Re: [HACKERS] Interval for launching the table sync worker

On Wed, Apr 12, 2017 at 11:46 PM, Peter Eisentraut
<peter.eisentraut@2ndquadrant.com> wrote:
> On 4/12/17 00:48, Masahiko Sawada wrote:
>> On Wed, Apr 12, 2017 at 1:28 PM, Peter Eisentraut
>>> Perhaps instead of a global last_start_time, we store a per relation
>>> last_start_time in SubscriptionRelState?
>>
>> I was thinking the same. But a problem is that the list of
>> SubscriptionRelState is refreshed whenever the syncing table state
>> becomes invalid (table_state_valid = false). I guess we need to
>> improve these logic including GetSubscriptionNotReadyRelations().
>
> The table states are invalidated on a syscache callback from
> pg_subscription_rel, which happens roughly speaking when a table
> finishes the initial sync.  So if we're worried about failing tablesync
> workers relaunching to quickly, this would only be a problem if a
> tablesync of another table finishes right in that restart window.  That
> doesn't seem a terrible issue to me.
>

I think the table states are invalidated whenever the table sync
worker starts, because the table sync worker updates its status of
pg_subscription_rel and commits it before starting actual copy. So we
cannot rely on that. I thought we can store last_start_time into
pg_subscription_rel but it might be overkill. I'm now thinking to
change GetSubscriptionNotReadyRealtions so that last_start_time in
SubscriptionRelState is taken over to new list.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center