectural style that treats everything as a resource. You use different HTTP verbs (GET, POST, DELETE) on distinct URLs (endpoints) to interact with these resources. For example, GET /users/123 would fetch a user, and GET /users/123/posts would fetch their posts. Its main drawback is over-fetching (getting more data than you need) or under-fetching (having to make multiple requests to get all the data you need).

GraphQL is a query language for your API. It uses a single endpoint (e.g., /graphql) and allows the client to specify the exact shape of the data it needs in a single request. This solves the over-fetching and under-fetching problem, making it very efficient for complex applications or mobile clients with limited bandwidth. However, it can add complexity on the server-side, especially around caching and query parsing.

Question 2: What is the N+1 query problem and how do you solve it?

What the Interviewer is Looking For:

This is a practical question that tests your real-world experience with databases and Object-Relational Mappers (ORMs). It’s a very common performance killer, and knowing how to spot and fix it is a sign of a competent developer.

Sample Answer:

The N+1 query problem occurs when your code executes one query to retrieve a list of parent items and then executes N additional queries (one for each parent) to retrieve their related child items.

For example, if you fetch 10 blog posts and then loop through them to get the author for each one, you’ll end up running 1 (for the posts) + 10 (one for each author) = 11 total queries. This is incredibly inefficient.

The solution is “eager loading” or “preloading.” Most ORMs provide a way to tell the initial query to also fetch the related data ahead of time. It effectively combines the N subsequent queries into a single, second query. Instead of 11 small queries, you would have just 2: one to get the 10 posts, and a second to get the 10 corresponding authors using a WHERE author_id IN (...) clause.

Question 3: Explain how you would implement JWT-based authentication.

What the Interviewer is Looking For:

This question tests your knowledge of modern, stateless authentication flows and core security concepts. A backend developer must be able to implement secure user login systems.

Sample Answer:

JWT, or JSON Web Token, is a standard for creating self-contained access tokens that are used to authenticate users without needing to store session data on the server. The flow works like this:

1. A user submits their credentials (e.g., email and password) to a login endpoint.
2. The server validates these credentials against the database.
3. If they are valid, the server generates a JWT. This token is a JSON object containing a payload (like { "userId": 123, "role": "admin" }) that is digitally signed with a secret key known only to the server.
4. The server sends this JWT back to the client.
5. The client stores the JWT (for example, in a secure cookie) and includes it in the Authorization: Bearer header of every subsequent request to a protected route.
6. For each incoming request, the server’s middleware inspects the token, verifies its signature using the secret key, and if it’s valid, grants access to the requested resource.

Career Advice & Pro Tips

Tip 1: Understand the Full System. Backend development doesn’t end when the code is written. Be prepared to discuss testing strategies (unit, integration), CI/CD pipelines for deployment, and the importance of logging and monitoring for application health.

Tip 2: Security First. Always approach problems with a security mindset. Mention things like input validation to prevent malicious data, using prepared statements to avoid SQL injection, and properly hashing passwords with a strong algorithm like bcrypt.

Tip 3: Go Beyond Your Framework. Whether you use Node.js, Python, or Go, understand the universal principles they are built on. Know how HTTP works, what database indexing is, and how different caching strategies (like Redis) can improve performance. This shows true depth of knowledge.

Conclusion

The backend interview is a chance to prove you can build the robust, logical core of an application. It’s about demonstrating your ability to manage data, secure endpoints, and build for scale. By mastering these foundational concepts and thinking like an architect, you can show that you have the skills to create reliable systems and thrive in your tech career.