Re: Synchronize the dump with a logical slot with --snapshot

On Sat, Sep 28, 2024 at 1:04 AM Durgamahesh Manne <maheshpostgres9(at)gmail(dot)com>
wrote:

> Hi Team
>
> Can anyone respond to my question from respected team members ?
>
> Durga Mahesh
>
> On Thu, Sep 26, 2024 at 2:23 AM Durgamahesh Manne <
> maheshpostgres9(at)gmail(dot)com> wrote:
>
>> Hi Team
>>
>> --snapshot=snapshotname
>> (Use the specified synchronized snapshot when making a dump of the
>> database
>>
>> This option is useful when needing to synchronize the dump with a logical
>> replication slot) as per the pgdg
>>
>> How do we synchronize the dump with a logical replication slot with
>> --snapshot?
>>
>> I am using the postgresql 14 version which supports only
>> pg_create_logical_replication_slot. How to generate a internal snapshot
>> with it ?
>>
>> Below CREATE_REPLICAION_SLOT not supported by postgresql 14
>>
>> example osdb_lsr=# CREATE_REPLICATION_SLOT lsr_sync_01 LOGICAL pgoutput;
>> slot_name | consistent_point | snapshot_name | output_plugin
>> -------------+------------------+---------------------+---------------
>> lsr_sync_01 | 0/C000110 | 00000003-00000002-1 | pgoutput
>>
>> Regards,
>>
>> Durga Mahesh
>>
>>
>>
>> On Fri, 20 Sept, 2024, 01:27 Durgamahesh Manne, <
>> maheshpostgres9(at)gmail(dot)com> wrote:
>>
>>> Hi Team
>>>
>>> --snapshot=*snapshotname*
>>>
>>> (Use the specified synchronized snapshot when making a dump of the
>>> database
>>>
>>> This option is useful when needing to synchronize the dump with a
>>> logical replication slot) as per the pgdg
>>>
>>> How do we synchronize the dump with a logical replication slot
>>> with --snapshot?
>>>
>>> I am using the postgresql 14 version which supports only
>>> pg_create_logical_replication_slot. How to generate a snapshot with it ?
>>>
>>> Below CREATE_REPLICAION_SLOT not supported by postgresql 14
>>>
>>> example osdb_lsr=# CREATE_REPLICATION_SLOT lsr_sync_01 LOGICAL pgoutput;
>>> slot_name | consistent_point | snapshot_name | output_plugin
>>> -------------+------------------+---------------------+---------------
>>> lsr_sync_01 | 0/C000110 | 00000003-00000002-1 | pgoutput
>>>
>>> Regards,
>>>
>>> Durga Mahesh
>>>
>>

Hi Durgamahesh,

I am not sure what you are after with matching pg_dump and replication slot
together unless you are trying to get a dump to handle the initial data
sync. There is not a benefit to doing that as the WAL is going to build up
on the publisher...

You have to create a snapshot using the export function

https://www.postgresql.org/docs/current/sql-set-transaction.html

https://www.postgresql.org/docs/17/functions-admin.html#FUNCTIONS-SNAPSHOT-SYNCHRONIZATION

Then you can create the logical replication slot with using that slotname
option
https://www.postgresql.org/docs/17/sql-createsubscription.html#SQL-CREATESUBSCRIPTION-PARAMS-WITH-SLOT-NAME
and no sync option.

Then you tell pg_dump to use that snapshot name snapshot with this option
--snapshot=snapshotname

https://www.postgresql.org/docs/current/app-pgdump.html

Once pg_restore is done on the destination , you can create a subscription
using that slotname option probably and specify copy_data = false.

Keep in mind the WAL will build up during this process, not sure what the
benefit would be just allowing logical replication to do the initial sync.

Thanks
Justin