Trident provides an abstraction for reading from and writing results to stateful sources. We can maintain the state either internally to the topology (memory), or we can store it in external sources (Memcached or Cassandra).

Let's consider that we are maintaining the output of the preceding hello world Trident topology in a database. Every time you process the tuple, the count of country present in a tuple is increased in the database. We can't achieve exactly-once processing by only maintaining a count in the database. The reason is that if any tuple failed during processing, then the failed tuple is retried. This gives us a problem while updating the state, because we are not sure whether the state of this tuple was updated previously or not. If the tuple has failed before updating the state, then retrying the tuple will increase the count in the database...