BackoffLib

---

A PHP library that implements various backoff (delay) routines, such as exponential backoff. The classes can be used for implementing various types of delays (i.e. using sleep(), delays between http requests). See below for a more descriptive usage example. To execute, call $class->backoff(); The backoff() function is declared abstract in BackoffBase and implemented in the various child classes.

• A exponential backoff delay with a default exponent of 2 will increment in the following way: 1, 2, 4, 16, 256

• An incremental backoff delay with a default increment of 1 will increment in the following way: 1, 2, 3, 4, 5, 6, 7

• A multiplicative backoff delay with a default exponent of 2 will increment in the following way: 0.5, 1, 2, 4, 8, 16, 32, 64

Example Usage: Using a backoff algorithm that slowly backs off api calls if they return a "busy" status code so the server isn't overwhelmed by repeated requests. Each time the server returns a "busy" status, the delay between api calls increases.

---

Classes

• BackoffBase - base class that all BackoffLib classes are inherited from.
• BackoffCaller - with a specified BackoffBase object: implements a delay handler callback and a OnFire callback function. Can be used to quickly implement a backoff with minimal code.
• BackoffExponential - exponential backoff
• BackoffExponentialMax- exponential backoff with maxiumum value
• BackoffIncremental - incremental backoff
• BackoffIncrementalMax - incremental backoff with maxiumum value
• BackoffMultiplicative - multiplicative backoff
• BackoffRandom - random backoff

---

Functions

• $Backoff->backoff(); - executes the backoff implementation.
• $Backoff->getInterval(); - implementation of the algorithm for the given class. Modifies $Backoff->interval.
• $Backoff->getTime(); - returns the value of the backoff (delay) $time after calling backoff().
• $Backoff->getIntervalValue(); - returns the value of the $interval for the class.

• $BackoffCaller($bo,$cb,$delayCb); - creates a BackoffCaller object with the specified BackoffBase object, a bool function($data); callback, and a int function($length) delay callback (return 0 on success).). See file header for more descriptive usage.
• $BackoffCaller->run(); - executes the callback in a loop until it returns true.

---

Interfaces

• IBackoffMaximum - implemented by classes having a maximum value.

---

Unit Tests

• Unit tests are in 'test/', and should be written and pass before any pull requests.
• PHPUnit is required to run the unit tests. Execute phpunit in the BackoffLib directory.

---

Examples

require('BackoffLib/Backoff.php');
$be = new BackoffLib\BackoffIncrementalMax(1, 2); //increment by 1, max 2
function beo($b) {
  $b->backoff();
  echo "\$b->time = ".$b->getTime() . PHP_EOL;
}
for($i = 1; $i <= 5; $i++)
  beo($be);
echo "be->getCount = ".$be->getCount() . PHP_EOL;

---

require('BackoffLib/Backoff.php');

$be = new BackoffLib\BackoffExponential(2.0);
function beo($b, $backoff=true) {
  if ($backoff)
    $b->backoff();
  echo "\$b->time = ".$b->getTime() . PHP_EOL;
}
beo($be,false);
beo($be);
beo($be);
beo($be);
beo($be);

---

Using BackoffCaller

  require_once('BackoffLib/Backoff.php');


  define('CB_MAX_COUNTER',      10);    //hard limit for number of times the callback fires
  define('CB_MAX_DELAY',        64);    //good limit for multiplicative backoff 

  //callback that fires on each backoff call
  $backoff_callback = function($delayValue) {
    static $counter = 0;
    $counter++; 

    if (is_numeric($delayValue))
      $delayValue = sprintf("%.2f", $delayValue); //truncate the value
      
    //output the call counter and the delay value 
    echo "[debug] callback #${counter}\t${delayValue} \n";

    if (isset($delayValue) && is_numeric($delayValue)) 
      if ($delayValue >= CB_MAX_DELAY) 
        return true;  //halt backoffCaller execution, the delay has reached its maximum allowed value
    if ($counter >= CB_MAX_COUNTER) 
      return true;    //halt execution, the callback counter has reached its maximum allowed value.
      
    return false;     //continue backoffCaller execution: the next delay->backoff calls will occur
  };

  $backoff_delay_callback = function($len) {
    sleep($len);  //implement a delay using sleep(): anything could be done here, i.e. limit the max delay.
    return 0;     //return 0 on success: the next backoff call will occur
  };
  
  //create a backoff that multiplies the interval by 2 each time it backs off
  $backoffClass = new BackoffLib\BackoffMultiplicative(2.0);  
  //create a caller class, that implements a basic delay-on-fire.  the delay is actually handled in the callback.
  $backoff = new BackoffLib\BackoffCaller($backoffClass, $backoff_callback, $backoff_delay_callback);

  $backoff->run();  //begin execution: can only be halted by returning TRUE from the callback.

---

License

BackoffLib is open source software, available under the MIT license. See the LICENSE file for more information.