Intermediate interview questions in Backend Dev, page 4
Lifecycle of a single row update
Buffer manager faults the page in, the row is modified in memory marking the page dirty, a WAL record is written, and commit fsyncs the log while the dirty page is flushed later by a checkpoint.
How would you override a FastAPI dependency during testing?
Tests your grasp of FastAPI's dependency override mechanism. A strong answer mentions app.dependency_overrides, notes that sub-dependencies are bypassed, and stresses clearing overrides after each test.
B+ Tree range queries across pages
Internal nodes are pages of keys guiding the search, all data sits in linked leaf pages; a range query descends to the start key then follows the leaf chain sequentially until the upper bound.
How do you use a Python class as a FastAPI dependency?
It tests whether you understand FastAPI DI beyond functions and when stateful encapsulation wins. Explain that Depends takes callable classes, centralizing setup and shared state in __init__. Red flag: claiming classes are pure syntactic sugar.
How does FastAPI execute setup and teardown in nested yield dependencies?
It tests FastAPI's dependency injection lifecycle and stack-like teardown for nested yield dependencies. Setup runs top-down; teardown runs bottom-up after the response. A red flag is claiming teardown order is arbitrary or follows garbage collection.
How does FastAPI cache dependencies within a single request?
Tests if you know FastAPI caches a dependency after the first call in a request and reuses it across the tree. A strong answer covers default use_cache=True, request-scoped lifetime, and disabling it.
Difference between errors.Is and errors.As in Go
This tests error-chain inspection in Go. errors.Is checks sentinel equality through wrapping, such as os.ErrNotExist. errors.As extracts a custom type, like os.PathError, into a variable. A red flag is using direct == or type assertions on wrapped errors.
What is the difference between unwrap and expect on Option and Result?
It tests your Rust error-handling discipline and justified panics. Both extract values or panic, but expect adds a custom message. Use them only for unrecoverable invariant violations, not routine errors. A red flag is using them as lazy error propagation.
How does the question mark operator use From to unify error types?
This tests Rust error conversion mechanics. You define a unified enum error and implement From for each source error so ? automatically converts via From::from. A red flag is suggesting manual match or map_err on every call instead of trait-based conversion.
Application-level vs router-level middleware
App.use() binds to the app and runs everywhere; router.use() binds to a Router and runs only for that router's routes.
Express error-handling middleware signature
(err, req, res, next) with err first, identified by arity, placed last after all routes.
Middleware execution order and sharing data via req
Middleware runs top-down in registration order; each calls next(); attach data like req.user that later handlers read.
Hash join versus sort-merge join
Hash join builds and probes a hash table, great for unsorted equality joins with enough memory; sort-merge sorts both inputs then merges, winning when inputs are already sorted or output must…
Sorting data larger than memory
External merge sort reads memory-sized chunks, sorts each in RAM and writes them as sorted runs to disk, then merges many runs together in passes until one sorted output remains.
How to apply a dependency to only one FastAPI router?
Pass dependencies=[Depends(auth)] to APIRouter for /users, omit it for /items, include both.
Explain Go's empty interface, safe usage, and runtime risks
Interface{} (alias any) holds values; unpack with type switches or ok assertions; risks are panics from bare assertions and nil interface vs nil concrete value confusion.
Manage dev, staging, and prod configs in a large FastAPI app
This tests separating environment config from code with pydantic-settings and env vars. A strong answer covers .env files for local dev, per-environment validation, and secret injection for prod. Red flags are hardcoded values or scattered conditionals.
Static dispatch with impl Trait versus dynamic dispatch with Box<dyn Trait>
Tests monomorphization versus vtables. Note: static dispatch monomorphizes for zero-cost abstraction but bloats code; dynamic dispatch uses vtables for smaller binaries but adds indirection.
Value versus pointer receivers and interface satisfaction
Value-receiver methods belong to both T and *T, but pointer-receiver methods belong only to *T, so a value of T may not satisfy an interface.
How do you apply a common path prefix across FastAPI routers?
Tests whether you know APIRouter decouples routes from path prefixes. Use relative paths in APIRouter, then mount with app.include_router(router, prefix="/api/v1"). This keeps modules reusable. Red flag: hardcoding the full absolute path in every decorator.
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