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Collect concurrent errors with Group

Concurrently Collecting Errors with a Group​

When you need to run multiple independent operations concurrently and collect any errors they produce, you can use a multierror.Group. This is useful for tasks like parallelizing API calls or processing items from a work queue, where you want all operations to complete regardless of individual failures.

The Group provides two primary methods: Go to start a function in a new goroutine, and Wait to block until all started goroutines have finished.

Waiting for Successful Operations​

If you start several functions with group.Go and none of them return an error, group.Wait will return nil. This confirms that all concurrent operations completed successfully. You can use sync/atomic to safely verify that all your goroutines executed.

package main

import (
	"sync/atomic"

	"github.com/hashicorp/go-multierror"
)

func main() {
	var group multierror.Group
	var ran atomic.Int32
	group.Go(func() error { ran.Add(1); return nil })
	group.Go(func() error { ran.Add(1); return nil })
	result := group.Wait()
	if result != nil || ran.Load() != 2 {
		panic("expected both functions and no errors")
	}
}

Aggregating Errors from Failed Operations​

When functions started with group.Go return non-nil errors, group.Wait collects them. After all goroutines finish, Wait returns a single error value that contains all the errors that occurred. If at least one function returns an error, the result of Wait will be non-nil.

The order in which the functions are executed or the errors are collected is not guaranteed. The primary purpose is to know that all tasks have finished and to receive a collection of any errors that happened along the way.

package main

import (
	"errors"
	"sync/atomic"

	"github.com/hashicorp/go-multierror"
)

func main() {
	var group multierror.Group
	var ran atomic.Int32
	group.Go(func() error { ran.Add(1); return errors.New("alpha") })
	group.Go(func() error { ran.Add(1); return errors.New("beta") })
	result := group.Wait()
	if result == nil || ran.Load() != 2 {
		panic("expected both functions and errors")
	}
}