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fix(aws): EC2 per-pool expiry ignores size flexibility and keys non-RDS services without the service #450
Description
Summary
Two known limitations of the EC2 per-pool expiry adjustment in providers/aws/recommendations/expiry.go. They predate go#449, which documents them in its godoc.
- Size flexibility. Pools key on the exact instance type. A replacement of a different size in the same family is not credited, and an expiring RI of one size is not netted against coverage of another size. Size-flexible regional Linux/default-tenancy RIs are therefore over-deducted, which makes the tool re-buy. The RDS family path (
family_nu.go) handles this with normalized units. - Key collision. Non-RDS services share the
commitmentPoolKeyshape (expiry.go:~153). If two services used an identical ResourceType string in the same region, their expiring counts would combine.
Expected
- Credit and net EC2 size-flexible RIs in normalized units within the family, as the RDS family path does.
- Include the service in the pool key, or prove it already is unique per service.
Test
- A large RI expiring, replaced by two medium RIs, produces no re-buy.
- Two services with the same ResourceType string in one region keep their expiring counts separate.
Activity
cristim commentedon Oct 16, 2026
Claiming: working on this.
cristim commentedon Oct 16, 2026
Plan (
go-450-ec2-expiry-sizeflex-key.md) findings on main:Part 1, the key collision, is not reachable today. The non-RDS services that share the 2-part
region:typekey have disjoint ResourceType shapes:m5.largecache.m5.largem5.large.searchdc2.*/ra3.*db.r6g.largeRDS uses its own 4-part
rdsPoolKey. Adding a service dimension would changepoolKey,lookupPoolKeyandcommitmentPoolKeytogether, and with them the exportedPoolCoverageMapshape that the ladder and the CLI read. So I'm doing a guard test over the real per-service shapes plus a comment oncommitmentPoolKey, with no key change.Part 2, EC2 size flexibility, needs maintainer decisions before implementation, because it changes recommended purchase quantities. Proposal:
d = max(0, E_nu - R_nu). That keeps fix(aws): credit replacement RIs bought inside the coverage lookback in the EC2 expiry deduction #449's rule of possibly under-buying but never over-buying.dback to the per-size recs.Decisions needed:
100*d/Tcoverage points (uniform), or shoulddbe weighted toward the expiring sizes?CoverageLookbackDays == 0, i.e. on expiring-vs-expiring netting alone?Part 2 is about 400 lines and may split into two PRs. It changes no API, so the CLI caller picks it up on upgrade. I'm leaving part 2 open pending these answers.
test(aws): guard that non-RDS expiry pool keys stay disjoint across s…
test(aws): guard that non-RDS expiry pool keys stay disjoint across s…