Top 80 Backend Developer Interview Questions & Answers for 2025

Landing a backend developer role in 2025 requires a solid understanding of everything from databases and APIs to system design and cloud infrastructure. Whether you’re a junior developer or a seasoned engineer, preparing for the technical interview is crucial. This comprehensive guide covers the top 80 backend interview questions with detailed, clear answers to help you ace your next interview. Let’s dive in! 🚀

## General & Conceptual Questions

This section covers high-level concepts that every backend developer should know.

1. What is a “backend”? The backend, or “server-side,” of an application is the part that users don’t see. It’s responsible for storing and organizing data, processing requests from the client (the frontend), and ensuring everything works correctly. It includes the server, the application logic, and the database.

2. Explain the Client-Server model. The Client-Server model is a computing model where a “client” requests resources or services from a “server.” The client is typically a user’s browser or mobile app, and the server is a powerful computer that hosts the application, data, and business logic. The client and server communicate over a network, usually the internet.

3. What’s the difference between monolithic, microservices, and serverless architectures?

• Monolithic: The entire application is built as a single, unified unit. All components (UI, business logic, data access layer) are tightly coupled and run as a single service. It’s simpler to develop and deploy initially.
• Microservices: The application is broken down into a collection of smaller, independent services. Each service handles a specific business function, has its own database, and can be developed, deployed, and scaled independently.
• Serverless: An architecture where you run your code without provisioning or managing servers. The cloud provider dynamically manages the allocation of machine resources. It’s often event-driven and scales automatically. Think AWS Lambda or Google Cloud Functions.

4. What are stateful and stateless applications?

• Stateful: A stateful application remembers data from previous interactions or transactions. It stores “state” information on the server. Examples include online shopping carts or a user’s login session.
• Stateless: A stateless application does not save any client data on the server between requests. Each request is treated as an independent transaction. REST APIs are a prime example of a stateless architecture.

5. Explain caching and why it’s important. Caching is the process of storing copies of files or data in a temporary storage location, called a “cache,” so they can be accessed more quickly. It’s crucial for improving application performance and reducing latency. By caching frequently accessed data, you reduce the load on your primary database and speed up response times for the user. Common caching tools include Redis and Memcached.

6. What is middleware? Middleware is software that acts as a bridge between an operating system or database and applications. In web frameworks (like Express.js), middleware refers to functions that have access to the request object (req), the response object (res), and the next function in the application’s request-response cycle. It’s often used for logging, authentication, and handling CORS.

7. What is the difference between a library and a framework? The key difference is inversion of control.

• Library: You are in control. Your code calls the library’s functions when you need them. (e.g., requests in Python, lodash in JavaScript).
• Framework: The framework is in control. It provides a structure and calls your code when it needs to. You fill in the blanks provided by the framework. (e.g., Django, Ruby on Rails, Spring).

8. What is long polling? Long polling is a technique where the server holds a client’s request open until new data is available. Once the server has new information, it sends a response to the client, closing the connection. The client can then immediately initiate another request. It’s a way to simulate a push mechanism from the server.

9. What are WebSockets? WebSockets provide a full-duplex communication channel over a single, long-lived TCP connection. This means the client and server can send data to each other at the same time without needing to re-establish connections. It’s ideal for real-time applications like chat apps, online gaming, and live sports updates.

10. What is a background job or worker process? A background job is a task that runs outside the main request-response cycle. These are used for time-consuming operations that shouldn’t block the main application thread, such as sending emails, processing images, or generating reports. Tools like Celery (Python) or Sidekiq (Ruby) are used to manage background jobs.

## Programming Language & Frameworks

These questions are specific to popular backend languages and frameworks like Node.js, Python, and Java.

11. Explain the Event Loop in Node.js. The Event Loop is the core of Node.js’s non-blocking I/O model. It allows Node.js to perform I/O operations (like reading a file or making a database query) on a single thread without getting blocked. When an async operation is initiated, it’s offloaded. The event loop continuously checks a queue for completed events and executes their corresponding callback functions.

