Goroutines in Go are effectively lightweight threads. The Go runtime will schedule goroutines across a number of real operating system threads (often based on the number of available CPU cores on the system) so that they run concurrently on multi-core systems. It’s common in Go to communicate between goroutines using channels, which are inherently thread-safe.
In the case that you have lots of similar tasks to be performed but you want to control the number of tasks that can be ran simultaneously, you can build a worker pool. You may wish to do this to cap memory consumption, network traffic or file descriptor use.
Start by defining a job, which contains all of the context required for a worker
to complete its work. In our simple example, this is simply text to print:
type job struct {
text string
}
Create 30 sample job items which we will insert into our pending queue shortly:
jobs := []job{}
for i := 1; i <= 30; i ++ {
jobs = append(jobs, job{
text: fmt.Sprintf("Position %d in queue", i),
})
}
Create the pending job queue, which is a buffered
channel. Using a buffered channel here
allows us to write into the channel without blocking. Important to note here is
that the buffered channel must be equal to or greater than len(jobs),
otherwise any writes to the channel above the designated capacity will block.
Once we are done, we must close() the channel so that the workers will be able
to determine when there is no more work pending in the queue:
pending := make(chan job, len(jobs))
for _, j := range jobs {
pending <- j
}
close(pending)
Define a maximum number of workers. If the number of pending jobs is less than the maximum number of allowed workers then we should reduce the maximum number of allowed workers to the number of pending jobs—this prevents us from starting more workers than we need:
numWorkers := 5
if len(jobs) < numWorkers {
numWorkers = len(jobs)
}
Create a wait group and add the number of workers to the wait group. This allows us to block whilst waiting on the worker pool finishing all of the pending tasks:
var wait sync.WaitGroup
wait.Add(numWorkers)
Define what the worker should do for each job. In our example, we will simply
print the text from within the job struct:
work := func(j job) {
fmt.Println(j.text)
}
Then define the worker itself. The range ch here will continuously pull jobs
from the receive-only pending queue, as passed to the worker function, in a
thread-safe fashion. The range ch will automatically finish when there are no
more pending work objects in the buffered channel, reaching the channel closure
from above. Once the range ch loop ends, the defer statement will notify the
wait group that the worker is no longer running:
worker := func(ch <-chan job) {
defer wait.Done()
for j := range ch {
work(j)
}
}
Now that the worker pool has been prepared and there is work to be done waiting in the pending queue, we can now start the required number of workers to complete the work:
for i := 0; i < numWorkers; i++ {
go worker(pending)
}
Finally, we will wait for the worker pool to finish. Each worker signals to the wait group that they are no longer running when there is no more work to do. The following statement will block until that happens:
wait.Wait()
In this Go Playground sample based on
the above code, there is a larger number of queued objects and a time.Sleep()
in the work function. This helps to show that only five workers, as defined in
numWorkers, are running at a time, until there is no more work to be done.
You now have a worker pool!