If a DDL operation is sent by the router to a databases's non-primary shard, the error is currently not retryable.

There should be a new kind of "retryable" exception in order to implement something similar to the following logic:

retries = 0;
do{
 try{
    // best effor get db primary
    // call DDL on db primary
 }catch(/* not primary node for db */){
     retries++;
 }
}while(retries < 3);