They are different databases. Riak is a key value store, not unlike the core of redis. The SQL database is obviously relational. Ignoring the difference for a

They are different databases. Riak is a key value store, not unlike the core of redis. The SQL database is obviously relational. Ignoring the difference for a minute. If you can fit data in Redis, chances are that they can fit in riak and a RDBMS is not needed at all. Why Riak only then? It is one of the very few databases that can be clustered. It is sharded, multi-master, highly available, to talk in buzzwords. It is very reliable and very robust, you can add/remove nodes as you wish, replication is highly configurable, etc... Postgre/Mysql/MariaDB + Redis have no HA. They are both the disaster in disaster recovery.