I'll confirm Alex's answer, there is no support for name parameters in the prepared query string itself (or rather, CQL doesn't support them and the driver religiously avoid parsing/messing with the query string).
But, if you prepare "insert into users (user_id, name) values (? , ?)", you can later do something like setString("name", "someName") on the BoundStatement corresponding.
But don't get me wrong, I'm not saying named parameters wouldn't be nice. For instance, if a SELECT statement has 2 bind markers that correspond to the same column name, you do have to resort to indexes, which is slightly annoying. But as I don't like forcing clients to mess with query strings, I'd like to see if it's not something that could be fixed in the CQL side first (so in Cassandra). It's not supported at this point in any case.
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Sylvain