====== Defining Time Schedules with CRON Syntax ======

In the Mervis system, you will frequently need to schedule recurring automated tasks—such as defining how often a **DataNode** communicates with an external API or database. To define these precise schedules, Mervis uses standard **CRON syntax**.

The most common place you will encounter this is in the **DataNode type properties** under the **`CommTaskCron`** field.

{{:en:mervis-scada:90-how-to:datanode-cron.png?direct&800 |DataNode CommTaskCron Example}}

===== Understanding CRON Structure =====

A CRON expression is a string consisting of **five fields** separated by spaces. Each field represents a specific unit of time. 

The standard format is: 
<code>* * * * *</code>

From left to right, the fields represent:
  - **Minute** (0 - 59)
  - **Hour** (0 - 23)
  - **Day of the Month** (1 - 31)
  - **Month** (1 - 12)
  - **Day of the Week** (0 - 7, where both 0 and 7 represent Sunday)

===== Special Characters =====

To create flexible schedules, you can use the following special characters within the fields:

  * **`*` (Asterisk):** Represents "every" or "any" value. If you put an asterisk in the Hour field, it means "every hour."
  * **`/` (Slash):** Used to specify increments or "step" values. For example, `*/15` in the Minute field means "every 15 minutes."
  * **`-` (Hyphen):** Defines a range of values. For example, `9-17` in the Hour field means "every hour from 9 AM to 5 PM."
  * **`,` (Comma):** Used to separate individual items in a list. For example, `1,3,5` in the Day of the Week field means "Monday, Wednesday, and Friday."

===== Common CRON Examples =====

Here is a cheat sheet of the most common CRON expressions used for DataNode communication and automated tasks:

^ CRON Expression ^ Description / Schedule ^
| **`0 */6 * * *`** | **Every 6 hours** (at minute 0). //(This is the example shown in the image above)// |
| `* * * * *` | **Every minute** of every day. |
| `*/5 * * * *` | **Every 5 minutes**. |
| `*/15 * * * *` | **Every 15 minutes**. |
| `0 * * * *` | **Every hour**, at the top of the hour (minute 0). |
| `30 * * * *` | **Every hour, at 30 minutes past the hour**. |
| `0 0 * * *` | **Every day at Midnight** (00:00). |
| `30 8 * * *` | **Every day at 8:30 AM**. |
| `0 8-18 * * *` | **Every hour between 8:00 AM and 6:00 PM**. |
| `0 8 * * 1-5` | **Every weekday (Monday to Friday) at 8:00 AM**. |
| `0 0 * * 0` | **Every Sunday at Midnight**. |
| `0 0 1 * *` | **The 1st day of every month at Midnight**. |

> **Important Note:** When writing CRON expressions, pay close attention to the spaces between the asterisks and numbers. Missing a space or adding an extra one will cause the schedule to fail.