How about delegating the tinyUrl expiration to the Storage Layer. Most key/value stores (including DynamoDB, Riak, Cassandra) support TTL at the record or value level. That should reduce maintenance quite a bit.
Also, why does nobody ever mentions HBase? 
Totally agree! Setting up the record with a TTL removes the burden of expiry and deletion from the application layer. Request the author to amend the course accordingly.
So the database handles deletion of records that have passed some expiration time? Excellent! Thank you for the heads up 
Hi Marceo,
If you refer the CAP theorum (https://www.educative.io/module/lesson/grokking-system-design-interview/m7kRvl0oBYA) HBase doesn’t fit well for high availability scenarios in distributed systems.