On Fri, Jul 17, 2015 at 6:53 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Fri, Jul 17, 2015 at 8:31 AM, Florent Guiliani <flor...@guiliani.fr> wrote: >> A pg_export_snapshot_for_slot(...) would work very well. >> >> Let me explain the use case. You have many downstream systems that are >> replicated with logical decoding. Using a dedicated replication slot >> for each target is not practical. A single logical replication slot is >> configured. It generates a stream of LSN-stamped transactions in >> commit order. Those transactions are published to all downstream >> nodes. >> >> The snapshot exported during the slot creation can be used to generate >> a complete dump that the replicated systems will load before applying >> the transaction stream. >> >> How do you individually reload/recover one of the downstream node? You >> can use the initial dump and reapply all transactions emitted since >> the slot's inception. It will quickly become impractical. What you >> need is to generate a newer dump and only apply the transactions from >> that point. > > I'd like to point out that I told Andres repeatedly during the > development of logical decoding that this exact thing was going to be > a problem. He told me fixing it was way too complicated, but I hope > he'll relent, because I still think this is important. > > (Not trying to be a jerk here.)
As a quick solution, I added a replication command named LOGICAL_DECODING_SNAPSHOT that does everything CREATE_REPLICATION_SLOT does for logical slots except that the slot is automatically dropped ( see attached patch or https://github.com/flyerman/postgres/compare/9.5...flyerman:export-logical-snapshot ). Feel free to review and comment my patch. I'd like to provide a better solution but I'd need some guidance on how to get the locking right between the snapshot creation and the LSN retrieval. -- Florent
logical-decoding-snapshot.patch
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