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Reimplement web-service throttle to use leaky bucket algorithm, and t…
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…o operate independently of logging.
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@trampgeek
trampgeek committed May 19, 2024
1 parent 15f42c2 commit 7266ee7
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Showing 3 changed files with 40 additions and 45 deletions.
28 changes: 14 additions & 14 deletions classes/external/run_in_sandbox.php
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Original file line number Diff line number Diff line change
Expand Up @@ -143,22 +143,22 @@ public static function execute(
throw new qtype_coderunner_exception(get_string('wsnolanguage', 'qtype_coderunner', $language));
}

if (get_config('qtype_coderunner', 'wsloggingenabled')) {
// Check if need to throttle this user, and if not or if rate
// sufficiently low, allow the request and log it.
$maxhourlyrate = intval(get_config('qtype_coderunner', 'wsmaxhourlyrate'));
if ($maxhourlyrate > 0) { // Throttling enabled?
if (!isset($SESSION->throttle)) {
$throttle = new \qtype_coderunner_wsthrottle();
} else {
$throttle = unserialize($SESSION->throttle);
}
if (!$throttle->logrunok()) {
throw new qtype_coderunner_exception(get_string('wssubmissionrateexceeded', 'qtype_coderunner'));
}
$SESSION->throttle = serialize($throttle);
// Check if need to throttle this submission.
$maxhourlyrate = intval(get_config('qtype_coderunner', 'wsmaxhourlyrate'));
if ($maxhourlyrate > 0) { // Throttling enabled?
if (!isset($SESSION->throttle)) {
$throttle = new \qtype_coderunner_wsthrottle();
} else {
$throttle = unserialize($SESSION->throttle);
}
if (!$throttle->logrunok()) {
throw new qtype_coderunner_exception(get_string('wssubmissionrateexceeded', 'qtype_coderunner'));
}
$SESSION->throttle = serialize($throttle);
}

// If logging enabled, log the run.
if (get_config('qtype_coderunner', 'wsloggingenabled')) {
$event = \qtype_coderunner\event\sandbox_webservice_exec::create([
'contextid' => $context->id]);
$event->trigger();
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51 changes: 23 additions & 28 deletions classes/wsthrottle.php
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Original file line number Diff line number Diff line change
Expand Up @@ -31,56 +31,51 @@
/**
* Class to manage the throttling of an individual webservice user to the rate given
* in the wsmaxhourlyrate config parameter.
* Uses the "leaky bucket" algorithm (https://en.wikipedia.org/wiki/Leaky_bucket). This allows
* a surge of run requests equal to the maxhourlyrate but thereafter the rate is limited
* to maxhourlyrate / 3600 runs per second.
* This class must be instantiated within the SESSION variable.
*/
class qtype_coderunner_wsthrottle {
private $timestamps;
private $maxhourlyrate;
private $head;
private $tail;
private $bucketlevel;
private $timestamp;

public function __construct() {
$this->init();
}

private function init() {
$this->maxhourlyrate = intval(get_config('qtype_coderunner', 'wsmaxhourlyrate'));
$this->timestamps = array_fill(0, $this->maxhourlyrate, 0);
$this->head = $this->tail = 0; // Head and tail indices for circular list.
$this->bucketlevel = 0; // Current level of virtual fluid in the bucket (runs in last hour).
$this->timestamp = time(); // When the bucket level was last updated.
}

/**
* Add a log entry to the circular list of timestamps, clearing any
* expired entries (i.e. entries more than 1 hour ago).
* Allow any drainage to occur from the bucket. Then, if it's still full,
* disallow the run, otherwise allow it.
* Return true if logging succeeds, false if user has reached their limit.
*/
public function logrunok() {
if (intval(get_config('qtype_coderunner', 'wsmaxhourlyrate')) != $this->maxhourlyrate) {
// Rate has been changed. Restart throttle.
$this->init();
}
$now = strtotime('now');

// Purge any non-zero entries older than 1 hour.
while ($this->expired($this->timestamps[$this->tail], $now)) {
$this->timestamps[$this->tail] = 0;
$this->tail = ($this->tail + 1) % $this->maxhourlyrate;
}
if ($this->timestamps[$this->head] == 0) { // Empty entry available?
$this->timestamps[$this->head] = $now;
$this->head = ($this->head + 1) % $this->maxhourlyrate;
// Allow any fluid to drain since the last time the level was computed.
$now = time();
$elapsedseconds = $now - $this->timestamp;
$drainage = $this->maxhourlyrate * $elapsedseconds / (60 * 60); // Change in bucket level.
$this->bucketlevel = max(0.0, $this->bucketlevel - $drainage);
$this->timestamp = $now;

// Now if there's enough space for 1 more run, allow it.
if ($this->bucketlevel + 1 <= $this->maxhourlyrate) { // Enough space in bucket?
$this->bucketlevel += 1; // Yes, another quantum of fluid gets added.
return true;
} else {
// List of timestamps is full. Need to throttle user.
} else { // Not enough space.
return false;
}
}

/**
*
* @param int $timestamp the timestamp of interest
* @param int $now current timestamp
* @return bool true if the timestamp is non-zero and older than 1 hour
*/
private function expired($timestamp, $now) {
return ($timestamp !== 0) && ($now - $timestamp) > 3600;
}
}
}
6 changes: 3 additions & 3 deletions lang/en/qtype_coderunner.php
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Original file line number Diff line number Diff line change
Expand Up @@ -1333,14 +1333,14 @@ function should be applied, e.g. <code>{{STUDENT_ANSWER | e(\'py\')}}</code> is
$string['wscputimeexcess'] = 'CPU time specified exceeds set maximum CPU time';
$string['wsdisabled'] = 'Sandbox web service disabled. Talk to a sysadmin';
$string['wsloggingenable'] = 'Log sandbox web service usage';
$string['wsloggingenable_desc'] = 'If this option is checked, every code execution via the sandbox web service will be logged. This option must be enabled if user rate throttling is to work.';
$string['wsloggingenable_desc'] = 'If this option is checked, every code execution via the sandbox web service will be logged.';
$string['wsmaxcputime'] = 'Max CPU time (secs)';
$string['wsmaxcputime_desc'] = 'Limits the maximum CPU time that a web service job can use, even if it explicitly sets the CPU time sandbox parameter.';
$string['wsmaxhourlyrate'] = 'Max hourly rate of submissions';
$string['wsmaxhourlyrate_desc'] = 'If a user attempts to exceed this rate of submissions in any given hour their submissions will be disallowed. 0 for no rate throttling.';
$string['wsmaxhourlyrate_desc'] = 'A burst of submissions of up to this value will be accepted but thereafter, if a user attempts to exceed this average rate of submissions, their submissions will be rejected. 0 for no rate throttling.';
$string['wsnoaccess'] = 'Only logged-in non-guest users can access this functionality';
$string['wsnolanguage'] = 'Language "{$a}" is not known';
$string['wssubmissionrateexceeded'] = 'You have exceeded the maximum hourly \'Try it!\' submission rate. Request denied.';
$string['wssubmissionrateexceeded'] = 'You have exceeded the maximum web-service job submission rate. Request denied.';

$string['xmlcoderunnerformaterror'] = 'XML format error in coderunner question';
$string['coderunner_grading_cache'] = 'Caches grading results so we can avoid going to Jobe so often';
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