This is a major difference between vanilla MySQL and Amazon Aurora (and a potentially major risk!):
because Aurora MySQL primary and replica instances share a storage layer, they share a set of undo logs. This means that, for a REPEATABLE READ isolation level, the storage instance must maintain undo logs at least as far back as could be required to satisfy transactional guarantees for the primary or any read replica instance. Long-running replica transactions can negatively impact writer performance in Aurora MySQL—finally, an explanation for the incident that spawned this investigation. The same scenario plays out differently in vanilla MySQL because of its different model for undo logs. Vanilla MYSQL: there are two undo logs – one on the writer, and one on the reader. The performance impact of an operation that prevents the garbage collection of undo log records will be isolated to either the writer or the reader. Aurora MySQL: there is a single undo log that is shared between the writer and reader. The performance impact of an operation that prevents the garbage collection of undo log records will affect the entire cluster.