Helidon Concurreny Limit module - Adding queueing support to AIMD

[logovind]

Environment Details

• Helidon Version: Helidon-4.1.x
• Helidon SE or Helidon MP: SE & MP
• JDK version: Java-21
• OS: Linux
• Docker version (if applicable):

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we tried to explore the AIMD feature introduced in the Concurrency limit module. So we tried out our existing Spectra UCP example with helidon-4.1.4.

From tests we observed that,

1. Significant volume of requests rejection with 503 , even for few bad requests above KPI.
   Ex : Even with backoff-ratio of 0.9, we see 68% request failures whereas only a small subset of 3% requests are above KPI of 1sec.

2. The requests also fail due to sudden spikes , as the increase in dynamic concurrency takes time to adjust for the high user concurrency.
   Ex: 50 users test , it takes ~4.3sec to reach max-limit of 50 from initial-limit of 25. During this period, we do see ~ 3% of errors.

So it will be good if we have queuing support like in Fixed Limit, to avoid such request rejection errors.