Add option for `@ConcurrencyLimit` to throw rejection exception

[mafr]

When using the @ConcurrencyLimit annotation added in 7.0, the underlying ConcurrencyThrottleInterceptor blocks (virtual) threads for a potentially very long time. In typical web scenarios, clients may no longer be interested in the result when the blocked methods finally execute, but they execute anyway, wasting resources.

As a user, I would expect this behavior to be documented in the @ConcurrencyLimit Javadoc (note that the way the Lock is used, there is no guarantee of fairness). Ideally, I would be able to specify a different behavior on the annotation (e.g. throw an exception that I can map to a 429 HTTP status code).

Thank you!

[sbrannen]

Hi @mafr,

Congratulations on submitting your first issue for the Spring Framework! 👍

The Javadoc for @ConcurrencyLimit currently states:

For synchronous invocations, this annotation provides equivalent behavior through org.springframework.aop.interceptor.ConcurrencyThrottleInterceptor.

And the Javadoc for ConcurrencyThrottleInterceptor states:

Interceptor that throttles concurrent access, blocking invocations if a specified concurrency limit is reached.

So, are you saying that you would like for the class-level Javadoc for @ConcurrencyLimit to be more explicit about the blocking nature of the feature?

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Ideally, I would be able to specify a different behavior on the annotation (e.g. throw an exception that I can map to a 429 HTTP status code).

Are you asking for a boolean attribute in @ConcurrencyLimit that would result in an exception being thrown if the concurrency limit is reached?

In other words... fail instead of blocking?

[jhoeller]

On a related note, we added a rejectTasksWhenLimitReached flag to SimpleAsyncTaskExecutor in 6.2.6, building on the same concurrency throttle mechanism: #34727 - While we cannot reuse the executor-oriented TaskRejectedException from there, we could introduce an analogous flag on ConcurrencyThrottleInterceptor and @ConcurrencyLimit, throwing a well-defined exception when the limit has been reached. We might have to introduce a new dedicated exception type for that.

Also, SyncTaskExecutor acquired a concurrency throttle capability in 7.0: #35460 - We should add a rejectTasksWhenLimitReached flag there as well, throwing TaskRejectedException just like SimpleAsyncTaskExecutor.

[mafr]

Thanks a lot for your response!

Yes, the Javadoc of '@ConcurrencyLimit' should mention this behavior explicitly (the request is blocked, not rejected). Not a lot of people will follow the links.

In my opinion, an exception to fail/reject excess traffic would be extremely useful for most web use cases (yes, a boolean attribute on the annotation would be great). Otherwise a traffic spikes leads to a buildup of large numbers of virtual threads, and until that is all processed, clients (even the ones arriving after the spike) will experience bad latency.