Goroutines

Start thousands of concurrent tasks with a single keyword, and coordinate them safely with sync.WaitGroup.

7 min readProgramming Foundations (Go)Code

The go Statement

Prefixing a function call with go starts a goroutine — a lightweight concurrent task managed by the Go runtime. Goroutines are much cheaper than OS threads: you can start hundreds of thousands of them.

Gostarting-a-goroutine.go

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Both `fetch` calls run concurrently. The `time.Sleep` in main prevents the program from exiting before the goroutines finish — but sleeping is a poor synchronization mechanism. We will use `sync.WaitGroup` instead.

Goroutines are not OS threads

The Go runtime schedules thousands of goroutines onto a small pool of OS threads using cooperative and preemptive multitasking. A goroutine starts with just a few kilobytes of stack that grows as needed. Creating a goroutine costs almost nothing compared to creating a thread.

Goroutine Scheduling

Goroutines are multiplexed onto OS threads by the Go scheduler. When a goroutine performs a blocking operation (I/O, channel send/receive, system call), the scheduler runs another goroutine on that thread.

Goconcurrent-vs-sequential-timing.go

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All three tasks run concurrently. The total time is dominated by the slowest (200ms), not the sum (450ms). This is the power of concurrent I/O-bound work.

sync.WaitGroup

sync.WaitGroup is a counter you increment before starting work and decrement when work finishes. wg.Wait() blocks until the counter reaches zero.

Gowaitgroup-pattern.go

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Three rules: call `wg.Add(1)` before `go`, call `wg.Done()` inside the goroutine with `defer`, and always pass `&wg` (pointer) — never copy a WaitGroup after first use.

Closures and Goroutines

A goroutine launched inside a loop that captures the loop variable captures the variable itself, not its current value. By the time the goroutine runs, the loop may have moved on.

Goloop-variable-capture-—-the-classic-bug-and-fix.go

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Pass the loop variable as a function argument to create a per-goroutine copy. Go 1.22 changed loop variable semantics so each iteration has its own variable — but passing the value explicitly remains the clearest and most portable idiom.

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Packages, Modules & Concurrency Intro