Usually, when you change the schema on a master and on its replica databases, you cannot do these changes simultaneously. Typically, if you have changed the schema on the master database and not the replica databases, you do not want a replica database to refresh the data until the schema on the replica database has the same change. You can use the bulletin board parameter SYNC_APP_SCHEMA_VERSION to prevent data synchronization when the master and replica databases do not have the same schema.

If either the master or the replica database has the SYNC_APP_SCHEMA_VERSION parameter set, then, when the master and replica databases try to synchronize data, they compare the values of this parameter. If the value on the master database does not match the value on the replica database, the servers do not synchronize data. Instead, the replica database stores the synchronization message. After you have updated the replica schema, you can re-send the synchronization message by using the MESSAGE FORWARD statement.

For example, if the value of SYNC_APP_SCHEMA_VERSION is set to Version2 on the master database, and the value is set to Version1 on the replica database, the databases will not synchronize data.

Note that the master and replica databases just compare values for this parameter; they do not compare their schemas. If you accidentally set the values of SYNC_APP_SCHEMA_VERSION to the same value on both master and replica database and the databases do not actually have the same schema, then the master and replica databases will try to synchronize.

The databases only use the SYNC_APP_SCHEMA_VERSION bulletin board parameter if it is explicitly set. There is no default value, and the servers do not automatically increment the value each time that their schemas are updated. The actual values are meaningless; the only thing that matters is whether or not the master and replica database have the same value for the parameter or a different value.