[2020-04-06] Parallelism vs. Concurrency

I noticed developers often confuse both terms, so I thought I'd clarify then and keep as a quick reference for myself.

Parallelism is about doing lot of things at once. This requires CPU has > 1 cores so we can run multiple threads in parallel to speed up our program.

Concurrency is about dealing with lot of things at once. Typically, concurrency is applied when:

1. we have a shared resource that 2 threads need to access or update
2. there are multiple threads that need to coordinate

Parallelism and concurrency usually go hand in hand, like in the following scenario:

• split the sequantial flow into independent components
• use threads or threadpools to parallelize these components (& thus to speed up)
• whenever shared resource is to be updated or accessed, or whenever multiple threads want to coordinate with each other, we use concurrency tools to ensure the program runs correctly.