MDL-45758 tool_monitor: Add a universal event observer

This observer does the following task:-
1) It monitors all events and stores relevant data in a temporary table
2) It doesn't write the events straight away, it buffers them, so there is only one db write per request
3) It calculates and sees if there are subscriptions related to those events who need to be notified
4) If there are such cases, it schedules a adhoc task for the same

Original issue - MDL-46132

admin/tool/monitor/classes/eventobservers.php

@@ -36,6 +36,15 @@
  */
 class eventobservers {
 
+    /** @var array $buffer buffer of events. */
+    protected $buffer = array();
+
+    /** @var int Number of entries in the buffer. */
+    protected $count = 0;
+
+    /** @var  eventobservers a reference to a self instance. */
+    protected static $instance;
+
     /**
      * Course delete event observer.
      * This observer monitors course delete event, and when a course is deleted it deletes any rules and subscriptions associated
@@ -49,4 +58,133 @@ public static function course_deleted(\core\event\course_deleted $event) {
             rule_manager::delete_rule($rule->id);
         }
     }
+
+    /**
+     * The observer monitoring all the events.
+     *
+     * This observers puts small event objects in buffer for later writing to the database. At the end of the request the buffer
+     * is cleaned up and all data dumped into the tool_monitor_events table.
+     *
+     * @param \core\event\base $event event object
+     */
+    public static function process_event(\core\event\base $event) {
+
+        if (empty(self::$instance)) {
+            self::$instance = new static();
+            // Register shutdown handler - this is useful for buffering, processing events, etc.
+            \core_shutdown_manager::register_function(array(self::$instance, 'process_buffer'));
+        }
+
+        self::$instance->buffer_event($event);
+
+        if (PHPUNIT_TEST) {
+            // Process buffer after every event when unit testing.
+            self::$instance->process_buffer();
+
+        }
+    }
+
+    /**
+     * Api to buffer events to store, to reduce db queries.
+     *
+     * @param \core\event\base $event
+     */
+    protected function buffer_event(\core\event\base $event) {
+
+        $eventdata = $event->get_data();
+        $eventobj = new \stdClass();
+        $eventobj->eventname = $eventdata['eventname'];
+        $eventobj->contextid = $eventdata['contextid'];
+        $eventobj->contextlevel = $eventdata['contextlevel'];
+        $eventobj->contextinstanceid = $eventdata['contextinstanceid'];
+        if ($event->get_url()) {
+            // Get link url if exists.
+            $eventobj->link = $event->get_url()->out();
+        } else {
+            $eventobj->link = '';
+        }
+        $eventobj->courseid = $eventdata['courseid'];
+        $eventobj->timecreated = $eventdata['timecreated'];
+
+        $this->buffer[] = $eventobj;
+        $this->count++;
+    }
+
+    /**
+     * This method process all events stored in the buffer.
+     *
+     * This is a multi purpose api. It does the following:-
+     * 1. Write event data to tool_monitor_events
+     * 2. Find out users that need to be notified about rule completion and schedule a task to send them messages.
+     */
+    public function process_buffer() {
+        global $DB;
+
+        $events = $this->flush(); // Flush data.
+
+        $select = "SELECT COUNT(id) FROM {tool_monitor_events} ";
+        $now = time();
+        $messagestosend = array();
+
+        // Let us now process the events and check for subscriptions.
+        foreach ($events as $eventobj) {
+            $subscriptions = subscription_manager::get_subscriptions_by_event($eventobj);
+            $idstosend = array();
+            foreach ($subscriptions as $subscription) {
+                $starttime = $now - $subscription->timewindow;
+                if ($subscription->courseid == 0) {
+                    // Site level subscription. Count all events.
+                    $where = "eventname = :eventname AND timecreated >  :starttime";
+                    $params = array('eventname' => $eventobj->eventname, 'starttime' => $starttime);
+                } else {
+                    // Course level subscription.
+                    if ($subscription->cmid == 0) {
+                        // All modules.
+                        $where = "eventname = :eventname AND courseid = :courseid AND timecreated > :starttime";
+                        $params = array('eventname' => $eventobj->eventname, 'courseid' => $eventobj->courseid,
+                                'starttime' => $starttime);
+                    } else {
+                        // Specific module.
+                        $where = "eventname = :eventname AND courseid = :courseid AND contextinstanceid = :cmid
+                                AND timecreated > :starttime";
+                        $params = array('eventname' => $eventobj->eventname, 'courseid' => $eventobj->courseid,
+                                'cmid' => $eventobj->contextinstanceid, 'starttime' => $starttime);
+
+                    }
+                }
+                $sql = $select . "WHERE " . $where;
+                $count = $DB->count_records_sql($sql, $params);
+                if (!empty($count) && $count >= $subscription->frequency) {
+                    $idstosend[] = $subscription->id;
+                }
+            }
+            if (!empty($idstosend)) {
+                $messagestosend[] = array('subscriptionids' => $idstosend, 'event' => $eventobj);
+            }
+        }
+
+        // Schedule a task to send notification.
+        if (!empty($messagestosend)) {
+            $adhocktask = new notification_task();
+            $adhocktask->set_custom_data($messagestosend);
+            $adhocktask->set_component('tool_monitor');
+            \core\task\manager::queue_adhoc_task($adhocktask);
+        }
+    }
+
+    /**
+     * Protected method that flushes the buffer of events and writes them to the database.
+     *
+     * @return array a copy of the events buffer.
+     */
+    protected function flush() {
+        global $DB;
+
+        // Flush the buffer to the db.
+        $events = $this->buffer;
+        $DB->insert_records('tool_monitor_events', $events); // Insert the whole chunk into the database.
+        $this->buffer = array();
+        $this->count = 0;
+        return $events;
+    }
 }

admin/tool/monitor/db/events.php

@@ -27,5 +27,9 @@
     array(
         'eventname'   => '\core\event\course_deleted',
         'callback'    => '\tool_monitor\eventobservers::course_deleted',
+    ),
+    array(
+        'eventname'   => '*',
+        'callback'    => '\tool_monitor\eventobservers::process_event',
     )
 );