Say, I we have master-master replication with only one master active at a time.
We have users table:
create table users ( id int auto_increment, email varchar(255), primary key id(id), unique key email(email) );
and web application cluster which inserts new record for new users.
Now we got error which stopped replication:
Error 'Duplicate entry 'firstname.lastname@example.org-' for key 'email'' on query. Default database: 'xxx'. Query: 'INSERT INTO users ...
Question is: is that true that this error in this setup is impossible, and probable reason of error is that we had two masters active at same time?