12. What are Promises and async/await in JavaScript/Node.js?

• Promises: A Promise is an object representing the eventual completion (or failure) of an asynchronous operation. It can be in one of three states: pending, fulfilled, or rejected.
• async/await: This is syntactic sugar built on top of Promises. The async keyword makes a function return a Promise. The await keyword pauses the function execution until a Promise is settled (resolved or rejected), making asynchronous code look and behave more like synchronous code.

13. What is the Global Interpreter Lock (GIL) in Python? The GIL is a mutex that protects access to Python objects, preventing multiple native threads from executing Python bytecodes at the same time. This means that even on a multi-core processor, only one thread can execute Python code at once. Because of the GIL, Python threads are best for I/O-bound tasks, not CPU-bound ones. For true parallelism, Python’s multiprocessing module is used.

14. What is dependency injection? Dependency Injection (DI) is a design pattern where an object’s dependencies (the other objects it works with) are “injected” into it rather than created by the object itself. This promotes loose coupling between components, making the code more modular, easier to test, and more maintainable. Frameworks like Spring (Java) and NestJS (Node.js) heavily use DI.

15. Explain the difference between == and is in Python.

• is checks for object identity. It returns True if two variables point to the exact same object in memory.
• == checks for equality. It returns True if the values of the two objects are equal.

16. What does it mean that Java is a statically-typed language? Statically-typed means that variable types are declared and checked at compile-time, not run-time. If you declare a variable as a String, you cannot later assign an Integer to it without causing a compilation error. This catches many potential bugs early in the development process.

17. What is a virtual environment in Python? A virtual environment is a self-contained directory that houses a specific version of Python plus a number of additional packages. It allows you to work on different projects with different dependencies without conflicts. Tools like venv or conda are used to manage them.

18. What is the difference between null and undefined in JavaScript?

• undefined means a variable has been declared but has not yet been assigned a value.
• null is an assignment value. It can be assigned to a variable as a representation of no value.

19. What is Object-Relational Mapping (ORM)? An ORM (like SQLAlchemy for Python or TypeORM for Node.js) is a technique that lets you query and manipulate data from a database using an object-oriented paradigm. Instead of writing raw SQL, you work with objects and classes, which the ORM translates into SQL queries.

20. What are decorators in Python? A decorator is a design pattern in Python that allows you to add new functionality to an existing object (like a function or class) without modifying its structure. They are a form of metaprogramming and are often used in web frameworks like Flask and Django for tasks like authentication and logging.

## Databases (SQL & NoSQL)

Data is the heart of most applications. These questions test your knowledge of how to store and retrieve it efficiently.

21. What is the difference between SQL and NoSQL databases?

• SQL (Relational Databases): Store data in a structured way using tables with rows and columns. They have a predefined schema (like MySQL, PostgreSQL). They are best for applications requiring complex queries and transactions (e.g., financial systems). They guarantee ACID properties.
• NoSQL (Non-relational Databases): Store data in various formats like document (MongoDB), key-value (Redis), wide-column (Cassandra), or graph (Neo4j). They have dynamic schemas and are built for scalability and flexibility. They often follow the BASE model.

22. What are database indexes and how do they work? A database index is a data structure that improves the speed of data retrieval operations on a database table. It works like an index in a book: instead of scanning the entire table (a “full table scan”), the database can use the index to quickly locate the data. However, indexes add overhead to write operations (INSERT, UPDATE, DELETE).

23. Explain the ACID properties. ACID is a set of properties of database transactions intended to guarantee validity even in the event of errors or power failures.

• Atomicity: All changes to data are performed as if they are a single operation. Either all of the changes are made, or none of them are.
• Consistency: A transaction brings the database from one valid state to another, maintaining database invariants.
• Isolation: The intermediate state of a transaction is invisible to other transactions. It’s as if transactions are running sequentially.
• Durability: Once a transaction has been committed, it will remain so, even in the event of power loss or a system crash.

24. What is the CAP Theorem? The CAP Theorem states that a distributed data store cannot simultaneously provide more than two out of the following three guarantees:

• Consistency: Every read receives the most recent write or an error.
• Availability: Every request receives a (non-error) response, without the guarantee that it contains the most recent write.
• Partition Tolerance: The system continues to operate despite an arbitrary number of messages being dropped (or delayed) by the network between nodes.

