Writing an "arrived" and "departed" query with siddhi using timeouts - wso2

Question

I'm looking to replace Esper with Siddhi in my application. The esper statement right now is a "timeout" type pattern, where I need to report back when events of a unique "name" and "type" (just string values I can look for on the incoming events) arrive and depart. I know that the event has arrived when the event first arrives in my firstunique window, and I assume the event departs if I don't see any events of the same name and type in a user-defined timeout value. Here's what my esper statements look like(note that there's a lot more going on in the actual esper, I've just simplified this for the sake of example):
create window events_1.std:firstunique(name, type) as NameEvent
insert into events_1 select * from EventCycle[events]
on pattern [every event1=events_1->(timer:interval(4.0 sec) and not events_1(name=event1.name, type=event1.type))]delete from events_1 where name = event1.name AND type=event1.type
I then select the irstream from events_1 and by getting the incoming and removed events I then get the "arrived" and "departed" events from the window.
For the siddhi, the firstunique window is fairly straightforward (I think?):
from EventCycle#window.firstUnique('name')[ type=='type' ] select name, type insert into NameEvent
but I'm really drawing a blank on how to replace that esper "on pattern" with Siddhi. Can I use a single "from every" statement for this or will I need a different approach with Siddhi?
Any help setting me on the right path here will be appreciated!

Answer 1

One way of achieving your requirement, is by checking the non-occurrence of an event.
I'm afraid, AFAIK, non-occurrence check is not supported in WSO2 CEP-3.1.0.
However it is supported in WSO2 CEP-4.0.0 (yet to be released as I'm writing this on 24th Aug 2015).
You may refer to non-occurrence detection sample [1].
Explanation:
Here we depart the first event if no unique event has occured 4 seconds (which is the timout) later since the latest unique event has occured.
So it appears like we need to check the non-occurance of an event.
In CEP 4.0.0, you could achieve your requirement as follows:
from EventCycle#window.firstUnique(name)[ type=='type' ]
select name, type
insert into NameEvents; -- Note: I renamed NameEvent in the question to NameEvents
-- After seeing the latest unique event (Query-A), 4 seconds later (Query-B), we're checking if no unique event has occured in between (Query-C and Query-D).
-- So, we're checking the non-occurance of an event here... See link [1] for a sample.
--Query-A
from EventCycle#window.unique(name)[ type=='type' ]
select name, type
insert into latestEvents;
-- Query-B
from latestEvents#window.time(4 seconds) -- Here, I've taken 4 seconds as the timeout.
select *
insert expired events into timedoutEvents;
-- Query-C
from every latestEvent = latestEvents[ type=='type' ] ->
keepAliveEvent = latestEvents[ latestEvent.name == keepAliveEvent.name and type=='type' ]
or timedoutEvent = timedoutEvents[ latestEvent.name == timedoutEvent.name and type=='type' ]
select latestEvent.name as name, keepAliveEvent.name as keepAliveName
insert into filteredEvents;
-- Query-D
from filteredEvents [ isNull(keepAliveName)]
select name
insert into departedLatestEvents;
-- Since we want the name from the NameEvents stream, we're joining it with the departedLatestEvents stream
from departedLatestEvents#window.length(1) as departedLatestEvent join
NameEvents#window.length(1) as NameEvent
on departedLatestEvent.name == NameEvent.name -- not checking type as both departedLatestEvents and NameEvents have events only with type 'type'
select NameEvent.name as name, 'type' as type
insert into departedFirstEvents;
Link referred in the code sample:
1 https://docs.wso2.com/display/CEP400/Sample+0111+-+Detecting+non-occurrences+with+Patterns
Hope this helps!