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CRON Configuration

We use CRON expressions configure the behavioral timing of our devices. This CRON configuration can be adjusted during device configuration using the Lobaro Maintenance Tool or remotely over the air.

The CRON expressions consists of 6 fields, separated by space: e.g. 0 0/15 * * * * , which describes time points every 15 minutes starting from minute 0.

CRON expression allow you to define specific POINTS in time they are NOT helpful to define time durations. They can be seen has the start time to trigger an action, for example initiating the sensor measurement or wireless sendout. The duration of an action is defined by a separate configuration parameter if needed.

Timezone and Daylight Saving

All times are referenced to UTC+0 timezone! Keep this in mind when setting CRON configurations on Lobaro devices.

This implementation detail implies that daylight saving time switches will change the behavior of the device by one hour.
Example for Central Europe:

  • Winter: UTC+1 (Central European Time)
  • Summer: UTC+2 (Central European Summer Time)

Meaning of the six field parameter​

0 0/15 * * * * - time points every 15 minutes starting from minute 0.

SecondMinuteHourDay of MonthMonth of yearDay of Week
Range(0-59)(0-59)(0-23)(1-31)(1-12)(1-7)
AlternativeJAN-DECMON-SUN
, - * / special charsyesyesyesyesyesyes
? special charyesyes
H L special charsyesyesyesyesyesyes
W special charyes
Example00/15****
warning

H L W are only supported on recent firmware releases, check product description for details.

tip

Day of Month and Day of Week should be treated mutually exclusive and one of them should be set to ?.
As an example the following CRON may not have the intended behaviour if not following this advice:

0 0 1 1 * MON Would only trigger on 1st of every month that is a Monday.

Star (Asterisk) *​

Used to select all values within a field. For example, "*" in the minute field means "every minute".

Question mark (?)​

Useful when you need to specify something in one of the two fields in which the character is allowed, but not the other. For example, if I want my trigger to fire on a particular day of the month (say, the 10th), but don’t care what day of the week that happens to be, I would put “10” in the day-of-month field, and “?” in the day-of-week field. See the examples below for clarification.

Dash, minus (-)​

Used to specify ranges. For example, “10-12” in the hour field means “the hours 10, 11 and 12”.

Comma (,)​

Used to specify multiple explicit values. For example, “MON,WED,FRI” in the day-of-week field means “the days Monday, Wednesday, and Friday”, "1,15" in the day-of-month field means "on the first and 15th of the month".

Slash (/)​

Used to specify increments. For example, "0/15" in the seconds field means "0 and then every 15", that is "the seconds 0, 15, 30, and 45". And "5/15" in the seconds field means "the seconds 5, 20, 35, and 50". You can also use "/" for other fields. "1/3" in the day-of-month field means "fire every 3 days starting on the first day of the month".

Weekday (W)​

Only valid in the day-of-month field. Used as a modifier added after a given day, e.g. 15W. This will execute on the nearest weekday (Monday to Friday) around that day. So 0 0 12 15W * * will execute on 12h UTC of the 15th of a month, if that day is a Monday, Tuesday, Wednesday, Thursday, of Friday. If the 15th is a Saturday, it will execute on 12h UTC on Friday the 14th. If the 15th is a Sunday, it will execute on 12h UTC on Monday the 16th. It will however never jump into another month. If you use 1W, and the first day of the month is a Saturday, it will execute on Monday the 3rd.

note
  • Can be used in combination with lists (comma ","), must be used for each individual value of the list you want to use it on, e.g. 1W,15W.
  • Cannot be used with ranges (dash, "-") or increments (slash "/").
  • Can be used in combination is hash ("H"), in the form HW or e.g. H(6-9)W.
  • Can be used in combination with "L", but only as LW, to execute on the last weekday of a month.

Last (L)​

Has a different meaning, depending on where you use it.

A simple L means "the last of something". This is useful for the day-of-month field, as the last day of a month is dependent on the current month and year. 0 0 12 L ? will execute on the 31st of January, on the 28th or 29th (for leap years) of February, on the 31st of March, on the 30th of April, and so on. You can also specify offsets from the last value, like this: 0 0 12 L-3 * ?. This executes on the 28th of January, on the 25th of 26th of February, and so on.

In the day-of-week field L simply refers to Sunday. But in combination with a number, it can be used to execute on the last specific weekday of a month, like this: 0 0 12 ? * 1L will execute at 12h on the last Monday of every month.

note
  • Can be used in combination with W, as LW, to execute on the last weekday of a month.

