Interview questions in Backend Dev, page 11
Embed or reference likes in a document database?
Embedding is fast for small bounded lists but unbounded likes hit document size limits; referencing scales for high-cardinality, write-heavy likes.
How do you dynamically discover and register FastAPI routers from a directory?
Scan app/routers/ with importlib, validate APIRouter objects, include_router with prefixes, and isolate failures.
Describe Rust's orphan rule and its ecosystem purpose
State that either trait or type must be local; explain this stops conflicting foreign impls; note crates.io would see silent impl collisions breaking downstream builds.
Status codes for successful POST and GET
201 Created for a successful POST (ideally with a Location header), 200 OK for a successful GET returning data.
What are eventual consistency and the BASE model?
Eventual consistency means replicas converge given no new writes; BASE is Basically Available, Soft state, Eventually consistent.
Associated types vs generic type parameters in traits
Associated types fix one type per implementer; generics allow many impls; Iterator::Item is the canonical example.
How do you manage service lifecycle with FastAPI Depends versus formal DI?
Tests scaling FastAPI DI beyond routes. Answer: use Depends(yield) for request-scoped DB sessions; use a formal container for deep singleton service graphs and lifespan wiring; hybrid is best. Red flag: using Depends for everything and ignoring testability.
Modularize routes with express.Router
Create a Router per resource in its own file, define routes on it, export it, and mount with app.use('/products', router).
Why fit Cassandra to a high-read, high-write workload?
Consistent-hash partitioning spreads load, replication and no single master give availability, log-structured writes are fast, tunable consistency balances per query.
Override a FastAPI dependency at the APIRouter level
Tests FastAPI DI scoping limits. Answer: APIRouter has no dependency_overrides; create a sub-app, apply overrides, mount it. Red flag: Claiming router-level overrides exist or mutating global app state.
What is the fundamental difference between a goroutine and an OS thread?
This tests your grasp of Go's M:N scheduler. A strong answer notes that goroutines are runtime-managed, multiplexed onto OS threads, and use far less memory per unit, enabling thousands of concurrent tasks.
Idempotency: PUT vs POST in REST
Idempotent means repeated identical requests leave the same server state; PUT is idempotent, POST is not. Use PUT to overwrite a resource at a known URL.
How do you keep consistency without multi-document transactions?
A Saga runs a sequence of local transactions, each with a compensating action to undo on failure, coordinated via choreography or orchestration.
What is the difference between async def and regular functions?
This tests async def call semantics. A strong answer contrasts regular functions, which execute immediately, with async def, which returns a coroutine object that does nothing until awaited or wrapped in create_task.
How do Send and Sync prevent data races? Give a Send-but-not-Sync example.
Explain Send moves values and Sync allows shared references across threads; give Cell as Send but not Sync because it has interior mutability.
404 vs 500: missing resource vs server failure
A missing resource returns 404 Not Found (client asked for something absent); a database failure returns 500 Internal Server Error (server-side fault).
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…
Explain await's purpose, awaitable types, and event loop signaling
This tests whether you understand await as a yield point. A strong answer lists three awaitables (coroutines, Tasks, Futures), explains await yields control to the event loop until completion, and warns that a bare coroutine call does not run it.
Buffered vs unbuffered Go channels and deadlock scenario
Tests channel synchronization semantics. Unbuffered channels block until sender and receiver rendezvous; buffered channels block only when full or empty. Deadlock: goroutine sends on unbuffered channel with no receiver.
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.
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