Session: stale-while-revalidate — 2026-09-10

Project: edge-cache · Branch: feat/swr · Mode: default · Model: claude-sonnet-4-6

Summary

Implemented stale-while-revalidate in the Go caching proxy: stale entries are served immediately while a single background refresh per key runs under singleflight, so a hot key never stampedes the origin.

Conversation

Turn 1 — User

Add stale-while-revalidate. Origin must not get a thundering herd.

Turn 1 — Assistant

Serve stale within the SWR window, trigger one refresh per key through singleflight.Group, and cap refresh concurrency with a semaphore.

func (c *Cache) serve(w http.ResponseWriter, r *http.Request, key string) {
    entry, ok := c.store.Get(key)
    if ok && entry.Fresh(time.Now()) {
        entry.WriteTo(w)
        return
    }
    if ok && entry.WithinSWR(time.Now()) {
        entry.WriteTo(w) // serve stale now
        go c.group.Do(key, func() (any, error) { return c.refresh(key, r) })
        return
    }
    c.fetchAndStore(w, r, key)
}

Tools used in this turn:

• Edit: cache/handler.go
• Write: cache/refresh.go
• Bash: go test ./... -race — ok

Connections

• [[edge-cache]]
• [[Go]]
• [[Singleflight]]
• [[StaleWhileRevalidate]]