25. What is the N+1 query problem? How can you solve it? The N+1 problem occurs when an ORM executes one query to retrieve the main objects (“1”) and then N subsequent queries to retrieve related data for each of the main objects (“N”). This is highly inefficient. It can be solved by using eager loading, where the ORM is instructed to fetch the main objects and their related data in a single query (often using a JOIN).

26. Differentiate between INNER JOIN, LEFT JOIN, and RIGHT JOIN.

• INNER JOIN: Returns records that have matching values in both tables.
• LEFT JOIN: Returns all records from the left table, and the matched records from the right table. The result is NULL from the right side if there is no match.
• RIGHT JOIN: Returns all records from the right table, and the matched records from the left table. The result is NULL from the left side if there is no match.

27. What is database normalization? Normalization is the process of organizing columns and tables in a relational database to minimize data redundancy. The goal is to divide larger tables into smaller, well-structured tables and define relationships between them. The most common forms are 1NF, 2NF, and 3NF.

28. What is sharding? Sharding is a type of database partitioning that separates one large database into many smaller, faster, more easily managed parts called “shards.” Each shard is a separate database, and they are often distributed across multiple machines. It’s a method of horizontal scaling.

29. What is a connection pool? A connection pool is a cache of database connections maintained so that the connections can be reused for future requests. Opening a new database connection is an expensive operation. A connection pool improves performance by reducing the time spent establishing connections and avoids exhausting the number of available connections.

30. When would you use a document database like MongoDB over a relational database like PostgreSQL? You would choose MongoDB when:

• Your data is unstructured or semi-structured.
• You need high write throughput and horizontal scalability.
• Your schema is likely to change frequently (high flexibility).
• You are building applications that don’t require complex transactions (e.g., content management systems, IoT data logging).

## API & Web Services

APIs are the backbone of modern web applications, enabling communication between different services.

31. What is an API? An API (Application Programming Interface) is a set of rules and protocols that allows different software applications to communicate with each other. It defines the methods and data formats that applications can use to request and exchange information.

32. What’s the difference between REST and GraphQL?

• REST (Representational State Transfer): An architectural style for designing networked applications. It’s resource-based, using standard HTTP methods (GET, POST, PUT, DELETE) and URLs to manipulate resources. A key challenge is over-fetching or under-fetching data.
• GraphQL: A query language for APIs and a runtime for fulfilling those queries with your existing data. It allows the client to ask for exactly the data it needs and nothing more, solving the over/under-fetching problem. It typically uses a single endpoint for all requests.

33. What are the common HTTP methods and what do they do?

• GET: Retrieve data from a specified resource.
• POST: Submit data to be processed to a specified resource (e.g., create a new user).
• PUT: Update a specified resource by replacing it entirely.
• PATCH: Apply partial modifications to a resource.
• DELETE: Delete a specified resource.

34. What is idempotency in the context of APIs? An operation is idempotent if making the same request multiple times produces the same result as making it once. GET, PUT, and DELETE requests are typically idempotent. POST is not idempotent (e.g., making the same POST request twice will create two resources). PATCH may or may not be idempotent.

35. Explain CORS (Cross-Origin Resource Sharing). CORS is a browser security feature that restricts web pages from making requests to a different domain than the one that served the page. To allow these requests, the server at the other origin must include specific CORS headers (like Access-Control-Allow-Origin: *) in its response.

36. What are HTTP status codes? Give examples. HTTP status codes are issued by a server in response to a client’s request. They are grouped into five classes:

• 1xx (Informational): Request received, continuing process.
• 2xx (Success): The action was successfully received, understood, and accepted. (e.g., 200 OK, 201 Created).
• 3xx (Redirection): Further action must be taken to complete the request. (e.g., 301 Moved Permanently).
• 4xx (Client Error): The request contains bad syntax or cannot be fulfilled. (e.g., 400 Bad Request, 401 Unauthorized, 404 Not Found).
• 5xx (Server Error): The server failed to fulfill an apparently valid request. (e.g., 500 Internal Server Error).

