Function validate_cache_pairs 

fn validate_cache_pairs(
    policy_nodes: &[NodeConfig],
) -> Result<Vec<CachePairWarning>, String>

Checks that every proxy-cache pair agrees about where it caches.

The two halves of a pair are linked only by their shared id; nothing else ties them together. If they disagree about policy or store, the store half writes somewhere the lookup half never reads, and the route compiles, serves traffic, and returns a permanent 100% miss with no error anywhere – it simply looks like a cache that is never warm. There is no configuration for which that is correct, so it is refused.

A half with no counterpart is returned as a warning instead: equally useless, but also what a policy looks like halfway through being built.