Added backpressure handling to prevent memory buildup and system degredation under high load

[ulischulte]

Closes #2897

[cdprete]

@ulischulte isn't this line useless given that the errorConsumer will just do the same but on ERROR level?

In the previous code there was just a warning, but now you end up with a warning and and error providing exactly the same information.
Actually I find the WARN level better in this case, because it's just one of the retries.

[ulischulte]

I wouldn't say useless. In the first step, I more or less adapted the error logging from the previous implementation. It also logs a warning in doBeforeRetry AND has a direct implementation of the errorConsumer in subscribe.

Please note, however, that you are reviewing a WIP/draft that has not yet been released for review.
I am in the process of creating failing tests for the known scenarios and, in the first step, I intentionally injected an errorConsumer for easier verifiability.

[cdprete]

I am in the process of creating failing tests for the known scenarios and, in the first step, I intentionally injected an errorConsumer for easier verifiability.

Ok, I see.
I wanted just to avoid to have double entries in my logs. :)

[ulischulte]

And please keep in mind. I didn't change the error logging here so far, regardless of wether it's verbose or not. The old and new IntervalCheck both log a warning before retry and both invoke an errorConsumer when ever there's an error signal.

[cdprete]

Yep. My comment was only about the code block that gets executed before the retry.

Now it logs a warning, the previous line, and it invokes as well the error consumer, which logs an error.

Or, that's what I see in the diff here honestly :)

[Copilot]

Pull Request Overview

This PR enhances the IntervalCheck service to handle backpressure and prevent overflow exceptions under high load or slow check operations. The changes introduce reactive backpressure handling using onBackpressureLatest() and switch from sequential to concurrent processing with flatMap.

Key changes:

• Switched from concatMap to flatMap with controlled concurrency to allow parallel check execution
• Added onBackpressureLatest() operator to prevent dropped checks and overflow exceptions under backpressure
• Exposed retryConsumer setter and lastChecked getter for improved testability
• Added comprehensive tests for overflow scenarios and backpressure handling

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 8 comments.

File	Description
spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/services/IntervalCheck.java	Enhanced with backpressure handling via onBackpressureLatest(), switched to concurrent processing with flatMap, and exposed internal state for testing
spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/services/IntervalCheckTest.java	Added three new tests to verify no overflow exceptions occur and checks aren't lost under backpressure scenarios

Comments suppressed due to low confidence (1)

spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/services/IntervalCheck.java:52

• getLastChecked exposes the internal representation stored in field lastChecked. The value may be modified after this call to getLastChecked.

	@Getter