Scheduled jobs

Author: simpleadm

1. Magento Architecture and Customization Techniques/6. Configure event observers and scheduled jobs.md

@@ -80,31 +80,61 @@ Example:
 
 ### Demonstrate how to configure a scheduled job
 
-`cron_groups.xml` - store view scope:
 
+##### Cron groups
+
+A cron group is a logical group that enables you to easily run cron for more than one process at a time. 
+Most Magento modules use the default cron group.
+
+To declare new group and specify settings, create `<module>/etc/cron_groups.xml` file (store view scope):
+
+```xml
+<?xml version="1.0"?>
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Cron:etc/cron_groups.xsd">
+    <group id="default">
+        <schedule_generate_every>15</schedule_generate_every>
+        <schedule_ahead_for>20</schedule_ahead_for>
+        <schedule_lifetime>15</schedule_lifetime>
+        <history_cleanup_every>10</history_cleanup_every>
+        <history_success_lifetime>10080</history_success_lifetime>
+        <history_failure_lifetime>10080</history_failure_lifetime>
+        <use_separate_process>0</use_separate_process>
+    </group>
+</config>
+```
+
+Existing groups:
 - default (no separate process)
 - index - mview, targetrule
 - catalog_event - catalog_event_status_checker - mark event open/closed
 - consumers - consumers_runner if configured to run by cron. `bin/magento queue:consumers:start`. PID file var/{$consumer}.pid
 - staging - staging_apply_version, staging_remove_updates, staging_synchronize_entities_period
 - ddg_automation (dotmailer)
 
-```
-    <group id="NAME">
-        <job name="NAME" instance="CLASS" method="METHOD">
+##### crontab.xml
+
+`crontab.xml` file is used to execute an action on schedule.
+This file always located in the <module>/etc/ folder (not in <module>/etc/{area}/)!
+
+```xml
+<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Cron:etc/crontab.xsd">
+    <group id="GROUP_NAME">
+        <job name="CRON_JOB_UNIQUE_ID" instance="CLASS" method="METHOD">
             <config_path>some/config/path</config_path>
         </job>
         <job name="NAME" instance="CLASS" method="METHOD">
             <schedule>* * * * *</config_path>
         </job>
     </group>
+</config>
 ```
 
 run:
 
 - magento cron:run [–group=”"]
 - pub/cron.php?[group=<name>] in a web browser, protect with basic auth
 
+Stack trace:
 ```
 \Magento\Cron\Console\Command\CronCommand::execute
 \Magento\Framework\App\Cron::launch
@@ -116,8 +146,8 @@ generate
 check for standalone process
 ```
 
-\Magento\Cron\Model\Config\Data extends \Magento\Cron\Model\Config\Data
-- merges \Magento\Cron\Model\Config\Reader\Db::get from Database
+[\Magento\Cron\Model\Config\Data](https://github.com/magento/magento2/blob/2.2-develop/app/code/Magento/Cron/Model/Config/Data.php) extends [\Magento\Cron\Model\Config\Data](https://github.com/magento/magento2/blob/2.2-develop/lib/internal/Magento/Framework/Config/Data.php)
+- merges [\Magento\Cron\Model\Config\Reader\Db::get](https://github.com/magento/magento2/blob/2.2-develop/app/code/Magento/Cron/Model/Config/Reader/Db.php#L51) from Database
 
   Sample DB structure:
 ```