How:

• init the cache with the content of the database (during registration) 
• during registration we should add the new alternateId if is not present
• cache entries with infinite TTL
• log if we went to the cache or the DB (debug level, for demo purposes)
• minimize calls to the DB

A/C:

• if the customer provides the same alternateId twice the second entry does not gets added (reported as an error)
• new error code & message should be agreed with the team & stakeholders
• document the new cache (is it possible to combine the two maps into one cache?)
• prove that the performance of the registration is not degraded significantly (registration perf. test)