Efficient Online Schema Upgrade In Distributed Databases

One or more non-transitory storage media storing sequences of instructions that, when executed by one or more computing devices, cause:

• receiving a request for a schema upgrade in a replicated database system comprising a first database server hosting a first replica in a first database and a second database server hosting a second replica within a second database, wherein the schema upgrade changes a first version of a database schema that defines one or more database objects stored in the first database to a second version of the database schema;
• performing the schema upgrade asynchronously, resulting in the second database being upgraded to the second version before the first database is upgraded to the second version;
• and
• during a schema upgrade window in which the schema upgrade of the first database is not completed:
• said first database server executing a first data manipulation language (DML) change against the first replica, said first DML change specifying a first change to a particular database object;
• generating a first logical change record (LCR) that specifies said first DML change;
• applying by said second database server said first LCR against said second replica;
• said second database server executing a second DML change against the second replica, said second DML change specifying a second change to said particular database object;
• generating a second LCR that specifies said second DML change;
• applying by said first database server said second LCR against said first replica;
• and
• said second database server preventing a third DML change against said second replica in response to determining that said third DML change is incompatible with the first version of the database schema.