Cumulocity CEL Check if event exists - complex-event-processing

Question

I am trying to implement that if I send Event A it will create a new Event B if it does not exist. If Event B already exists it should update it instead.
I tried making a counter that counts occurences of the event but it didnt work.
Also it seems that "not exists" should be implemented but I do not get it to work.
Thanks in advance Greets

Answer 1

If I got it well. Your are trying to create o update an event depend if an event with the same "key" exist.
One approach is to create your custom "key" like so:
insert into
CreateEvent
select
"testtype" as type,
part.event.source as source,
"testext" as text,
{
"mycustomkey" , "customValue",
} as fragments
from EventCreated;
Here we create an event with a custom key => "mycustomkey" with a custom value => "customValue".
Then when a new event is create you can use the "where" clause to know is you need to create a new event or update other event.
// create a new event if the firstEvent function return an event which
// the "mycustomkey" key if different to "customValue"
insert into
CreateEvent
select
"testtype" as type,
part.event.source as source,
"testext" as text,
{
"mycustomkey" , "customValue",
} as fragments
from EventCreated e
where getString(cast(
findFirstEventByFragmentTypeAndType("mycustomkey", "testtype"),
com.cumulocity.model.event),
"mycustomkey") != "customValue";
Here just update the event if the if the firstEvent function return an event which the "mycustomkey" if equal to "customValue"
insert into
EventUpdated
select
"testtype" as type,
part.event.source as source,
"testext" as text,
{
"mycustomkey" , "customValue",
} as fragments,
cast(findFirstEventByFragmentTypeAndType("mycustomkey",
"testtype"), com.cumulocity.model.event), "mycustomkey").getId() as
id
from EventCreated e
where getString(cast(
findFirstEventByFragmentTypeAndType("mycustomkey", "testtype"),
com.cumulocity.model.event),
"mycustomkey") = "customValue";
This solution needs that "customValue" must we know it and also the findFirstEventByFragmentTypeAndType function assumes it is going to return the event you need. We can improve this solution buy calling other function like findOneEventByFragmentType, findEventByFragmentTypeAndSourceAndTimeBetweenAndType etc..(you can find more information here) and find the event you need using a javascript function like :
insert into
CreateEvent
select
"testtype" as type,
part.event.source as source,
"testext" as text,
{
"mycustomkey" , "customValue",
} as fragments
from EventCreated e
where findTheCorrectEvent(
findEventByFragmentTypeAndSourceAndTimeBetweenAndType(
"mycusto mkey", "source", datefrom, dateTo).toJSON(),
"customValue") = true;
insert into
EventUpdated
select
"testtype" as type,
part.event.source as source,
"testext" as text,
{
"mycustomkey" , "customValue",
} as fragments
from EventCreated e
where findTheCorrectEvent(
findEventByFragmentTypeAndSourceAndTimeBetweenAndType(
"mycusto mkey", "source", datefrom, dateTo).toJSON(),
"customValue") = false;
create expression Boolean findTheCorrectEvent(evens, "customValue") [
var result = _findTheCorrectEvent(events, "customValue")
function _findTheCorrectEvent(events, referenceKey){
var _events = JSON.parse(events)
var result = false
_events.forEach(function(event){
if(event.mycustomkey === referenceKey) result = true
})
return result
}
result
];
Other approach, which in long term is easier, is create a microservice that do this. One microservice could check the last events created and check is the keys already exist and then update the same event otherwise create a new event.
In both solutions we need to create a "customkey" with a "customvalue" in other to find out if the event already exist. This "customkey" can be created using the a unique key like currentime which is a number.
Hope this can help you.
Good luck!