Practice — Concurrency vs Parallelism (5 questions)

Define concurrency and parallelism precisely, and explain the relationship between them. Give one concrete example where a system is concurrent but not parallel.

Compare threads and processes across memory, communication cost, and crash isolation. When would you deliberately choose multiple processes over multiple threads?

A Python service has two hot paths:

1. A route that computes SHA-256 hashes over large in-memory buffers in a tight Python loop.
2. A route that fans out to five external HTTP APIs and aggregates the responses.

For each path, would you use threads, asyncio, or multiprocessing? Justify each answer in terms of the GIL.

The following async handler crashes under load with InvalidRequestError: This session is provisioning a new connection; concurrent operations are not permitted:

a, b, c = await asyncio.gather(
    get_top_subjects(db),
    get_top_questions(db),
    get_categories(db),
)

Explain the root cause in terms of concurrency, and give two different correct fixes.

If an asyncio web server runs on a single thread with a single event loop, how can it serve thousands of simultaneous users without blocking? What is the eventual bottleneck for database-heavy endpoints?