On Tue, Jun 18, 2024 at 3:29 PM shveta malik <shveta.ma...@gmail.com> wrote: > On Tue, Jun 18, 2024 at 11:34 AM Dilip Kumar <dilipbal...@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
