What is a runtime?

Tokio is the async runtime for Rust.

• Think: thread pool + task scheduler + event loop.
• It drives your futures to completion.
• It gives you #[tokio::main] to start your async main().
• It provides async I/O (files, network, timers, etc.).

Tokio's Job:

• Poll your futures repeatedly.
• Wait for external events (e.g., HTTP done, file ready).
• Wake up your future and call poll() again only when needed.

One-liner intuition:

poll() = "Are you done yet?"
Future: "No." → Pending
Future: "Yes!" → Ready(output)

FuturesUnordered