On 1 April 2016 at 14:52, Andres Freund <and...@anarazel.de> wrote: > Hi, > > On 2016-04-01 08:46:01 +0200, Andres Freund wrote: > > That's a fundamental misunderstanding on your part (perhaps created by > > imprecise docs). > > > > Speaking of which, did you see the proposed README I sent for > > > src/backend/replication/logical ? > > > > I skimmed it. But given we have a CF full of patches, some submitted > > over a year ago, it seems unfair to spend time on a patch submitted a > > few days ago. > > For that purpos > > WRT design readme, it might be interesting to look at 0009 in > > http://archives.postgresql.org/message-id/20140127162006.GA25670%40awork2.anarazel.de > > That's not up2date obviously, but it still might help. >
Thanks, I've been reading it and the posts it references. Most of it was familiar by this point, but would've been a good reference earlier on. The snapshot builder docs in README.SNAPBUILD.txt are handy and help glue a few separate pieces together better for me, and the invalidations section was brief but informative. The very last point looks interesting, but only really alludes to what's going on: +== Restartable Decoding == + +As we want to generate a consistent stream of changes we need to have the +ability to start from a previously decoded location without waiting possibly +very long to reach consistency. For that reason we dump the current visibility +information to disk whenever we read an xl_running_xacts record. I don't feel like I've grasped this properly yet. I think it's referring to the pg_logical/snapshots/ serialization, the use of which allows us to avoid doing extra work in SnapBuildFindSnapshot(...), but doesn't seem to be crucial for correct function. After all, decoding still restarts at the restart_lsn and feeds relevant xact info into the snapshot builder, accumulates invalidation information, etc. After 9.6 I'd like to go through that, update it, and get it in as a README for logical decoding. It would've done me a lot of good when getting up to speed. -- Craig Ringer http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services
