Backend & APIs interview questions

.NET is Microsoft's free, open-source developer platform for building web, desktop, mobile, cloud and game applications, mainly in C#.

.NET Framework is the original Windows-only version, now in maintenance mode. .NET, formerly .NET Core, is the modern, cross-platform and open-source successor, where all new development happens.

The web framework of modern .NET. It is used to build REST APIs, server-rendered websites, real-time apps and Blazor web interfaces.

No. C# is by far the most used, but F# and Visual Basic also run on .NET, and all of them can use the same libraries.

An API is a set of rules that lets one piece of software request data or actions from another in a predictable, documented way.

No. API is the general concept; a REST API is one style of API built on HTTP that follows REST principles.

An API key is a secret value sent with requests so the API can identify the caller and control or limit access.

An API gateway is a server that sits in front of a group of backend services and acts as the single entry point that receives, checks, and routes API requests.

A load balancer spreads traffic across several copies of the same service to share the load. An API gateway routes requests to different services based on the API path and adds features like authentication and rate limiting, and it often sits in front of load balancers.

An API gateway is a kind of reverse proxy. A plain reverse proxy mainly forwards traffic, while an API gateway adds API-specific features such as API key checks, per-client rate limits, and request transformation.

Not always. A single monolithic application can often handle authentication and rate limiting itself, but a gateway becomes valuable once clients would otherwise call many separate services directly.

API versioning is the practice of labeling and managing changes to an API so existing clients keep working while new versions add or change features.

There is no single best way, but URL path versioning such as /v1/ is the most common because it is simple, visible, and easy to cache and test. Header-based versioning keeps URLs clean but is harder to try out in a browser.

Create one only for breaking changes, such as removing or renaming fields, changing data types, or adding required parameters. Backward-compatible changes like new optional fields or new endpoints can ship in the current version.

It depends on your users, but public APIs commonly give at least 6 to 12 months of notice before retiring a version. Announce the timeline, send deprecation headers, and track which clients still use the old version.

An application server is software that runs an application's code and business logic, turning requests into dynamic responses, usually behind a web server.

A web server handles HTTP connections, HTTPS and static files, while an application server runs the code that builds dynamic responses. In most deployments, the web server sits in front and forwards dynamic requests to the application server.

Loosely, yes, and many people call it one. Strictly, Tomcat is a servlet container that implements only the web parts of Jakarta EE, while full application servers such as WildFly or GlassFish also implement the rest of the platform, such as transactions and messaging.

Every web application needs something that runs its code, but today it is often built in rather than installed separately. Spring Boot, ASP.NET Core and Node.js apps include their own server, and containers have largely replaced deploying many apps to one shared Java application server.

The backend is the server side of an application: the code, databases and services that store data, apply business rules and answer the frontend's requests.

Almost any general-purpose language: JavaScript or TypeScript with Node.js, Python, Java, C#, Go, PHP and Ruby are all common. The choice usually depends on the team, the libraries available and how fast the service needs to be.

A purely static site, or an app that keeps everything on the device, can work without a backend of its own. Many apps instead rely on managed services, such as a backend-as-a-service or serverless functions, so someone else runs the servers.

A background job is a task that a server runs outside the normal request-response cycle, so slow work like sending emails doesn't make users wait.

A cron job runs on a fixed schedule, such as every night at 2 a.m. A background job is usually created on demand by an event, such as a user uploading a file, and runs as soon as a worker is free, although many job systems also support scheduled jobs.

A worker is a separate process that waits for jobs on a queue, runs them, and reports whether they succeeded. You can run several workers in parallel to process jobs faster.

Use one when work takes more than a fraction of a second, depends on a slow external service, or doesn't need to finish before the user gets a response. Sending emails, processing uploads, and generating reports are classic examples.

Backpressure is a mechanism that lets a slow consumer signal a fast producer to slow down, so data doesn't pile up faster than it can be processed.

Data piles up in memory between the fast and slow parts of the system. Latency climbs as the backlog grows, and eventually the process runs out of memory, crashes, or starts dropping data.

No. Rate limiting applies a fixed, predefined cap on requests, while backpressure is a dynamic signal based on how much the consumer can handle right now. Systems often use both.

Consumers pull messages at their own pace, often with a limit on how many they take at once, so the queue absorbs bursts. A growing backlog, called consumer lag, is the signal to add consumers or slow producers, and bounded queues can reject new messages when full.