Vercel ha annunciato di aver implementato il request collapsing per impedire valanghe di richieste contemporanee in caso di cache miss.
Raramente si conoscono i dettagli implementativi di una CDN, quindi è interessante leggere l'approccio basato su lock distribuiti:
Request collapsing works through a two-level distributed locking system.
At the node level, each CDN node maintains an in-memory lock. When multiple requests for the same uncached path arrive at that node, the lock ensures that only one proceeds. Others wait until the cache fills. This prevents a single node from firing multiple function invocations for the same path.
At the regional level, every region enforces its own lock across all nodes. After acquiring the node lock, a request tries to get the regional lock. Only one request per region can hold both locks simultaneously, meaning only one function invocation per region can regenerate a page.
This design makes collapsing scalable. Without the node-level grouping, hundreds of concurrent requests could all compete for the regional lock simultaneously. This would create a thundering herd problem where the lock coordination itself becomes a bottleneck.