Coroutine

You might say

Do not block the execution path while an endpoint waits on external services. Suspend the coroutine and let other work proceed.

A coroutine is a lightweight execution unit that can suspend at explicit points and resume later.When an endpoint calls three external services, coroutines can yield at each await so the same thread advances other work, then resume when responses arrive and combine results. A coroutine is not a new thread and does not guarantee parallelism; CPU-heavy work is not automatically faster.

Know first

Thread

How a coroutine suspends and resumes around an external wait

Async request schedulingThread 1

Request Awaits 2s for API

Request Bqueued behind

CPUidle while waiting

EvidenceBlocking wait occupies the execution path

Later work cannot use the I/O wait.

How coroutines use I/O wait time

Problem and key mechanism: When a request synchronously holds its execution path while waiting for an external API, later work queues behind it; at each await, a coroutine preserves its progress, suspends, and yields execution, then resumes when the response arrives so other coroutines can use the I/O wait time.

Verify overlapping waits: With one slow response, other coroutines still advance and suspension is traceable. This does not prove CPU work runs in parallel.

Chat attachment uploads also suit coroutines: When a chat app uploads several attachments, each upload coroutine can suspend while waiting on the network. Other uploads continue and the suspended one resumes when its response arrives.

You can say this to an AI Agent

Run these three external API calls as coroutines that suspend during I/O, yield execution, then resume and combine results. Record suspension, resumption, and elapsed time. Add one slow response and verify other work is not blocked by synchronous waiting. Do not claim CPU-heavy work becomes faster automatically.

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An endpoint waits for three external APIs and does almost no computation while waiting. Which change uses coroutines appropriately?

A. Suspend at waits, let the executor advance other coroutines, then resume on responseB. Call every async function a coroutine and assume each runs on its own threadC. Wrap heavy CPU computation in a coroutine and assume it becomes parallel and faster