Using iCal in Scheduling – ICS Definitive Tip #3

In ICS Definitive Guide #2 we looked at the scheduling of integrations and mentioned the possibility of using iCal. In this tip we shall explore advanced iCal facility for defining schedules within ICS.

The iCal format is probably best known for sharing meeting events in tools such as outlook. The format is actually a standard defined by the IETF originally in 1998 (https://www.ietf.org/rfc/rfc2445.txt)  and updated several times the latest being  2015 (https://tools.ietf.org/html/rfc7529). The goal of the definition has been for the purpose of driving standardisation across PIM tools (Personal Information Management) tools which includes Outlook and Thunderbird etc. It is a substantial standard the original document ran to 150 pages, but the section supported by the schweduler’s advanced option purely relates to the reoccurence definition covered by section 4.3.10 of the original RFC, and only takes five pages. If you want to know lots about iCal and the varying revisions then you may find https://icalendar.org/ helpful.

It is worth mentioning that today ICS under the hood exploits Oracle’s Enterprise Scheduler Service (ESS). ESS documentation identifies that it works to the 2445 version of the specification (see here). ESS became a 1st class citizen component with the start of the 12c middleware product stack. As a result, if you see any errors processing an iCal configuration the error codes will reference ESS does.

So why support for this part of the iCal standard. Well unlike the UI provided, the FREQ definition with all the elements of the definition that can be provided mean some very sophisticated schedules can be defined.  For example you could create a definition that says the scheduler should only trigger 9.15, 9.30, 14.15, 14.30 on Wednesdays for week numbers 7, 8, 21 and 49.

Syntax

For ease of reference, the definition of how a FREQ element should be created is, according for RFC2445:

“FREQ”=freq *(

; either UNTIL or COUNT may appear in a ‘recur’,
; but UNTIL and COUNT MUST NOT occur in the same ‘recur’

( “;” “UNTIL” “=” enddate ) /
( “;” “COUNT” “=” 1*DIGIT ) /

; the rest of these keywords are optional,
; but MUST NOT occur more than once

( “;” “INTERVAL” “=” 1*DIGIT ) /
( “;” “BYSECOND” “=” byseclist ) /
( “;” “BYMINUTE” “=” byminlist ) /
( “;” “BYHOUR” “=” byhrlist ) /
( “;” “BYDAY” “=” bywdaylist ) /
( “;” “BYMONTHDAY” “=” bymodaylist ) /
( “;” “BYYEARDAY” “=” byyrdaylist ) /
( “;” “BYWEEKNO” “=” bywknolist ) /
( “;” “BYMONTH” “=” bymolist ) /
( “;” “BYSETPOS” “=” bysplist ) /
( “;” “WKST” “=” weekday ) /

( “;” x-name “=” text )
)

freq = “SECONDLY” / “MINUTELY” / “HOURLY” / “DAILY”/ “WEEKLY” / “MONTHLY” / “YEARLY”
enddate = date
enddate =/ date-time ;An UTC value
byseclist = seconds / ( seconds *(“,” seconds) )
seconds = 1DIGIT / 2DIGIT ;0 to 59
byminlist = minutes / ( minutes *(“,” minutes) )
minutes = 1DIGIT / 2DIGIT ;0 to 59
byhrlist = hour / ( hour *(“,” hour) )
hour = 1DIGIT / 2DIGIT ;0 to 23
bywdaylist = weekdaynum / ( weekdaynum *(“,” weekdaynum) )
weekdaynum = [([plus] ordwk / minus ordwk)] weekday
plus = “+”
minus = “-”
ordwk = 1DIGIT / 2DIGIT ;1 to 53
weekday = “SU” / “MO” / “TU” / “WE” / “TH” / “FR” / “SA”
;Corresponding to SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY,
;FRIDAY, SATURDAY and SUNDAY days of the week.

bymodaylist = monthdaynum / ( monthdaynum *(“,” monthdaynum) )
monthdaynum = ([plus] ordmoday) / (minus ordmoday)
ordmoday = 1DIGIT / 2DIGIT ;1 to 31
byyrdaylist = yeardaynum / ( yeardaynum *(“,” yeardaynum) )
yeardaynum = ([plus] ordyrday) / (minus ordyrday)
ordyrday = 1DIGIT / 2DIGIT / 3DIGIT ;1 to 366
bywknolist = weeknum / ( weeknum *(“,” weeknum) )
weeknum = ([plus] ordwk) / (minus ordwk)
bymolist = monthnum / ( monthnum *(“,” monthnum) )
monthnum = 1DIGIT / 2DIGIT ;1 to 12
bysplist = setposday / ( setposday *(“,” setposday) )
setposday = yeardaynum

Applying the syntax to get complex schedules

With all of that you have if careful all the information to create the most complex schedules you like.  To illustrate here is a validating example:

FREQ=YEARLY;INTERVAL=3;BYMONTH=5;BYDAY=2,7;

This translates to a schedule triggering every 3 years on May 2nd and 7th. As you can see trying to express this using calendars and menus would probably prove to be fairly complex. But pretty simple in text once the notation is understood.
We should address a couple of challenges that exist.

Avoiding the traps

Firstly (the current ICS documentation here) shows the example expression as:

FREQ=MONTHLY;BYMONTHDAY=1,10,15;BYHOUR=5,10,15,20;BYMINUTE=15

However the example omits a key terminating semi colon (essentially each part of the expression needs to be terminated by a semi colon). So, the correct representation is:

FREQ=MONTHLY;BYMONTHDAY=1,10,15;BYHOUR=5,10,15,20;BYMINUTE=15;

Whilst not too serious, an easy mistake to make. Unfortunately today the error messages from ESS back to ICS are rather uninformative with an error message and code of

ESS-09402 Failed to compile recurrence expression

So if you see this error, our advise is to simplify the expression or build it up a piece at a time if you can.
The last issues we have yet to understand relate to the definitions support for extension attributes shown as x-name in the specification. Well to be honest, this is not important as ICS and ESS do not appear to have any extension elements. The slightly more troubling question, which we have yet to get to the bottom of, is the support for COUNT in the expression which does not appear to be accepted.