Looks like we stumbled across a bug in our production middleware that was resulting in some unwarranted internal service errors - looks to us like there's an overflow in the min / max value calculation for random mutually exclusive groups.
I'll use an example from our production code that was giving us trouble to explain the problem.
1) We have a mutually exclusive group (random assignment) with exactly 6 tenants
2) shortlistedCampaigns would then have their weights adjusted to 16 based on the following:
5) which, looking at the algo from _generateBucketValue, looks to default to a max of 10000 based on Constants.MAX_TRAFFIC_PERCENT. This leaves a 4% chance that an assigned bucketed value falls outside the acceptable bounds of all active tests in a group and returns null - failing on this line:
Looks like we stumbled across a bug in our production middleware that was resulting in some unwarranted internal service errors - looks to us like there's an overflow in the min / max value calculation for random mutually exclusive groups.
I'll use an example from our production code that was giving us trouble to explain the problem.
1) We have a mutually exclusive group (random assignment) with exactly 6 tenants 2)
shortlistedCampaignswould then have their weights adjusted to 16 based on the following:https://github.com/wingify/vwo-node-sdk/blob/16df6ebc5fac741b9eac091242b10149250745de/lib/utils/DecisionUtil.js#L1016 3) based on that weight, the following would adjust the lower and upper bounds of
startVariationAllocation&endVariationAllocationrespectively:https://github.com/wingify/vwo-node-sdk/blob/16df6ebc5fac741b9eac091242b10149250745de/lib/utils/CampaignUtil.js#L41 4) this all results in a maximum upper bound of
9600when it comes time to decide on a variation:https://github.com/wingify/vwo-node-sdk/blob/16df6ebc5fac741b9eac091242b10149250745de/lib/core/BucketingService.js#L62-L64
5) which, looking at the algo from
_generateBucketValue, looks to default to a max of10000based onConstants.MAX_TRAFFIC_PERCENT. This leaves a 4% chance that an assigned bucketed value falls outside the acceptable bounds of all active tests in a group and returns null - failing on this line:https://github.com/wingify/vwo-node-sdk/blob/efee33f24955c3c869185ae87f5dc0514ca0a230/lib/utils/DecisionUtil.js#L1033
Here's a snapshot of the overflow in action on our production site: