= metronome

== Description

Metronome is an interval scheduler and task runner.  It can be used
locally as a cron replacement, or as a network-wide job executor.

Events are stored via simple database rows, and optionally managed
via AMQP events.  Interval/time values are expressed with intuitive
English phrases, ie.: 'at 2pm', or 'Starting in 20 minutes, run every 10
seconds and then finish in 2 days', or 'execute 12 times during the next
minute'.

It includes an executable under bin/:

metronome-exp::
  A simple tester for trying out interval expression parsing.


== Synopsis

Here's an example of a cron clone:

	require 'symphony/metronome'

	Symphony.load_config

	Symphony::Metronome.run do |opts, id|
		Thread.new do
			pid = fork { exec opts.delete('command') }
			Process.waitpid( pid )
		end
	end


And here's a simplistic AMQP message broadcaster, using existing
Symphony connection information:

	require 'symphony/metronome'

	Symphony.load_config

	Symphony::Metronome.run do |opts, id|
		key = opts.delete( :routing_key ) or next
		exchange = Symphony::Queue.amqp_exchange
		exchange.publish( 'hi from Metronome!', routing_key: key )
	end


== Adding Actions

There are two primary components to Metronome -- getting schedules into
its database, and performing some task with those actions when the time
is appropriate.

By default, Metronome will start up an AMQP listener, attached to your
Symphony exchange, and wait for new scheduling messages.  There are two
events it will take action on:

metronome.create::
  Create a new scheduled event.  The payload should be a hash.  An
  'expression' key is required, that provides the interval description.
  Anything additional is serialized to 'options', that are passed to the
  block when the interval fires.  You can populate it with anything
  your task requires to execute.

metronome.delete::
  The payload is the row ID of the action.  Metronome removes it from
  the database.


If you'd prefer not to use the AMQP listener, you can put actions into
Metronome using any database methodology you please.  When the daemon
starts up or receives a HUP signal, it will re-read and schedule out
upcoming work.


== Performing Work

Calling 'run' on the Metronome class is a blocking call, waking up upon
a scheduled event.  The run method expects a ruby block, and it receives
the payload and the database ID of the scheduled event.  Metronome is
unopinioned, what you do within this block is entirely up to you.  See
the Synopsis section above for some examples.


== Options

Metronome uses
Configurability[https://rubygems.org/gems/configurability] to determine
behavior.  The configuration is a YAML[http://www.yaml.org/] file.  It
shares AMQP configuration with Symphony, and adds metronome specific
controls in the 'metronome' key.

	metronome:
		splay: 0
		listen: true
		db: sqlite:///tmp/metronome.db


=== splay

Randomize all start times for actions by this many seconds on either
side of the original execution time.  Defaults to none.

=== listen

Start up an AMQP listener using Symphony configuration, for remote
administration of schedule events.  Defaults to true.

=== db

A {Sequel}[https://rubygems.org/gems/sequel] connection URI.  Currently,
Metronome is tested under SQLite and PostgreSQL.  Defaults to a SQLite
file at /tmp/metronome.db.


== Scheduling Examples

Note that Metronome is designed as an interval scheduler, not a
calendaring app.  It doesn't have any concepts around phrases like "next
tuesday", or "the 3rd sunday after christmas".  If that's what you're
after, check out the {chronic}[http://rubygems.org/gems/chronic]
library instead.

Here are a small set of example expressions.  Feel free to use the
+metronome-exp+ utility to get a feel for what Metronome anticipates.

  in 30.5 minutes
  once an hour
  every 15 minutes for 2 days
  at 2014-05-01
  at 2014-04-01 14:00:25
  at 2pm
  starting at 2pm once a day
  start in 1 hour from now run every 5 seconds end at 11:15pm
  every other hour
  run every 7th minute for a day
  once a day ending in 1 week
  run once a minute for an hour starting in 6 days
  10 times a minute for 2 days
  run 45 times every hour
  30 times per day
  start at 2010-01-02 run 12 times and end on 2010-01-03
  starting in an hour from now run 6 times a minute for 2 hours
  beginning a day from now, run 30 times per minute and finish in 2 weeks
  execute 12 times during the next 2 minutes
  once a minute beginning in 5 minutes

In general, you can use reasonably intuitive phrasings.  Capitalization,
whitespace, and punctuation doesn't matter.  When describing numbers,
use digit/integer form instead of words, ie: '1 hour' instead of 'one
hour'.


== Installation

    gem install symphony-metronome


== Contributing

You can check out the current development source with Mercurial via its
{project page}[http://bitbucket.org/mahlon/symphony-metronome]. 

After checking out the source, run:

    $ rake

This task will run the tests/specs and generate the API documentation.

If you use {rvm}[http://rvm.io/], entering the project directory will
install any required development dependencies.


== License

Copyright (c) 2014, Mahlon E. Smith
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are met:

* Redistributions of source code must retain the above copyright notice,
  this list of conditions and the following disclaimer.

* Redistributions in binary form must reproduce the above copyright notice,
  this list of conditions and the following disclaimer in the documentation
  and/or other materials provided with the distribution.

* Neither the name of the author/s, nor the names of the project's
  contributors may be used to endorse or promote products derived from this
  software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE
FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
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