Re: Conflict Detection and Resolution - Mailing list pgsql-hackers

On Wed, Jun 19, 2024 at 1:52 PM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> On Tue, Jun 18, 2024 at 3:29 PM shveta malik <shveta.malik@gmail.com> wrote:
> > On Tue, Jun 18, 2024 at 11:34 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> >
> > I tried to work out a few scenarios with this, where the apply worker
> > will wait until its local clock hits 'remote_commit_tts - max_skew
> > permitted'. Please have a look.
> >
> > Let's say, we have a GUC to configure max_clock_skew permitted.
> > Resolver is last_update_wins in both cases.
> > ----------------
> > 1) Case 1: max_clock_skew set to 0 i.e. no tolerance for clock skew.
> >
> > Remote Update with commit_timestamp = 10.20AM.
> > Local clock (which is say 5 min behind) shows = 10.15AM.
> >
> > When remote update arrives at local node, we see that skew is greater
> > than max_clock_skew and thus apply worker waits till local clock hits
> > 'remote's commit_tts - max_clock_skew' i.e. till 10.20 AM. Once the
> > local clock hits 10.20 AM, the worker applies the remote change with
> > commit_tts of 10.20AM. In the meantime (during wait period of apply
> > worker)) if some local update on same row has happened at say 10.18am,
> > that will applied first, which will be later overwritten by above
> > remote change of 10.20AM as remote-change's timestamp appear more
> > latest, even though it has happened earlier than local change.
>
> For the sake of simplicity let's call the change that happened at
> 10:20 AM change-1 and the change that happened at 10:15 as change-2
> and assume we are talking about the synchronous commit only.

Do you mean "the change that happened at 10:18 as change-2"

>
> I think now from an application perspective the change-1 wouldn't have
> caused the change-2 because we delayed applying change-2 on the local
> node

Do you mean "we delayed applying change-1 on the local node."

>which would have delayed the confirmation of the change-1 to the
> application that means we have got the change-2 on the local node
> without the confirmation of change-1 hence change-2 has no causal
> dependency on the change-1.  So it's fine that we perform change-1
> before change-2

Do you mean "So it's fine that we perform change-2 before change-1"

>and the timestamp will also show the same at any other
> node if they receive these 2 changes.
>
> The goal is to ensure that if we define the order where change-2
> happens before change-1, this same order should be visible on all
> other nodes. This will hold true because the commit timestamp of
> change-2 is earlier than that of change-1.

Considering the above corrections as base, I agree with this.

> > 2)  Case 2: max_clock_skew is set to 2min.
> >
> > Remote Update with commit_timestamp=10.20AM
> > Local clock (which is say 5 min behind) = 10.15AM.
> >
> > Now apply worker will notice skew greater than 2min and thus will wait
> > till local clock hits 'remote's commit_tts - max_clock_skew' i.e.
> > 10.18 and will apply the change with commit_tts of 10.20 ( as we
> > always save the origin's commit timestamp into local commit_tts, see
> > RecordTransactionCommit->TransactionTreeSetCommitTsData). Now lets say
> > another local update is triggered at 10.19am, it will be applied
> > locally but it will be ignored on remote node. On the remote node ,
> > the existing change with a timestamp of 10.20 am will win resulting in
> > data divergence.
>
> Let's call the 10:20 AM change as a change-1 and the change that
> happened at 10:19 as change-2
>
> IIUC, although we apply the change-1 at 10:18 AM the commit_ts of that
> commit_ts of that change is 10:20, and the same will be visible to all
> other nodes.  So in conflict resolution still the change-1 happened
> after the change-2 because change-2's commit_ts is 10:19 AM.   Now
> there could be a problem with the causal order because we applied the
> change-1 at 10:18 AM so the application might have gotten confirmation
> at 10:18 AM and the change-2 of the local node may be triggered as a
> result of confirmation of the change-1 that means now change-2 has a
> causal dependency on the change-1 but commit_ts shows change-2
> happened before the change-1 on all the nodes.
>
> So, is this acceptable? I think yes because the user has configured a
> maximum clock skew of 2 minutes, which means the detected order might
> not always align with the causal order for transactions occurring
> within that time frame.

Agree. I had the same thoughts, and wanted to confirm my understanding.

>Generally, the ideal configuration for
> max_clock_skew should be in multiple of the network round trip time.
> Assuming this configuration, we wouldn’t encounter this problem
> because for change-2 to be caused by change-1, the client would need
> to get confirmation of change-1 and then trigger change-2, which would
> take at least 2-3 network round trips.


thanks
Shveta