Threads, channels, locks, atomics and Tokio, with a bias towards the bug you will hit at 3am.
Retries fix transient failures and amplify real outages. The right combination is a timeout per attempt, capped jittered backoff, a total deadline and a breaker that stops hammering.
A pool hands out reusable connections, caps how many exist and takes them back on drop. Building a small one explains the knobs on SQLx and deadpool.
A token bucket allows bursts up to a capacity and a steady refill rate. The whole algorithm is a timestamp and a float, and the async part is waiting for the next token.
A chat room is a one to many fan out. Tokio's broadcast channel gives each client its own receiver, and lagging clients are handled for you.
The core of every network service is an accept loop that spawns a task per connection. This walks through an echo server and a line protocol with Tokio, then adds limits.
io_uring submits I/O to the kernel in batches and reads completions from a ring. Rust crates for it exist, but the ownership model differs from Tokio's, and the gains are workload specific.
tokio::spawn demands Send because tasks can move between threads. LocalSet pins tasks to one thread so Rc, RefCell and other !Send types can cross awaits.
The default of one worker per core is right for most services. Change it when a container limit misleads Tokio, or when measurements show queueing or contention.
The numbers that tell you a runtime is struggling are queue depth, worker busy time and poll counts. Tokio exposes them, and a small task exports them to Prometheus.
A hang is usually one of four things: a blocked runtime thread, a lock held across an await, a future that never wakes or a channel nobody closes. Here is the order to check them.
An executor is a loop that polls tasks when their wakers fire. Build one with the standard library alone and Tokio stops looking like magic.
An async fn is a state machine behind the Future trait. Writing a future yourself, with poll and a waker, removes the mystery from await.
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