what is lock, reentrantlock, synchronized, how to make threads run in sequence, wildcards, eg. of wildcards, bus ticketing design system, what is diff between treemap, hashmap, concurrenthashmap, how to handle service which has used up full memory, lifecycle of bean, transactional annotation and its attributes, what is microservice, how 2 or more service behaves in case of failure of any one of the service, how to handle it, long to short url design