https://github.com/jnthn/oo-monitors

Monitors with condition variables for Raku
https://github.com/jnthn/oo-monitors

Last synced: 5 days ago
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Monitors with condition variables for Raku

• Host: GitHub
• URL: https://github.com/jnthn/oo-monitors
• Owner: jnthn
• Created: 2014-10-11T13:36:46.000Z (about 10 years ago)
• Default Branch: master
• Last Pushed: 2021-05-24T23:00:02.000Z (over 3 years ago)
• Last Synced: 2023-04-23T05:27:53.141Z (over 1 year ago)
• Language: Raku
• Homepage:
• Size: 19.5 KB
• Stars: 11
• Watchers: 5
• Forks: 7
• Open Issues: 4
• Metadata Files:
  • Readme: README.markdown

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README

        
# OO::Monitors

[![Build Status](https://travis-ci.org/jnthn/oo-monitors.svg?branch=master)](https://travis-ci.org/jnthn/oo-monitors)

A monitor provides per-instance mutual exclusion for objects. This means that

for a given object instance, only one thread can ever be inside its methods at

a time. This is achieved by a lock being associated with each object. The lock

is acquired automatically at the entry to each method in the monitor. Condition

variables are also supported.

## Basic Usage

A monitor looks like a normal class, but declared with the `monitor` keyword.

    use OO::Monitors;

    

    monitor IPFilter {

        has %!active;

        has %!blacklist; 

        has $.limit = 10;

        has $.blocked = 0;

        

        method should-start-request($ip) {

            if %!blacklist{$ip} || (%!active{$ip} // 0) == $.limit {

                $!blocked++;

                return False;

            }

            else {

                %!active{$ip}++;

                return True;

            }

        }

        

        method end-request($ip) {

            %!active{$ip}--;

        }

    }

That's about all there is to it. The monitor meta-object enforces mutual

exclusion.

## Conditions

Condition variables are declared with the `conditioned` trait on the monitor.

To wait on a condition, use `wait-condition`. To signal that a condition has

been met, use `meet-condition`. Here is an example of a bounded queue.

    monitor BoundedQueue is conditioned(< not-full not-empty >) {

        has @!tasks;

        has $.limit = die "Must specify a limit";

        

        method add-task($task) {

            while @!tasks.elems == $!limit {

                wait-condition ;

            }

            @!tasks.push($task);

            meet-condition ;

        }

        method take-task() {

            until @!tasks {

                wait-condition ;

            }

            meet-condition ;

            return @!tasks.shift;

        }

    }

When `wait-conditon` is used, the lock is released and the thread blocks until

the condition is met by some other thread. By contrast, `meet-condition` just

marks a waiting thread as unblocked, but retains the lock until the method

is over.

## Circular waiting

Monitors are vulnerable to deadlock, if you set up a circular dependency. Keep

object graphs involving monitors simple and cycle-free, so far as is possible.