Advanced interview questions in Backend Dev, page 4
What consistency do you sacrifice in an AP system?
You give up linearizability and often sequential consistency, accepting stale reads and conflicts, then mitigate with quorums, vector clocks or CRDTs, and…
Centralized error handling in an Express API
A final four-arg error middleware, an asyncHandler wrapper to funnel promise rejections via next, a custom error class with statusCode, returning uniform JSON.
API versioning: URL vs header strategies
Version via URL path (/v1/), a custom or Accept header, or a query param; URL is visible and cache-friendly, headers keep URLs clean but are less discoverable.
Unhandled exception in asyncio.create_task(): consequence and detection
Tests Task exception capture vs propagation. Good answer: exceptions are stored in the Task object, the loop keeps running, and the creator must await the task or call task.exception() to retrieve it; unretrieved ones may be logged.
When should Rust atomics replace a Mutex, and how do orderings work?
Atomics replace mutexes for counters; Relaxed is atomicity only, SeqCst adds global order, weaker ones skip fences on ARM.
How do you gracefully cancel and clean up an asyncio task?
This tests asyncio cooperative cancellation and cleanup. A strong answer covers catching CancelledError at await points, using try/finally or async context managers for cleanup, and re-raising.
Solving the N+1 query problem in Sequelize
Define N+1 as one parent query plus one per child, detect it via SQL logging, fix with eager loading using include.
Why separate storage and compute in a cloud warehouse?
Data lives in cheap shared object storage while independent compute clusters scale separately, enabling elastic, concurrent, isolated workloads and pay-per-use.
Why are contextvars better than threading.local in async Python?
This tests whether you know async tasks share OS threads, making thread-local storage unsafe for request state. A great answer notes ContextVar is task-local and resets automatically, while threading.local bleeds across concurrent coroutines.
Atomic order creation with Sequelize transactions
Wrap dependent writes in sequelize.transaction, pass the transaction to each query, let managed transactions auto-commit or roll back.
What is an OLAP cube and its operations?
A cube pre-aggregates measures across dimensions; operations are slice, dice, drill-down, roll-up, and pivot.
MongoDB aggregation pipeline for total sales
Explain the pipeline as ordered stages, use $group with $sum to total per productId, $match on the computed total, then $sort descending.
How do you manage multiple related Rust crates as a single unit?
Tests Cargo workspaces for multi-crate Rust projects. Strong answers cite the [workspace] section, shared Cargo.lock, unified target directory, inherited metadata, and workspace-wide commands.
Purpose and mechanism of a Rust build.rs script
Build.rs compiles and runs before the crate, emitting cargo: directives via stdout to set link flags, env vars, and rerun triggers; used to compile C, generate code, or probe the system.
Role-based access control middleware in Express
Authenticate first to set req.user, then a parameterized role-check middleware that compares req.user.role and returns 403 if it fails, applied to protected routes.
How do you atomically create an order and update inventory?
Tests transaction boundaries and SQLAlchemy 2.0 session lifecycle in FastAPI. A strong answer wraps both writes in session.begin(), flushes to catch constraint errors early, and uses exceptions to trigger rollback.
What does go mod tidy do beyond adding dependencies?
Tests reproducible Go module graph knowledge. A strong answer covers that tidy reconciles imports with go.mod, prunes unused modules, and ensures go.sum contains every checksum for the minimal build list.
SQLAlchemy connection pooling across Uvicorn workers
Each worker has its own pool; total DB connections equal workers times (pool_size plus max_overflow); overflow connections are temporary; misconfiguration exhausts DB…
Strategies for revoking stateless JWTs
Short-lived access tokens with refresh-token rotation, or a server-side denylist of revoked token ids, weighing statelessness against immediacy.
Durable write path in a sharded KV store
Route by key hash to the shard leader, append to WAL and fsync, replicate to two followers, ack on quorum, then confirm.
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