> For the complete documentation index, see [llms.txt](https://trizen.gitbook.io/perl6-rosettacode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://trizen.gitbook.io/perl6-rosettacode/programming_tasks/m/metered_concurrency.md).

# Metered concurrency

Uses a buffered channel to hand out a limited number of tickets.

```perl
class Semaphore {
    has $.tickets = Channel.new;
    method new ($max) {
        my $s = self.bless; 
        $s.tickets.send(True) xx $max;
        $s;
    }
    method acquire { $.tickets.receive }
    method release { $.tickets.send(True) }
}

sub MAIN ($units = 5, $max = 2) {
    my $sem = Semaphore.new($max);

    my @units = do for ^$units -> $u {
        start {
            $sem.acquire; say "unit $u acquired";
            sleep 2;
            $sem.release; say "unit $u released";
        }
    }
    await @units;
}
```

#### Output:

```
unit 0 acquired
unit 1 acquired
unit 0 released
unit 1 released
unit 3 acquired
unit 2 acquired
unit 3 released
unit 2 released
unit 4 acquired
unit 4 released
```