Backend Developer 路 Intern to senior
Backend interviews test how you store, process and serve data reliably: language fundamentals, SQL and indexing, concurrency, API design and how systems behave under load.
131 questions
A HashMap stores entries in an array of buckets; it computes the key's hash code to choose a bucket, resolves collisions with a linked list that becomes a red-black tree when a bucket grows large, and resizes when the load factor of 0.75 is exceeded.
A decorator is a function that takes another function and returns a modified version of it, letting you add behaviour such as logging, caching or access control without changing the original code.
A generator is a function that uses yield to produce values one at a time and remembers its state between calls, so it can process large or infinite sequences without holding everything in memory.
CPython frees objects mainly through reference counting, deleting an object as soon as nothing refers to it, and uses a cyclic garbage collector to clean up groups of objects that reference each other.
Choose SQL when your data is relational and you need transactions, joins and strong consistency; choose NoSQL when you need flexible schemas, very high write throughput or horizontal scaling for a specific access pattern.
Caching stores frequently read data in fast storage such as memory to reduce latency and database load; the main strategies are cache-aside, write-through, write-back and time-based expiry (TTL).
REST exposes resources over HTTP URLs and is simple and cache-friendly, GraphQL lets clients ask for exactly the fields they need from a single endpoint, and gRPC uses binary Protocol Buffers over HTTP/2 for fast service-to-service calls.
An operation is idempotent if repeating it any number of times has the same effect as doing it once; it matters because network retries can otherwise create duplicate orders or payments.
Choose an algorithm such as token bucket or sliding window, store a counter per client (by user ID, API key or IP) in a fast shared store like Redis, and return HTTP 429 with a Retry-After header when the limit is exceeded.
git revert creates a new commit that undoes an earlier one and is safe on shared branches; git reset moves the branch pointer back to an earlier commit, effectively removing later commits from that branch鈥檚 history.
Docker Compose defines a multi-container application (services, networks and volumes) in one YAML file so you can start, stop and rebuild the whole stack with a single command, typically for local development and testing.
Treat a flaky test as a bug: quarantine it so it stops blocking the pipeline, reproduce it by running it repeatedly, fix the root cause (usually timing, shared state or external dependencies), then bring it back.
Describe a real technical or process disagreement, show that you listened and argued with evidence rather than opinion, explain how a decision was reached, and how you supported it afterwards even if it was not your idea.
Describe a problem you noticed, why you decided to act, how you coordinated with others, and the impact of taking ownership.
Explain how you identified ambiguity, clarified the important decisions, made reasonable assumptions and kept delivery moving.
Explain how you assessed urgency, impact, dependencies and risk, then communicated the resulting priorities.
Show respectful disagreement, evidence-based reasoning, willingness to listen and commitment after the final decision.
Identify a recurring source of wasted time or risk, make a focused improvement, and demonstrate its impact.
Explain the manual problem, why automation was worthwhile, how you implemented it safely and what time or error reduction resulted.
Show how you adapted your explanation to the audience, focused on the outcome and avoided unnecessary technical jargon.