How to write a Splunk alert query to trigger alert when there are no transactions?

[aaa2324]

I want to create Splunk alert when there are no transactions continuously for 30mins. Kindly assist.

[mehmetgunertr]

index="abc" (EVENT=1 OR EVENT=2)| transaction MACHINE startswith=(EVENT=1) endswith=(EVENT=2)|where duration> 1800 | table  duration  EVENT MACHINE NAME DESCR

[aaa2324]

Can you please explain how the below query works , what is event=1 and 2 and what is machine ?

[ITWhisperer]

Create a report that counts the transactions in the previous 30 minutes and trigger the alarm when the count is zero