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From: Tomas Vondra <[email protected]>
To: Amit Kapila <[email protected]>
To: Alvaro Herrera <[email protected]>
Cc: Peter Eisentraut <[email protected]>
Cc: Pg Hackers <[email protected]>
Subject: Re: bogus: logical replication rows/cols combinations
Date: Sun, 8 May 2022 20:11:08 +0200
Message-ID: <[email protected]> (raw)
In-Reply-To: <CAA4eK1Le0Z6W_+oY+M4vV=fjwFMJRg3WguPr3LQAfEqATrSLbg@mail.gmail.com>
References: <CAA4eK1KQimd=0gLS1DHTK2HrZA3kPdaY3sDY9Jko8o-nXRQw9A@mail.gmail.com>
	<[email protected]>
	<CAA4eK1Le0Z6W_+oY+M4vV=fjwFMJRg3WguPr3LQAfEqATrSLbg@mail.gmail.com>

On 5/7/22 07:36, Amit Kapila wrote:
> On Mon, May 2, 2022 at 6:11 PM Alvaro Herrera <[email protected]> wrote:
>>
>> On 2022-May-02, Amit Kapila wrote:
>>
>>
>>> I think it is possible to expose a list of publications for each
>>> walsender as it is stored in each walsenders
>>> LogicalDecodingContext->output_plugin_private. AFAIK, each walsender
>>> can have one such LogicalDecodingContext and we can probably share it
>>> via shared memory?
>>
>> I guess we need to create a DSM each time a walsender opens a
>> connection, at START_REPLICATION time.  Then ALTER PUBLICATION needs to
>> connect to all DSMs of all running walsenders and see if they are
>> reading from it.  Is that what you have in mind?  Alternatively, we
>> could have one DSM per publication with a PID array of all walsenders
>> that are sending it (each walsender needs to add its PID as it starts).
>> The latter might be better.
>>
> 
> While thinking about using DSM here, I came across one of your commits
> f2f9fcb303 which seems to indicate that it is not a good idea to rely
> on it but I think you have changed dynamic shared memory to fixed
> shared memory usage because that was more suitable rather than DSM is
> not portable. Because I see a commit bcbd940806 where we have removed
> the 'none' option of dynamic_shared_memory_type. So, I think it should
> be okay to use DSM in this context. What do you think?
> 

Why would any of this be needed?

ALTER PUBLICATION will invalidate the RelationSyncEntry entries in all
walsenders, no? So AFAICS it should be enough to enforce the limitations
in get_rel_sync_entry, which is necessary anyway because the subscriber
may not be connected when ALTER PUBLICATION gets executed.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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