You probably noticed that our app is misbehaving.
Even though we just requested 10 unique cities, our app used A LOT of memory over time.
When we take a heap dump we can see that our City class has a lot of instances. (Precisely 10K instances after 10K requests)
Moreover, its "Surviving Generation" metrics keeps on growing and growing, a strong indication of a memory leak.
The root cause is that our CityId class implemented hashcode without equals.
Failing to do so causes our Cache (that is based on a HashMap) to misbehave and act like a List, storing duplicate values.
It fails to correctly .get on our map, so we end up storing all of the cities in the map until we crash with an OutOfMemoryException.
Changing our CityId class to a case class will solve the issue.
case class CityId(id: Long)
In a case class, equals and hashcode are overridden for us, by default.
If you will compile and run the app again you will see that memory graph looks like it should, and
if you will take a heap dump will see exactly 10 instances of City