How to send email by determining the need to send at runtime?

Question

Java

Hello, I encountered the need to send an email notification with minimal user action.
The situation is as follows: conditionally, there is some field in the database that can change by itself (imitation of a sensor), as soon as the value of the temperature sensor becomes critical, the user receives a letter to the mail, they say go to the site, send instructions.
Ideally, this should be done without any user action.
I found 2 options
1) https://o7planning.org/ru/11145/spring-email-tutorialwith sending a letter when going to some url (which would be convenient given the secure context of the spring), that is, for an authorized user, we can always get a context by which we will already find the user in the database and pull out his mail, then we will send a letter to this mail.
But here, as you can see, the user needs to go to the site, which does not satisfy the requirement
2) https://www.baeldung.com/spring-email simple initialization of sending in the component, as I understand it, will be performed once, but if we constantly initialize this bean, then it can be performed with a certain frequency.
I encounter such a task for the first time and as I see it: while the application is running, a certain component continuously passes through all users, pulls out sensors for each user and checks the value for each sensor
sends a letter, if necessary, when it reaches the end, it falls asleep for a while and then again. But it sounds a little expensive, you have to spend a lot of memory to take it all out + a lot of time to get it all. In general, I think this is not the best way to affect performance.
But there are services that are able to send mail without user actions, so I ask you to explain on what basis they do this, thanks in advance

Answer 1

I'm not sure, but if I understand the task correctly, then in theory Spring Email should help you, @Scheduledfor example, for launching by cron, and @Asyncfor asynchronous sending. But perhaps there are better options.

Answer 2

Spring Integration will help with tracking changes in the database and sending email.