Application schema upgrades v5.6

Similar to the upgrade of EDB Postgres Distributed, there are two approaches to upgrading the application schema. The simpler option is to stop all applications affected, preform the schema upgrade, and restart the application upgraded to use the new schema variant. This approach imposes some downtime.

To eliminate this downtime, EDB Postgres Distributed offers useful tools to perform a rolling application schema upgrade.

The following recommendations and tips reduce the impact of the application schema upgrade on the cluster.

Rolling application schema upgrades

By default, DDL is automatically sent to all nodes. You can control this behavior manually, as described in DDL replication. You can use this approach to create differences between database schemas across nodes.

PGD is designed to allow replication to continue even with minor differences between nodes. These features are designed to allow application schema migration without downtime or to allow logical standby nodes for reporting or testing.

See Replicating between nodes with differences for details.

When one node runs DDL that adds a new table, nodes that haven't yet received the latest DDL need to handle the extra table. In view of this, the appropriate setting for rolling schema upgrades is to configure all nodes to apply the skip resolver in case of a target_table_missing conflict. Perform this configuration before adding tables to any node. This setting is intended to be permanent.

Execute the following query separately on each node. Replace node1 with the actual node name.

SELECT bdr.alter_node_set_conflict_resolver('node1',
		'target_table_missing', 'skip');

When one node runs DDL that adds a column to a table, nodes that haven't yet received the latest DDL need to handle the extra columns. In view of this, the appropriate setting for rolling schema upgrades is to configure all nodes to apply the ignore resolver in case of a target_column_missing conflict. Perform this before adding columns to one node. This setting is intended to be permanent.

Execute the following query separately on each node. Replace node1 with the actual node name.

SELECT bdr.alter_node_set_conflict_resolver('node1',
		'target_column_missing', 'ignore');

When one node runs DDL that removes a column from a table, nodes that haven't yet received the latest DDL need to handle the missing column. This situation causes a source_column_missing conflict, which uses the use_default_value resolver. Thus, columns that don't accept NULLs and don't have a DEFAULT value require a two-step process:

1. Remove the NOT NULL constraint, or add a DEFAULT value for a column on all nodes.
2. Remove the column.

You can remove constraints in a rolling manner. There's currently no supported way for handling adding table constraints in a rolling manner, one node at a time.

When one node runs a DDL that changes the type of an existing column, depending on the existence of binary coercibility between the current type and the target type, the operation might not rewrite the underlying table data. In that case, it's only a metadata update of the underlying column type. Rewriting a table is normally restricted. However, in controlled DBA environments, you can change the type of a column to an automatically castable one by adopting a rolling upgrade for the type of this column in a non-replicated environment on all the nodes, one by one. See ALTER TABLE for more details.

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• Rolling application schema upgrades