Hash (H)​

The character H can be used in place of a number in any field of the cron. The H wil be replaced with a consistent pseudo random number, that fits into that field. On any given device this number will always be the same for a field, but it will differ for different devices. This can be used as a simple mechanism for load balancing, or to avoid collisions, if you use a lot of devices.

Numbers will be 0-59 for second and minute fields, 0-23 for the hour field, 0-28 for the day-of-month field (it will never activate on 29th, 30th, or 31st), 1-12 for the month field, and 1-7 for the day-of-week field.

When you, for example, set many devices to the cron 0 0 4 * * *, they will all activate at 4 AM UTC in the same second. For LoRaWAN devices, this will lead to a lot of packet collisions and therefore data loss, if the devices are in the same area. For NB-IoT/LTE-M devices, it will lead to a concentrated workload on the server handling the requests and the system evaluating the data. This can lead to bad system performance during that spike, while the system is idle most of the day.

If instead of 0 0 4 * * * you set the devices to H H 4 * * *, the two H characters will be replaced with a per-device-consistent random number in the range of 0-59, for example 31 47 4 * * * for one and 17 0 4 * * * for another device. With this, the devices will all activate in the hour between 4 AM and 5 AM UTC and distribute the workload.

You can use H to pick a pseudo random number from a specific range by specifying the range in parenthesis like this: H(4-8) will be replaced with any of the numbers 4, 5, 6, 7, 8. The range must only use numbers that are valid for the field H is used in.

note
  • Can be used inside lists (comma ","), but the generated value could collide with other entries in list (see below).
  • Cannot be used with ranges (dash, "-").
  • Can be used with increments (slash, "/"), but only before the slash, as in "H(0-2)/3", not after it.

Pitfalls with using H​

warning

Working with H can be confusing and lead to unexpected results. It can be used in many places, but not all of them are useful.

Writing 0 H/10 * * * * will on most devices create a cron that does not activate every 10 minutes. It could evaluate to the expression 0 47/10 * * * *, which will only activate twice each hour, in minutes 47 and 57, but not during minutes 0-46 at all. To have devices activate every 10 minutes at distributed times, use 0 H(0-9)/10 * * * * or better yet: H H(0-9)/10 * * * *.

Using H in a list works, but it might lead to fewer activations as intended. Putting it in a list twice will not produce multiple activations, because both H will evaluate to the same number: 0 0 H,H * * * could turn into 0 0 3,3 * * *, which is the same as 0 0 3 * * *. Using 0 0 H(0-4),H(8-12) * * * would work to produce two activations per day.

Examples​

Standard CRON Syntax​

Cron definition
DescriptionTrigger time points (hh:mm:ss)
0 5 * * * * Hourly at minute 5, second 000:05:00, 01:05:00, 02:05:00...
0 1/10 * * * * Every 10 minutes starting from minute 1, second 000:01:00, 00:11:00, 00:21:00, [...], 01:01:00, [...]
0 0 6 * * * Daily on hour 6, minute 0, second 006:00:00
0 0 13 1,15 * * Hour 13, minute 0, second 0 on day 1 and 1513:00:00 at 1st and 15th of month
0 15 9 1-5 * * Hour 9, minute 15, second 0 on day 1 to 509:15:00 at 1st to 5th of month
0 0 6 * * 1 Weekly on Monday at 06:00:00Monday, 06:00:00

Extended CRON Syntax​

Cron definition
Description
0 0 11 1W,15W * * 11am on the days closest to the 1st and the 15th day each month that are not Saturdays or Sundays.
0 0 15 1,L * * 3pm on the first and last day of each month.
0 0 1 ? * 2L 1am on the last Tuesday of each month.
H H H(0-2) * * * Every day at a fixed pseudo random time between 00:00:00 and 02:59:59.Same time each day for a given device, but different time for different devices.
H H(0-9)/10 * * * *Every 10 minutes, but at a fixed time, randomly but consistently picked for each device.
info

The Extended CRON Syntax will only work on recent firmware versions, e.g. the app-nrf9160-wmbus firmware versions 0.15.4 and newer. Check the specific product manual to see if the extended syntax is implemented in the firmware.

Further Reading​

All our CRON expressions are in the same format as the Java Quarz scheduler, without the optional year field and a different way of numbering weekdays. Also the extended CRON field have been added by Lobaro. A good documentation including examples can be found here:

http://www.quartz-scheduler.org/documentation/quartz-2.3.0/tutorials/crontrigger.html

Online CRON Generators​