I've used cassandra quite a bit and even I had to go back and figure out what this primary key means:
((channel_id, bucket), message_id)
The primary key consists of partition key + clustering columns, so this says that channel_id & bucket are the partition key, and message_id is the one and only clustering column (you can have more).
They also cite the most common cassandra mistake, which is not understanding that your partition key has to limit partition size to less than 300MB, and no surprise: They had to craft the "bucket" column as a function of message date-time because that's usually the only way to prevent a partition from eventually growing too large. Anyhow, this is incredibly important if you don't want to suffer a catastrophic failure months/years after you thought everything was good to go.
They didn't mention this part: Oh, I have to include all partition key columns in every query's "where" clause, so... I have to run as many queries as are needed for the time period of data I want to see, and stitch the results together... ugh... Yeah it's a little messy.