DDL Replication v3.7

pglogical3 supports transparent DDL replication, where schema change commands run on provider node(s) are automatically captured and replicated to subscriber node(s) without the need for wrapper functions like pglogical.replicate_ddl_command or external schema management tools.

The main difference from normal replication of table rows ("table replication") is that DDL replication replicates statements themselves rather than the effects of those statements. Normal data replication replicates the changes made by a statement, e.g. it sends the rows that got UPDATEd by an UPDATE command rather than replicating and executing the UPDATE statement itself. pglogical's DDL replication captures, replicates and executes the text of the DDL statement itself.

Minimal example

Enabling DDL replication on a pglogical provider can be as simple as:

SELECT * FROM pglogical.replication_set_add_ddl('ddl_sql', 'all', NULL, NULL);

to replicate any captureable DDL statements executed by any user on the provider database to any subscriber(s) that subscribe to the enabled-by-default ddl_sql replication set.

However it's generally recommended to enable replication of a targeted subset of DDL instead.

There are also caveats relating to replication of changes to "global objects" like roles, the handling of some ALTER TABLE modes, etc that are important to understand. See "Restrictions" below.

How to use DDL replication

Transparent DDL replication in pglogical builds on the same Replication Sets model that's used by replication of table contents. The same replication set(s) may be used for both replicating table contents and for DDL.

To replicate future DDL commands on a provider, a DDL replication filter must be added to the replication set(s) used by subscribers that should receive and apply the DDL.

The DDL filter can specify a command_tag and role_name to allow replication of only some DDL statements. The command_tag is same as those used by . The role_name is used for matching against the current role which is executing the command. Both command_tag and role_name are evaluated as regular expressions which are case sensitive.

Functions for managing DDL replication filters

The following functions are provided for managing DDL replication filters using replication sets:

pglogical.replication_set_add_ddl

Adds a DDL replication filter to a replication set.

Synopsis

pglogical.replication_set_add_ddl (
    set_name name,
    ddl_filter_name text,
    command_tag text,
    role_name text
)

Parameters

  • set_name - name of the existing replication set
  • ddl_filter_name - name of the new DDL replication filter
  • command_tag - regular expression for matching command tags
  • role_name - regular expression for matching role name

The command_tag and role_name parameters can be set to NULL in which case they will match any command tag or role respectively. They are both regular expressions, so you can use patterns like 'CREATE.*' or '(CREATE|DROP).*'.

The target object identity (oid, name, etc) are not exposed, so you cannot filter on them.

pglogical.replication_set_remove_ddl

Remove a DDL replication filter from replication set.

Synopsis

pglogical.replication_set_remove_ddl(set_name name, ddl_filter_name text)

Parameters

  • set_name - name of the existing replication set
  • ddl_filter_name - name of the DDL replication filter to be removed from the set

Additional functions and views

pglogical.ddl_replication

This view lists ddl replication configuration as set up by current ddl_filters.

pglogical.ddl_replication Columns

NameTypeDescription
set_ddl_namenameName of DDL filter
set_ddl_tagtextWhich command tags it applies to (regular expression)
set_ddl_roletextWhich roles it applies to (regular expression)
set_namenameName of the replication set for which this filter is defined

pglogical.ddl_replication

This view lists ddl replication configuration as set up by current ddl_filters.

pglogical.ddl_replication Columns

NameTypeDescription
set_ddl_namenameName of DDL filter
set_ddl_tagtextWhich command tags it applies to (regular expression)
set_ddl_roletextWhich roles it applies to (regular expression)
set_namenameName of the replication set for which this filter is defined

pglogical.replicate_ddl_command

This function can be used to explicitly replicate a command as-is using the specified set of replication sets. The command will also be executed locally.

In most cases pglogical.replicate_ddl_command is rendered obsolete by pglogical's support for DDL replication filters.

Synopsis

pglogical.replicate_ddl_command(command text, replication_sets text[])`

Parameters

  • command - DDL query to execute
  • replication_sets - array of replication sets which this command should be associated with; default "{ddl_sql}"

Restrictions

When the DDL replication filter matches a DDL command it will modify the search_path configuration parameter used to execute the DDL on both provider and subscriber(s) to include only pg_catalog i.e. the system catalogs. This means that all the user objects referenced in the query must be fully schema qualified. For example CREATE TABLE foo... will raise an error when executed and has to be rewritten as CREATE TABLE public.foo....

DDL that matches the DDL replication filter and does not comply with this requirement will fail with an error like this:

ERROR:  no schema has been selected to create in
STATEMENT: CREATE TABLE foo (id integer);

or will raise an ERROR message complaining that data types, tables, etc referenced by the DDL statement do not exist even when they can be queried normally, are shown by psql, etc.

For example, attempting to drop some table public.a will fail:

ERROR: table "a" does not exist
STATEMENT: DROP TABLE a;

and must be reframed as:

DROP TABLE public.a;

The same restriction applies to any command executed using the pglogical.replicate_ddl_command function. The function call has the additional restriction that it cannot execute special commands which need to be run outside of a transaction. Most notably CREATE INDEX CONCURRENTLY will fail if run using pglogical.replicate_ddl_command but will work via DDL replication sets.

For testing purposes it can be useful to simulate the behaviour of DDL replication capture manually in psql. To do so, set the search_path to the empty string, e.g.

BEGIN;
SET LOCAL search_path = '';
CREATE TABLE mytable(id integer);
COMMIT;

will fail with ERROR: no schema selected to create in.

Considerations with global objects

Because PostgreSQL has objects that exist within one database, objects shared by all databases, and objects that exist outside the catalogs, some care is required when you may potentially replicate a subset of DDL or replicate DDL from more than one database:

  • pglogical can capture and replicate DDL that affects global objects like roles, users, groups, etc, but only if the commands are run in a database with pglogical ddl replication enabled. So it's easy to get into inconsistent states if you do something like CREATE ROLE in the postgres db then ALTER ROLE in the my_pglogical_enabled. The resulting captured DDL may not apply on the downstream, requiring a transaction to be skipped over or non-replicated DDL to be run on the downstream to create the object that's targeted by the replicated DDL.

  • pglogical can also capture and replicate DDL that references global objects that may not exist on the other node(s), such as tablespaces and users/roles. So an ALTER TABLE ... OWNER TO ... can fail to apply if the role, a global object, does not exist on the downstream. You may have to create a dummy global object on the downstream or if absolutely necessary, skip some changes from the stream.

  • DDL that references local paths like tablespaces may fail to apply on the other end if paths differ.

In general you should run all your DDL via your pglogical-enabled database, and ensure that all global objects exist on the provider and all subscribers. This may require the creation of dummy roles, dummy tablespaces, etc.

pglogical.tables

This view lists information about table membership in replication sets. If a table exists in multiple replication sets it will appear multiple times in this table.

pglogical.tables Columns

NameTypeDescription
relidoidThe OID of the relation
nspnamenameName of the schema relation is in
relnamenameName of the relation
set_namenameName of the replication set
set_opstext[]List of replicated operations
rel_columnstext[]List of replicated columns (NULL = all columns) (*)
row_filtertextRow filtering expression

pglogical.queue

DDL can also be queued up with a message to state the replication information. This can be seen in ascending order, on this view.

pglogical.queue_truncate

A function that erase's all the logging information of the view.