37. What is API versioning? Why is it important? API versioning is the practice of managing changes to your API so that you don’t break existing client applications when you introduce updates. It’s important because it allows you to evolve your API while providing a stable endpoint for older clients. Common strategies include versioning in the URL (/v1/users), via a query parameter (?version=1), or using custom headers.

38. How do you handle authentication in an API? Common methods include:

• API Keys: A unique string that a client provides when making API calls.
• Basic Authentication: Using a username and password encoded in the Authorization header.
• Session-based Authentication: The server creates a session for the user after they log in and stores a session ID in a cookie.
• Token-based Authentication (e.g., JWT): After logging in, the server provides a signed token (like a JSON Web Token) to the client. The client includes this token in the Authorization header for subsequent requests. This is stateless and works well for microservices.
• OAuth 2.0: An authorization framework that enables a third-party application to obtain limited access to a user’s account on another service.

39. What is rate limiting? Rate limiting is a strategy for limiting network traffic. It’s used to prevent abuse of an API by restricting how many requests a user can make in a given time frame. This protects the service from denial-of-service (DoS) attacks and ensures fair usage for all clients.

40. What is serialization? Serialization is the process of converting a data object (like a Python dict or a Java Object) into a format that can be stored or transmitted (e.g., a JSON string) and then reconstructed later. The reverse process is called deserialization. It’s fundamental for API communication.

## System Design

System design questions evaluate your ability to think about architecture, scalability, and reliability at a high level.

41. How would you design a URL shortener like TinyURL?

1. Functional Requirements: Shorten a long URL, redirect a short URL to the original URL, custom URLs.
2. Non-Functional Requirements: High availability, low latency, scalability.
3. High-Level Design:
   • API Endpoints: POST /api/shorten (with long_url in body), GET /{short_url}.
   • URL Shortening Logic: Generate a unique 6-8 character hash for each long URL. A common approach is to use a counter (which can be distributed using something like Zookeeper) and convert its value to a Base62 encoding ([a-zA-Z0-9]).
   • Database: A key-value store like Redis or Cassandra is ideal. The key would be the short hash, and the value would be the long URL. (short_hash -> long_url). We would also need a table to look up if a long URL has already been shortened to avoid duplicates.
   • Redirection: When a GET /{short_url} request comes in, the web server looks up the short_url in the database, retrieves the long_url, and sends back an HTTP 301 Moved Permanently redirect response.
   • Scaling: Use a load balancer to distribute traffic, replicate the database across multiple data centers, and use a CDN to cache popular links.

42. What is a load balancer? What are different load balancing algorithms? A load balancer distributes incoming network traffic across multiple servers. This improves responsiveness and availability by ensuring no single server is overloaded. Common algorithms include:

• Round Robin: Distributes requests to servers in a cyclical order.
• Least Connections: Sends traffic to the server with the fewest active connections.
• IP Hash: The client’s IP address is used to determine which server receives the request. This ensures a user is consistently sent to the same server.

43. Explain the difference between vertical and horizontal scaling.

• Vertical Scaling (Scaling Up): Increasing the resources of a single server, such as adding more CPU, RAM, or storage. This is often easier but has a hard physical limit and can be more expensive.
• Horizontal Scaling (Scaling Out): Adding more servers to your pool of resources. This is more complex to manage but is highly scalable and fault-tolerant. Modern cloud applications are designed for horizontal scaling.

44. What is a CDN? A Content Delivery Network (CDN) is a geographically distributed network of proxy servers. It caches static content (like images, CSS, JavaScript files) closer to users around the world. When a user requests content, the CDN serves it from the nearest server, which significantly reduces latency and offloads traffic from your origin server.

