Interview questions in Backend Dev, page 6
Reading a POST body from the request stream
Body arrives in chunks via data events, accumulate them, on the end event concatenate and JSON.parse inside try/catch.
Composite index column order for multi-column filters
Create one index on (last_name, first_name); order matters because the index serves leftmost-prefix lookups.
Rust binary and library crates in one project
A binary crate has main and produces an executable; a library crate has lib.rs and is reusable; put logic in the lib and a thin main that calls it.
How do you define a Pydantic model and use it in FastAPI?
Subclass BaseModel with name str and age int, then type-hint the parameter with the model.
path.resolve vs path.join
Join concatenates and normalizes relative segments, resolve builds an absolute path right to left from cwd and resets on any absolute segment.
Clustered versus non-clustered indexes
A clustered index orders the table's actual rows, one per table; a non-clustered index is a separate structure pointing to rows.
How would you use a Pydantic response_model to enforce output structure?
Tests separation of internal models from API contracts. Define a Pydantic output model with only safe fields, set it as the endpoint response_model, and let FastAPI filter and validate.
What is the purpose of the internal directory in Go?
Tests Go visibility boundaries beyond exported vs unexported. A strong answer states that internal is compiler-enforced module privacy, while lowercase is only package-private. Red flag: calling internal a naming convention rather than a build boundary.
fs.watch vs fs.watchFile
Watch uses OS event notifications, efficient but inconsistent across platforms, watchFile polls stat at an interval, reliable but slower.
What is a query execution plan?
The plan is the optimizer's chosen tree of operators; run EXPLAIN or EXPLAIN ANALYZE; watch for sequential scans, bad row estimates, and costly joins.
Define a FastAPI endpoint with path and query parameters
Tests if you know FastAPI infers parameter location from the route string. Good answer: route with {item_id}, signature item_id: int, q: str | None = None, noting any param not in the path becomes a query param.
Explain the difference between mod and use in Rust
Mod tells the compiler to compile a file into the crate tree; use brings an existing path into scope as a shortcut.
Scaling across cores with cluster and os
Os.cpus gives core count, the primary forks one worker per core, all workers share the listening port, the OS load-balances connections.
Trade-offs of adding indexes to a table
Indexes speed reads but slow writes since every INSERT, UPDATE, and DELETE must maintain them; they consume storage and can be unused on low-selectivity columns.
Rust workspace versus single crate for plugins
A workspace gives incremental compilation, enforced API boundaries via a shared api crate, and per-plugin deps; a single crate is simpler but recompiles wholesale and blurs boundaries.
Implement a FastAPI file upload endpoint with form data
Tests FastAPI multipart literacy. A strong answer names python-multipart, uses Annotated[UploadFile, File()] for the image, and Annotated[str, Form()] for user_id.
Backpressure in Node.js streams
Backpressure pauses the readable when the writable buffer fills, pipe and pipeline manage it automatically, pipeline also propagates errors and cleans up.
What is a covering index?
A covering index contains every column a query needs so the engine answers from the index alone, skipping the table heap; build it by including filter, join, and selected columns.
How do you create a reusable current-user dependency in FastAPI?
Tests DRY auth with FastAPI Depends. Answer: create get_current_user that Depends on OAuth2PasswordBearer, verifies token, returns User model, inject into routes. Red flag: middleware or manual header parsing in each endpoint.
Describe Go's memory management, garbage collection, and trade-offs
Explain Go's GC recycles heap memory, the compiler stack-allocates locals, and automatic collection costs runtime overhead.
We are hiring for this. Every open role lists the topics its interview covers, so you can prepare for the real thing rather than guessing.
See open roles