Re: Support logical replication of DDLs

On Sat, Jul 23, 2022 at 11:33 AM Joe Conway <mail@joeconway.com> wrote:
>
> On 7/22/22 17:18, Zheng Li wrote:
> > Here is a patch that supports replication of global object commands,
> > these include ROLE statements, database statements and tablespace statements.
> > The patch should be applied on top of the v13 DDL replication patch set that
> > ZJ Hou sent in the previous email.
> >
> > Global objects commands are different from other DDL commands in
> > that:
> > 1. Global objects commands are allowed to be executed in any databases
> > 2. Global objects are not schema qualified
> > 2. Global objects commands are not captured by event triggers
> >
> > This patch supports global objects commands replication by WAL
> > logging the command using the same function for DDL logging -
> > LogLogicalDDLMessage, towards the end of standard_ProcessUtility.
> > Because global objects are not schema qualified, we can skip the deparser
> > invocation and directly log the original command string for replay on
> > the subscriber.
>
> I have not looked at the patch but +1 for the general concept. Seems
> like you might want to start a separate thread, perhaps after the
> currently running commitfest is over.

Thanks for the suggestion. I'll start a new thread on the replication
of global objects
commands. I think it's different enough to get its own attention.

>
> > A key problem is global objects can get inconsistent between the
> > publisher and the subscriber if a command changes the global object
> > in a database (on the source side) which doesn't configure logical replication.
> > I think we can work on the following directions in order to avoid such
> > inconsistency:
> >
> > 1. Introduce a publication option for global objects command replication
> > and document that logical replication of global objects commands is preferred
> > to be configured on all databases. Otherwise inconsistency can happen
> > if a command changes the global object in a database which doesn't configure
> > logical replication.
> >
> > 2. Introduce database cluster level logical replication to avoid
> > such inconsistency, this is especially handy when there is a large
> > number of databases to configure for logical replication.
>
> I would strongly favor #2, although I admittedly have no idea what
> complexities it adds.

I will also start a new thread once we have more concrete plans on this.

Regards,
Zheng