cache: Coalesce in-flight identical upstream fetches #35

New issue

Closed

opened 2026-09-04 18:51:03 +00:00 by eva · 0 comments

eva commented

2026-09-04 18:51:03 +00:00

Owner

Problem

A depot chunk miss under several clients at once can stampede Steam CDNs (same object, N upstream GETs).

Why

Single-flight fill cuts wasted upstream bandwidth and load under concurrent miss storms.

Scope

Coalesce identical in-flight upstream fetches so only one fill runs; others wait on that result
Same cache path; no second stack / new APIs

Acceptance

Concurrent identical misses result in one upstream GET (covered by test)
Waiters receive the same filled object / error
README or metrics note if useful for ops

Out of scope

#31 HTTP HEAD
New API surface

Size: M. From Forge Meeting (Ash). Related: Moss CDN alias keying, Pike negative cache.

## Problem A depot chunk miss under several clients at once can stampede Steam CDNs (same object, N upstream GETs). ## Why Single-flight fill cuts wasted upstream bandwidth and load under concurrent miss storms. ## Scope - Coalesce identical in-flight upstream fetches so only one fill runs; others wait on that result - Same cache path; no second stack / new APIs ## Acceptance - Concurrent identical misses result in one upstream GET (covered by test) - Waiters receive the same filled object / error - README or metrics note if useful for ops ## Out of scope - #31 HTTP HEAD - New API surface Size: M. From Forge Meeting (Ash). Related: Moss CDN alias keying, Pike negative cache.