From: | Joe Conway <mail(at)joeconway(dot)com> |
---|---|
To: | Zheng Li <zhengli10(at)gmail(dot)com>, "houzj(dot)fnst(at)fujitsu(dot)com" <houzj(dot)fnst(at)fujitsu(dot)com> |
Cc: | Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Japin Li <japinli(at)hotmail(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Dilip Kumar <dilipbalaut(at)gmail(dot)com>, rajesh singarapu <rajesh(dot)rs0541(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Peter Smith <smithpb2250(at)gmail(dot)com> |
Subject: | Re: Support logical replication of DDLs |
Date: | 2022-07-23 15:33:43 |
Message-ID: | a702a699-bd10-cc89-b6be-03dfa749609d@joeconway.com |
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Lists: | pgsql-general pgsql-hackers |
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.
> 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.
--
Joe Conway
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
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