5 exercises — core backend terms: middleware, caching, ORM, idempotency, and the N+1 query problem. Essential vocabulary for backend interviews and team discussions.
In backend development, middleware is best described as:
Middleware is a function in a request pipeline that can inspect, transform, or terminate a request/response — it runs before the final route handler. Examples: authentication middleware (checks the JWT), logging middleware (records the request), rate-limiting middleware, CORS middleware. In Express.js: app.use(myMiddleware). In Django: MIDDLEWARE = [...]. The term can also refer to infrastructure middleware (message brokers, API gateways), but in backend code the most common meaning is "pipeline layer".
2 / 10
Complete the sentence using the correct backend term: "To avoid overloading the database, we added a _____ layer using Redis so that frequently-requested data is served without hitting the DB every time."
Caching — storing the result of an expensive operation so future requests can be served from the fast cache instead of re-computing. Redis and Memcached are the most common caching solutions. Related terms: cache invalidation (the hard problem — deciding when to expire/update cached data); cache hit/miss (hit = found in cache; miss = had to fetch from DB); TTL (Time To Live — how long a cache entry lives); write-through vs. write-behind (when the cache is updated relative to the DB). Load balancing distributes traffic; sharding splits a database; indexing speeds up queries — all distinct.
3 / 10
What is an ORM (Object-Relational Mapper)?
An ORM maps database tables to objects in the application language, allowing queries like User.findAll({ where: { active: true } }) instead of raw SQL. Popular ORMs: Sequelize (Node.js), Prisma (Node.js), SQLAlchemy (Python), Django ORM (Python), Hibernate (Java), ActiveRecord (Ruby). Trade-offs: ORMs reduce boilerplate and SQL injection risk, but can generate inefficient queries and abstract away performance-critical details. Schema migration tools (like Alembic, Flyway, Liquibase) are a separate concern — though many ORMs include migration features.
4 / 10
In the context of APIs, what does idempotency mean?
Idempotency: performing the same operation multiple times has the same effect as performing it once. HTTP method idempotency: GET, PUT, DELETE, HEAD are idempotent (calling them again doesn't change state). POST is NOT idempotent (sending the same POST twice creates two resources). Why it matters: in distributed systems, network retries can cause operations to execute more than once. Idempotent operations are safe to retry. Related: Idempotency-Key header — used in payment APIs (Stripe) to safely retry a charge without double-charging.
5 / 10
What is N+1 query problem in backend development?
The N+1 problem: 1 query to get N records, then N separate queries to get related data for each record — N+1 total. Example: fetch 100 users (1 query), then for each user fetch their profile (100 queries) = 101 queries instead of 2. Fix: use eager loading (JOIN or include in ORM), or a DataLoader pattern (batch + cache). This is one of the most common backend performance problems and a frequent interview question. ORMs make N+1 easy to accidentally create because the lazy-loading behaviour is implicit.
6 / 10
Sarah from the QA team just posted this comment on your code review: 'I'm getting a 500 error when trying to access the /users endpoint. The logs show a NullPointerException in the UserService class, specifically within the `getUserById` method. Can you investigate?' What does Sarah's message primarily indicate about the backend issue?
Sarah's message highlights a production issue: a 500 error. This indicates that something went wrong during request processing – most likely an unhandled exception. The NullPointerException specifically points to a problem within the `getUserById` method of the `UserService` class, suggesting a logic or data access flaw.
7 / 10
During your daily standup, you're asked: 'What did you work on yesterday and what are you planning for today?' You reply: 'I implemented a new API endpoint for user profile updates. I'm using DELETE to remove old data from the database and ensuring idempotency.' Complete the sentence by adding the correct backend term that describes this action.
Idempotency in an API means that multiple identical requests have the same effect as a single request. 'Rate limiting' is a common technique used to achieve this by restricting the number of calls to an endpoint within a given timeframe, preventing accidental or malicious overuse and ensuring predictable behavior; using DELETE alone doesn't guarantee idempotency.
8 / 10
The following API response was received from the microservice: `{"status": "200", "data": {"user_id": 123, "username": "john.doe", "email": "john.doe@example.com"}}`. What does this JSON response primarily convey about the backend operation?
The response code `200` signifies a successful operation. The `data` field contains structured information about the user (user ID, username, and email), confirming that the backend successfully retrieved and returned this data to the client. This is a standard representation of a successful API call.
9 / 10
You're writing a PR description for a change that adds support for asynchronous processing of image uploads. The PR includes a message to the Slack channel: 'Implemented background job queue using Celery to handle large image uploads, improving API response times.' What is the primary technical benefit of using a job queue in this scenario?
A job queue decouples the API from the image processing task. By placing the image upload in a background job, the API can respond quickly without waiting for the potentially lengthy process to complete. This significantly reduces server load and improves overall user experience by preventing slow API responses.
10 / 10
You're debugging a performance issue with your backend service. You discover that the application is making multiple database queries to retrieve related data for each user profile. This is causing significant delays. What is this problem most likely referred to as?
The 'N+1 query problem' occurs when a single query triggers N additional queries to retrieve related data. In this case, fetching user profiles often requires one initial query followed by one query for each profile to retrieve associated information – creating a performance bottleneck.
These modules build the same on-the-job skills as Backend Development Vocabulary
— work through them together for a fuller vocabulary set.
Database— useful for Backend fundamentals (Full-Stack Developer)
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