What is the difference between concurrency and parallelism?

technical-conceptual · Junior level · software-engineering

What the interviewer is really asking

Tests understanding of fundamental execution models that affect how engineers write and reason about multi-threaded or async code.

What to say

• Concurrency = dealing with multiple things at once (structure); parallelism = doing multiple things at once (execution). An analogy: one barista switching between coffee orders = concurrent; two baristas = parallel.
• Point out that concurrency is possible on a single core (via context switching), while parallelism requires multiple cores.
• Mention a real-world example: Node.js is concurrent but single-threaded; Go goroutines can be both concurrent and parallel.

What to avoid

• Treat the terms as synonyms.
• Say parallelism is always faster without acknowledging overhead (thread management, synchronization).
• Ignore the distinction between I/O-bound and CPU-bound workloads.

Example answers

Strong: A web server handling 1,000 simultaneous requests on a single thread via async I/O is concurrent — it interleaves progress without running them truly at the same time. If it used a thread pool across 8 cores, it's also parallel.

Weak: Concurrency is when your program uses multiple threads. Parallelism is the same thing.