Never thought about that scenario. Not an expert but we are using Aurora MySQL, too. At the moment we distribute known read only selects to the cluster-ro endpoint. We do not specify aurora in the JDBC connection url. My assumption was that you always end up getting a read-only slave with the ro-endpoint. So it does the load-balancing but not the failover?

I bookmarked this issue because it might be important to us. Sorry, I can't be of help.

I'm facing the exact same issue.
For now, I am using the jdbc:mariadb endpoint (instead of jdbc:mariadb:aurora) for the read-only endpoint.
Even though I'm not sure that's the correct way to use it, it seems to be working fine.

Per the documentation, if you use aurora mode and set the connection to read-only, it would automatically route to slave instances (aka readers):
https://mariadb.com/kb/en/about-mariadb-connector-j/

aurora This mode supports connection failover in an Amazon Aurora cluster. This mode does support load-balancing reads on slave instances if the connection is set to read-only before executing the read. The connector performs load-balancing by randomly picking a slave instance to execute read queries for a connection.
This mode has been available since MariaDB Connector/J 1.2.0

This was working fine with 1.7.2. I'm now trying to test 2.5.4 and I have some evidence that it is not always the case but according to the docs it should work.

Peter, I done a quick check and it seems the load balancing in Aurora may have stopped working between v 2.4.3 & 2.4.4. I haven't had the chance to deep dive but there are a couple of commits in 2.4.4 related to Aurora that may have contributed. From what I can tell releases 2.4.4+ are not working correctly for loadbalancing using the mariadb:aurora: connection string.

2.4.4 release notes: https://mariadb.com/kb/en/mariadb-connector-j-244-changelog/
https://mariadb.com/kb/en/mariadb-connector-j-releases/

Was anyone able to find a definitive answer to this question? Using "aurora", read-only connections, and a aurora read-only cluster endpoint result in an error on my end, as it can't connect to the master node under the read-only aurora endpoint.

I was able to get it working against ro endpoint by dropping aurora protocol from the connection string, e.g.
"jdbc:mariadb://my-database.cluster-ro-cp12345dovz.us-east-1.rds.amazonaws.com:3306/my_schema"

aurora protocol enables failover, and for this the driver needs to read the topology on the cluster, so it makes a query to a writer node that is not available via a -ro endpoint. In case of -ro end point, you do not really need any failover, as Aurora itself will re-route the calls to an available node in the cluster.

Hope, it helps.

Thanks Peter! The bit about needing to connect to the master to read back the topology to provide failover support makes sense.

By dropping the protocol, it seems like the driver won't load balance reads across replicas, or the include ability to blacklist failed read replicas via "autoReconnect." Ideally, both load balanced reads and blacklisting of read replicas would be available under a read-only endpoint, but I suspect there is some detail I'm missing.

The read-only end point sits in front of actual nodes, so the load balancing is happening on aurora side, at least this is how it was explained to me. Here is my understanding how it works but I could be wrong:

If you have a writer node and two reader nodes, and use -ro endpoint, your connections will be balanced between reader nodes. If one reader node goes down, your connections will go to the node that is still up. If the second reader node goes down, the connections will go to the writer.

I was not able to verify the load balancing between two reader nodes because by the time we upgraded to a newer mariaDB driver and changed our connection URL to use -ro end point,. we made application changes, so we only needed one reader node to handle the load.

We do have a setup, though, when we do not have a reader node, so the -ro endpoint correctly routes connections to the writer.