How does cache invalidation work, and why is deciding when to clear a cache often considered one of the harder problems in caching?

Cache invalidation means removing or updating cached data once it becomes stale or outdated, and it is considered difficult because you must carefully track every place that could change the underlying data and ensure the corresponding cache entry is cleared or refreshed at exactly the right moment, since caching data for too long risks showing users outdated information, while invalidating too aggressively defeats much of the performance benefit caching was meant to provide in the first place.

function updateProduct($id, $data) {
    $database->update($id, $data);
    $cache->delete('product_' . $id);
}