A map of cache bin to default cache backend service name.
All mappings in $settings takes precedence over this, but this can be used to optimize cache storage for a Drupal installation without cache customizations in settings.php. For example, this can be used to map the 'bootstrap' bin to 'cache.backend.chainedfast', while allowing other bins to fall back to the global default of 'cache.backend.database'.
Type: array
File
core/lib/Drupal/Core/Cache/CacheFactory.php, line 34
Class
CacheFact