45. Design a social media feed like Twitter.

1. Core Features: Post a tweet, view a timeline (tweets from people you follow), follow a user.
2. High-Level Design:
   • Post Tweet (Write Path): When a user posts a tweet, it’s written to a database. A background job then “fans out” this tweet to the timelines of all their followers.
   • Timeline (Read Path): A user’s timeline can be pre-computed and stored in a cache (like Redis). When a user requests their timeline, it’s read directly from this cache, making it very fast. This is a “fan-out on write” approach.
   • The “Celebrity” Problem: For users with millions of followers (celebrities), fan-out on write is too expensive. The solution is a hybrid approach: for most users, use fan-out on write. For celebrities, don’t fan out. When a user requests their timeline, fetch tweets from the people they follow and merge them with tweets from any celebrities they follow at read time (“fan-out on read”).
   • Database: A mix of SQL (for user data, relationships) and NoSQL (for tweets, timelines) can be effective.

46. What is a message queue? A message queue (like RabbitMQ or Kafka) is a component that enables asynchronous communication between different services. A “producer” service sends a message to the queue. A “consumer” service listens for messages on the queue and processes them. This decouples services, improves scalability, and provides reliability (if a consumer fails, the message remains in the queue to be processed later).

47. What is database replication? Database replication is the process of copying data from a primary database server to one or more secondary (replica) servers. This is done to:

• Improve Read Performance: Read queries can be distributed among the replicas.
• Increase Availability: If the primary server fails, a replica can be promoted to become the new primary (failover).

48. What is eventual consistency? Eventual consistency is a consistency model used in distributed systems where, if no new updates are made to a given data item, all accesses to that item will eventually return the last updated value. In the short term, different nodes might return different values. This is a trade-off made to achieve high availability.

49. How do you handle failure in a distributed system?

• Redundancy: Have multiple copies of services and data.
• Failover: Automatically switch to a standby system if the primary one fails.
• Timeouts and Retries: Don’t wait forever for a response from another service. If a request times out, retry it (with exponential backoff).
• Circuit Breaker Pattern: If a service is consistently failing, a “circuit breaker” can trip and stop sending requests to it for a while, preventing cascading failures.

50. What is a single point of failure (SPOF)? A SPOF is a part of a system that, if it fails, will stop the entire system from working. System design aims to eliminate SPOFs by introducing redundancy for every component (e.g., multiple web servers behind a load balancer, replicated databases).

## Concurrency & Multithreading

These questions test your understanding of how to manage multiple tasks running at the same time.

51. What is the difference between a process and a thread?

• Process: A program in execution. Each process has its own separate memory space.
• Thread: A single execution sequence within a process. Multiple threads within the same process share the same memory space, which makes communication between them easier but also introduces challenges like race conditions.

52. What is a race condition? A race condition occurs when two or more threads try to access and manipulate the same shared resource (e.g., a variable) at the same time, and the result of the operation depends on the unpredictable timing of their execution.

53. What is a deadlock? A deadlock is a situation where two or more threads are blocked forever, each waiting for the other to release a resource that it needs. For example, Thread A holds Lock 1 and is waiting for Lock 2, while Thread B holds Lock 2 and is waiting for Lock 1.

54. What are mutexes and semaphores? Both are synchronization primitives used to prevent race conditions.

• Mutex (Mutual Exclusion): It’s like a key to a room. Only one thread can hold the key (lock the mutex) at a time. Any other thread that wants the key must wait until the first thread is done and releases it.
• Semaphore: It’s like a counter for a fixed number of permits. Threads can “acquire” a permit (decrementing the counter). If the counter is zero, the thread blocks until a permit is “released” (incrementing the counter). It allows a limited number of threads to access a resource simultaneously. A mutex is essentially a semaphore with a count of 1.

55. What is the producer-consumer problem? This is a classic concurrency problem. A “producer” thread generates data and puts it into a shared buffer. A “consumer” thread takes data from the buffer and processes it. The challenges are to ensure that the producer doesn’t try to add data to a full buffer and the consumer doesn’t try to remove data from an empty buffer, all without race conditions. This is often solved using a message queue or semaphores.

56. What is non-blocking I/O? Non-blocking I/O means that when a program makes a request for an I/O operation (like reading from a network socket), it doesn’t wait for the operation to complete. Instead, it moves on to other tasks. The program is notified later when the I/O operation is finished. This is the model used by Node.js to achieve high concurrency on a single thread.

