Summary

The single-document lookup currently handles StaleConfig with a bespoke catch -> onStaleCollectionVersion -> reset acquisition (yields the lock) -> drop memoized CRI -> kNotHandled -> fallback re-routes. This is correct and CAR-accepted. Investigate folding it into CollectionRouter for a cleaner shared mechanism.

Proposed direction

Wrap the lookup in routeWithRoutingContext(callback, onException):

return router.routeWithRoutingContext([&](routingCtx){
    auto d = eligibility.check(routingCtx.getCri(nss), documentKey);  // CRI injected, not re-fetched
    if (!d.isLocal()) return kNotHandled;
    ScopedSetShardRole scope(...); acquisition = acquirer.acquireCollection(nss); runExecutor();
}, /*onException*/ [&]{ acquisition.reset(); });                       // yield lock; router drives refresh/retry

Benefits: the router owns refresh/retry; a single CRI is shared by both the router and LocalLookupEligibility (no double CatalogCache fetch).

Likely CollectionRouter API extensions (CAR review)

• An onException/onSuccess hook on route/routeWithRoutingContext to yield then reacquire locks.
• Avoid recreating RoutingContext/NssRoutingInfoMap on every per-document call (cache across invocations – this is per-document, not per-query).