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

On Wed, Jun 19, 2024 at 2:36 PM shveta malik <shveta.malik@gmail.com> wrote:
>
> 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.

Oops lot of mistakes in the usage of change-1 and change-2, sorry about that.

> > 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"

Right

> >
> > 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."

Right

> >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"

Right

> >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.

+1

> > > 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.

Okay

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com