57. What is thread starvation? Thread starvation occurs when a thread is perpetually denied necessary resources to process its work, often because other “greedy” threads are monopolizing them.

58. What is context switching? Context switching is the process of storing the state of a process or thread so that it can be restored and resume execution at a later point. This allows multiple processes to share a single CPU. Context switching has an overhead cost.

59. What are atomic operations? An atomic operation is an operation that is guaranteed to execute as a single, indivisible unit. It cannot be interrupted by another thread. In multi-threaded programming, simple operations like x++ are often not atomic and need to be protected by a lock.

60. What is a thread pool? A thread pool is a collection of pre-instantiated, idle threads that stand ready to be given work. This avoids the overhead of creating a new thread for every task. When a task arrives, it’s assigned to an available thread from the pool.

## Security

Security is not an afterthought. A backend developer must know how to protect the application and its data.

61. What is SQL Injection? How do you prevent it? SQL Injection is an attack where malicious SQL code is inserted into input fields, which is then executed by the database. This can be used to bypass authentication, read sensitive data, or even delete the entire database. Prevention: The primary way to prevent it is by using prepared statements (also called parameterized queries). With prepared statements, the SQL query template is sent to the database separately from the user input, so the input is treated as data and not as executable code.

62. What is Cross-Site Scripting (XSS)? XSS is an attack where a malicious script is injected into a trusted website. When an unsuspecting user visits the page, the script runs in their browser, allowing the attacker to steal information like session cookies or perform actions on behalf of the user. Prevention: The key is to sanitize and escape user input before rendering it on a page. This means treating user input as plain text, not HTML.

63. Explain hashing vs. encryption.

• Hashing: A one-way function that converts an input of any size into a fixed-size string of characters. You cannot reverse-engineer the original input from the hash. It’s used for storing passwords. You hash the user’s password and store the hash; you never store the plain-text password. Use strong, salted hashing algorithms like Argon2 or bcrypt.
• Encryption: A two-way process. Data is encrypted using a key, and it can be decrypted back into its original form using the same (or a different) key. It’s used for protecting data in transit (TLS/SSL) or at rest (database encryption).

64. What is OAuth 2.0? OAuth 2.0 is an authorization framework, not an authentication protocol. It allows an application (the “client”) to obtain limited access to a user’s account on an HTTP service (like Google or Facebook) without exposing the user’s password to the client application.

65. What is a CSRF (Cross-Site Request Forgery) attack? CSRF is an attack that tricks a victim into submitting a malicious request. It inherits the identity and privileges of the victim to perform an undesired function on their behalf (e.g., changing their email address). Prevention: Use anti-CSRF tokens. The server generates a unique, unpredictable token and embeds it in a hidden field in the form. When the form is submitted, the server checks if the token matches.

66. What is HTTPS/TLS? HTTPS (Hypertext Transfer Protocol Secure) is the secure version of HTTP. It uses TLS (Transport Layer Security), formerly SSL, to encrypt the communication between the client’s browser and the web server. This ensures confidentiality (prevents eavesdropping) and integrity (prevents data tampering).

67. What is “salting” a password? Salting is the process of adding a unique, random string of characters (the “salt”) to each user’s password before it is hashed. The salt is then stored along with the hash. This ensures that even if two users have the same password, their stored hashes will be different. This prevents rainbow table attacks.

68. What are some common security headers you should set in your application’s response?

• Strict-Transport-Security (HSTS): Forces browsers to use HTTPS.
• X-Content-Type-Options: nosniff: Prevents the browser from MIME-sniffing a response away from the declared content-type.
• X-Frame-Options: deny: Prevents clickjacking attacks by disallowing the page from being rendered in an <frame> or <iframe>.
• Content-Security-Policy (CSP): Helps prevent XSS by specifying which dynamic resources are allowed to load.

69. What is a Man-in-the-Middle (MITM) attack? An MITM attack is where an attacker secretly relays and possibly alters the communication between two parties who believe they are directly communicating with each other. HTTPS/TLS is the primary defense against this.

