Re: bogus: logical replication rows/cols combinations
От | Tomas Vondra |
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Тема | Re: bogus: logical replication rows/cols combinations |
Дата | |
Msg-id | dea2d025-9084-fb59-c6ee-da2fc19985b1@enterprisedb.com обсуждение исходный текст |
Ответ на | Re: bogus: logical replication rows/cols combinations (Amit Kapila <amit.kapila16@gmail.com>) |
Ответы |
Re: bogus: logical replication rows/cols combinations
(Amit Kapila <amit.kapila16@gmail.com>)
|
Список | pgsql-hackers |
On 5/9/22 05:45, Amit Kapila wrote: > On Sun, May 8, 2022 at 11:41 PM Tomas Vondra > <tomas.vondra@enterprisedb.com> wrote: >> >> On 5/7/22 07:36, Amit Kapila wrote: >>> On Mon, May 2, 2022 at 6:11 PM Alvaro Herrera <alvherre@alvh.no-ip.org> 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, >> > > Yes, that should be sufficient to enforce limitations in > get_rel_sync_entry() but it will lead to the following behavior: > a. The Alter Publication command will be successful but later in the > logs, the error will be logged and the user needs to check it and take > appropriate action. Till that time the walsender will be in an error > loop which means it will restart and again lead to the same error till > the user takes some action. > b. As we use historic snapshots, so even after the user takes action > say by changing publication, it won't be reflected. So, the option for > the user would be to drop their subscription. > > Am, I missing something? If not, then are we okay with such behavior? > If yes, then I think it would be much easier implementation-wise and > probably advisable at this point. We can document it so that users are > careful and can take necessary action if they get into such a > situation. Any way we can improve this in future as you also suggested > earlier. > >> which is necessary anyway because the subscriber >> may not be connected when ALTER PUBLICATION gets executed. >> > > If we are not okay with the resultant behavior of detecting this in > get_rel_sync_entry(), then we can solve this in some other way as > Alvaro has indicated in one of his responses which is to detect that > at start replication time probably in the subscriber-side. > IMO that behavior is acceptable. We have to do that check anyway, and the subscription may start failing after ALTER PUBLICATION for a number of other reasons anyway so the user needs/should check the logs. And if needed, we can improve this and start doing the proactive-checks during ALTER PUBLICATION too. regards -- Tomas Vondra EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
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