In MapReduce, distinct/unique count of large data set is very common but unfortunately it is not scalable because it requires one reducer. It gets worse when you have to perform unique count across different aggregation/segment groups. Again, unique counting across different aggregation granularities whether in terms of time dimension or in combination with other demographic attributes is a common practice in big data analytics. Eg. In your massive data sets, let’s find unique counts of users for every hour, every day, every week, every month and etc.
HyperLogLog approximate unique counting can be used to provide a scalable solution. Keep in mind, the unique count here is a probabilistic approximate counts.You can implement either a map side hyperloglog counting or reduce side hyperloglog counting. However, map side counting requires more memory and beware of out of memory issue if you have many segments to perform unique count on.
For more information, you can check out the following pages