Definition
Plain language
Multiple things happening at the same time inside a program, which can produce bugs that depend on the exact order.
As stated in the literature
A programming regime where multiple threads or processes can execute interleaved or simultaneously, often producing bugs visible only under specific interleavings.
Why it matters: Concurrency bugs are notoriously hard to find with normal testing, which makes them a high-value target for both AI agents hunting for vulnerabilities and AI tools writing safer code.
For example, two threads both check 'is the file open?' at once, both see 'no,' and both try to open it, producing a bug that only shows up under load.
Heard on the show
“They tried more aggressive static drafters and shelved them, because at high concurrency they strictly hurt throughput.”Episode 179 — How DeepSeek Made One User Faster Without Slowing Down the Crowd