Medhod CachedDNSToSwitchMapping::resolve() can return NULL which requires careful handling. Null can be returned in two cases:
• An error occurred with topology script execution (script crashes).
• Script returns wrong number of values (other than expected)
Critical handling is in the DN registration code. DN registration code is responsible for assigning proper topology paths to all registered datanodes. Existing code handles this NULL pointer on the following way (resolveNetworkLocation method):
The line of code that is assigning default rack:
can cause a serious problem. This means if somehow we got NULL, then the default rack will be assigned as a DN's network location and DN's registration will finish successfully. Under this circumstances, we will be able to load data into cluster which is working with a wrong topology. Wrong topology means that fault domains are not honored.
For the end user, it means that two data replicas can end up in the same fault domain and a single failure can cause loss of two, or more, replicas. Cluster would be in the inconsistent state but it would not be aware of that and the whole thing would work as if everything was fine. We can notice that something wrong happened almost only by looking in the log for the error: