diff --git a/classes/external/run_in_sandbox.php b/classes/external/run_in_sandbox.php
index a224b1620..d4cb77e96 100644
--- a/classes/external/run_in_sandbox.php
+++ b/classes/external/run_in_sandbox.php
@@ -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();
diff --git a/classes/wsthrottle.php b/classes/wsthrottle.php
index 8afce7c38..a421d7493 100644
--- a/classes/wsthrottle.php
+++ b/classes/wsthrottle.php
@@ -31,12 +31,16 @@
 /**
  * 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();
@@ -44,12 +48,13 @@ public function __construct() {
 
     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() {
@@ -57,30 +62,20 @@ public function logrunok() {
             // 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;
-    }
-}
+}
\ No newline at end of file
diff --git a/lang/en/qtype_coderunner.php b/lang/en/qtype_coderunner.php
index a1851f6b8..d9163fda9 100644
--- a/lang/en/qtype_coderunner.php
+++ b/lang/en/qtype_coderunner.php
@@ -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';