70. What are environment variables and why are they important for security? Environment variables are variables set outside the application code, in the operating system or a configuration file. They are crucial for security because they allow you to store sensitive information like API keys, database credentials, and secret keys outside of your version control system (like Git). This prevents secrets from being accidentally exposed in your codebase.

## DevOps & Cloud

Modern backend development is intertwined with deployment, operations, and cloud services.

71. What is CI/CD?

• CI (Continuous Integration): The practice of frequently merging all developers’ working copies of code to a shared mainline. Each integration is verified by an automated build (including tests) to detect integration errors as quickly as possible.
• CD (Continuous Delivery/Deployment): Continuous Delivery is the practice of keeping the application in a state where it can be released to production at any time. Continuous Deployment takes this a step further by automatically deploying every change that passes the tests to production.

72. What are Docker and containers? A container is a lightweight, standalone, executable package of software that includes everything needed to run it: code, runtime, system tools, system libraries, and settings. Docker is the most popular platform for creating, deploying, and running applications in containers. Containers ensure that an application works uniformly across different environments (development, staging, production).

73. What is Kubernetes? Kubernetes (K8s) is an open-source container orchestration platform. If you have many containers running your microservices, Kubernetes helps you manage them. It handles tasks like scheduling containers onto nodes in a cluster, service discovery, load balancing, and scaling.

74. What is Infrastructure as Code (IaC)? IaC is the practice of managing and provisioning infrastructure (networks, virtual machines, load balancers) through machine-readable definition files, rather than through physical hardware configuration or interactive configuration tools. Tools like Terraform and AWS CloudFormation allow you to define your entire cloud infrastructure in code.

75. What is server monitoring? What are some key metrics to track? Server monitoring is the process of collecting and analyzing data about a server’s performance and health. Key metrics include:

• CPU Usage: To check for processes hogging the CPU.
• Memory Usage: To prevent out-of-memory errors.
• Disk I/O and Space: To monitor storage performance and capacity.
• Network Traffic: To track data in and out.
• Application Metrics: Error rate, request latency (e.g., p95, p99), and throughput (requests per second).

## Coding & Problem-Solving

While less common in senior interviews, you might still get a quick coding challenge.

76. Write a function to check if a string is a palindrome. A palindrome is a word that reads the same forwards and backwards.

Python

def is_palindrome(s):
  # Clean the string: remove non-alphanumeric chars and convert to lowercase
  cleaned_s = ''.join(filter(str.isalnum, s)).lower()
  # Check if the cleaned string is equal to its reverse
  return cleaned_s == cleaned_s[::-1]

def is_palindrome(s):
  # Clean the string: remove non-alphanumeric chars and convert to lowercase
  cleaned_s = ''.join(filter(str.isalnum, s)).lower()
  # Check if the cleaned string is equal to its reverse
  return cleaned_s == cleaned_s[::-1]

77. What is the Two Sum problem? Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target.

• Logic: The efficient solution uses a hash map (or a dictionary in Python). Iterate through the array. For each element num, calculate the complement (target - num). Check if the complement exists in the hash map. If it does, you’ve found the pair. If not, add the current num and its index to the hash map. This gives a time complexity of O(n).

78. How would you reverse a string? In Python, this is famously simple: my_string[::-1]. In other languages, you might use a loop, iterating from the end of the string to the beginning and building a new string, or swap characters in place using two pointers.

79. Explain FizzBuzz. This is a classic screening question. Write a program that prints the numbers from 1 to 100. But for multiples of three, print “Fizz” instead of the number, and for the multiples of five, print “Buzz”. For numbers which are multiples of both three and five, print “FizzBuzz”.

• Logic: Use a loop from 1 to 100. Inside the loop, use the modulo operator (%). First, check for the most specific condition (num % 15 == 0), then the less specific ones (num % 3 == 0 and num % 5 == 0).

80. What is Big O notation? Big O notation is used to describe the performance or complexity of an algorithm. It specifically describes the worst-case scenario and can be used to describe the execution time required or the space used (e.g., in memory or on disk) by an algorithm. For example, O(n) means the complexity grows linearly with the size of the input n. O(1) means constant time, regardless of the input size.

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