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From Justin
Subject Re: Synchronize the dump with a logical slot with --snapshot
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Msg-id CALL-XeNd=8v7BF1RyamWK7ur9rjbbC40A3N8GFjBC+ZX3azeVQ@mail.gmail.com
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In response to Re: Synchronize the dump with a logical slot with --snapshot  (Durgamahesh Manne <maheshpostgres9@gmail.com>)
Responses Re: Synchronize the dump with a logical slot with --snapshot
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On Sat, Sep 28, 2024 at 1:04 AM Durgamahesh Manne <maheshpostgres9@gmail.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@gmail.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@gmail.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
 

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