Re: Synchronizing slots from primary to standby

On Mon, Feb 19, 2024 at 5:32 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> Few comments on 0001

Thanks for the feedback.

> ====================
> 1. I think it is better to error out when the valid GUC or option is
> not set in ensure_valid_slotsync_params() and
> ensure_valid_remote_info() instead of waiting. And we shouldn't start
> the worker in the first place if not all required GUCs are set. This
> will additionally simplify the code a bit.

Sure, removed 'ensure' functions. Moved the validation checks to the
postmaster before starting the slot sync worker.

> 2.
> +typedef struct SlotSyncWorkerCtxStruct
> {
> - /* prevents concurrent slot syncs to avoid slot overwrites */
> + pid_t pid;
> + bool stopSignaled;
> bool syncing;
> + time_t last_start_time;
> slock_t mutex;
> -} SlotSyncCtxStruct;
> +} SlotSyncWorkerCtxStruct;
>
> I think we don't need to change the name of this struct as this can be
> used both by the worker and the backend. We can probably add the
> comment to indicate that all the fields except 'syncing' are used by
> slotsync worker.

Modified.

> 3. Similar to above, function names like SlotSyncShmemInit() shouldn't
> be changed to SlotSyncWorkerShmemInit().

Modified.

> 4.
> +ReplSlotSyncWorkerMain(int argc, char *argv[])
> {
> ...
> + on_shmem_exit(slotsync_worker_onexit, (Datum) 0);
> ...
> + before_shmem_exit(slotsync_failure_callback, PointerGetDatum(wrconn));
> ...
> }
>
> Do we need two separate callbacks? Can't we have just one (say
> slotsync_failure_callback) that cleans additional things in case of
> slotsync worker? And, if we need both those callbacks then please add
> some comments for both and why one is before_shmem_exit() and the
> other is on_shmem_exit().

I think we can merge these now. Earlier 'on_shmem_exit' was needed to
avoid race-condition between startup and slot sync worker process to
drop 'i' slots on promotion. Now we do not have any such scenario.
But I need some time to analyze it well. Will do it in the next
version.

> In addition to the above, I have made a few changes in the comments
> and code (cosmetic code changes). Please include those in the next
> version if you find them okay. You need to rename .txt file to remove
> .txt and apply atop v90-0001*.

Sure, included these.

Please find the patch001 attached. I will rebase the rest of the
patches and post them tomorrow.

thanks
Shveta