Re: Synchronizing slots from primary to standby

On Thu, Feb 1, 2024 at 11:21 AM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
>
> Here are some review comments for v740001.

Thanks Peter for the feedback.

> ======
> src/sgml/logicaldecoding.sgml
>
> 1.
> + <sect2 id="logicaldecoding-replication-slots-synchronization">
> + <title>Replication Slot Synchronization</title>
> + <para>
> + A logical replication slot on the primary can be synchronized to the hot
> + standby by enabling the <literal>failover</literal> option during slot
> + creation and setting
> + <link linkend="guc-enable-syncslot"><varname>enable_syncslot</varname></link>
> + on the standby. For the synchronization
> + to work, it is mandatory to have a physical replication slot between the
> + primary and the standby, and
> + <link linkend="guc-hot-standby-feedback"><varname>hot_standby_feedback</varname></link>
> + must be enabled on the standby. It is also necessary to specify a valid
> + <literal>dbname</literal> in the
> + <link linkend="guc-primary-conninfo"><varname>primary_conninfo</varname></link>
> + string, which is used for slot synchronization and is ignored
> for streaming.
> + </para>
>
> IMO we don't need to repeat that last part ", which is used for slot
> synchronization and is ignored for streaming." because that is a
> detail about the primary_conninfo GUC, and the same information is
> already described in that GUC section.

Modified in v75.

> ======
>
> 2. ALTER_REPLICATION_SLOT slot_name ( option [, ...] ) #
>
> <para>
> - If true, the slot is enabled to be synced to the standbys.
> + If true, the slot is enabled to be synced to the standbys
> + so that logical replication can be resumed after failover.
> </para>
>
> This also should have the sentence "The default is false.", e.g. the
> same as the same option in CREATE_REPLICATION_SLOT says.

I have not added this. I feel the default value related details should
be present in the 'CREATE' part, it is not meaningful for the "ALTER"
part. ALTER does not have any defaults, it just modifies the options
given by the user.

> ======
> synchronize_one_slot
>
> 3.
> + /*
> + * Make sure that concerned WAL is received and flushed before syncing
> + * slot to target lsn received from the primary server.
> + *
> + * This check will never pass if on the primary server, user has
> + * configured standby_slot_names GUC correctly, otherwise this can hit
> + * frequently.
> + */
> + latestFlushPtr = GetStandbyFlushRecPtr(NULL);
> + if (remote_slot->confirmed_lsn > latestFlushPtr)
>
> BEFORE
> This check will never pass if on the primary server, user has
> configured standby_slot_names GUC correctly, otherwise this can hit
> frequently.
>
> SUGGESTION (simpler way to say the same thing?)
> This will always be the case unless the standby_slot_names GUC is not
> correctly configured on the primary server.

It is not true. It will not hit this condition "always" but has higher
chances to hit it when standby_slot_names is not configured. I think
you meant 'unless the standby_slot_names GUC is correctly configured'.
I feel the current comment gives clear info (less confusing) and thus
I have not changed it for the time being. I can consider if I get more
comments there.

> 4.
> + /* User created slot with the same name exists, raise ERROR. */
>
> /User created/User-created/

Modified.

> ~~~
>
> 5. synchronize_slots, and also drop_obsolete_slots
>
> + /*
> + * Use shared lock to prevent a conflict with
> + * ReplicationSlotsDropDBSlots(), trying to drop the same slot while
> + * drop-database operation.
> + */
>
> (same code comment is in a couple of places)
>
> SUGGESTION (while -> during, etc.)
>
> Use a shared lock to prevent conflicting with
> ReplicationSlotsDropDBSlots() trying to drop the same slot during a
> drop-database operation.

Modified.

> ~~~
>
> 6. validate_parameters_and_get_dbname
>
> strcmp() just for the empty string "" might be overkill.
>
> 6a.
> + if (PrimarySlotName == NULL || strcmp(PrimarySlotName, "") == 0)
>
> SUGGESTION
> if (PrimarySlotName == NULL || *PrimarySlotName == '\0')
>
> ~~
>
> 6b.
> + if (PrimaryConnInfo == NULL || strcmp(PrimaryConnInfo, "") == 0)
>
> SUGGESTION
> if (PrimaryConnInfo == NULL || *PrimaryConnInfo == '\0')

Modified.

thanks
Shveta