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

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.

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 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 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.

> 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. 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.

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