Concurrency
also: concurrent, concurrently
When several tasks are in progress during the same stretch of time, instead of strictly one after another. Your app might be handling hundreds of people's requests at once.
A system's ability to have multiple tasks in progress with overlapping lifetimes, such as many requests handled by one server. Tasks may take turns on one processor or truly run in parallel on several. Shared data then needs coordination.
A barista juggling five orders: steaming milk for one while espresso pours for another. All five orders are in progress at once, even though her hands do one thing at a time.
In production, your app is never used by one person at a time. When your agent talks about concurrency, it's asking what happens when many things touch the same data at once. That's where double bookings and lost updates come from.
I've limited concurrency to 5 so we stay under the email provider's rate limit. Raising it would be faster but risks getting temporarily blocked.
This import is slow. Can we process rows concurrently? First explain what could go wrong if two rows update the same customer at once.
Thinking concurrency only matters for huge apps. Even two users, or one user with two browser tabs, can have two requests changing the same data at once.