PhabricatorMetronome.php
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	<?php

	/**
	* Tick at a given frequency with a specifiable offset.
	*
	* One use case for this is to flatten out load spikes caused by periodic
	* service calls. Give each host a metronome that ticks at the same frequency,
	* but with different offsets. Then, have hosts make service calls only after
	* their metronome ticks. This spreads service calls out evenly more quickly
	* and more predictably than adding random jitter.
	*/
	final class PhabricatorMetronome
	extends Phobject {

	private $offset = 0;
	private $frequency;

	public function setOffset($offset) {
	if (!is_int($offset)) {
	throw new Exception(pht('Metronome offset must be an integer.'));
	}

	if ($offset < 0) {
	throw new Exception(pht('Metronome offset must be 0 or more.'));
	}

	// We're not requiring that the offset be smaller than the frequency. If
	// the offset is larger, we'll just clamp it to the frequency before we
	// use it. This allows the offset to be configured before the frequency
	// is configured, which is useful for using a hostname as an offset seed.

	$this->offset = $offset;

	return $this;
	}

	public function setFrequency($frequency) {
	if (!is_int($frequency)) {
	throw new Exception(pht('Metronome frequency must be an integer.'));
	}

	if ($frequency < 1) {
	throw new Exception(pht('Metronome frequency must be 1 or more.'));
	}

	$this->frequency = $frequency;

	return $this;
	}

	public function setOffsetFromSeed($seed) {
	$offset = PhabricatorHash::digestToRange($seed, 0, 0x7FFFFFFF);
	return $this->setOffset($offset);
	}

	public function getFrequency() {
	if ($this->frequency === null) {
	throw new PhutilInvalidStateException('setFrequency');
	}
	return $this->frequency;
	}

	public function getOffset() {
	$frequency = $this->getFrequency();
	return ($this->offset % $frequency);
	}

	public function getNextTickAfter($epoch) {
	$frequency = $this->getFrequency();
	$offset = $this->getOffset();

	$remainder = ($epoch % $frequency);

	if ($remainder < $offset) {
	return ($epoch - $remainder) + $offset;
	} else {
	return ($epoch - $remainder) + $frequency + $offset;
	}
	}

	public function didTickBetween($min, $max) {
	if ($max < $min) {
	throw new Exception(
	pht(
	'Maximum tick window must not be smaller than minimum tick window.'));
	}

	$next = $this->getNextTickAfter($min);
	return ($next <= $max);
	}

	}

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