I'd like to submit the replication solution which is based on the pglogical_output [1] module (which is obviously needed for this to compile).

The pglogical contrib module provides extension which does the master-slave logical replication based on the logical decoding.

The basic documentation is in README.md, I didn't bother making sgml docs yet since I expect that there will be ongoing changes happening and it's easier for me to update the markdown docs than sgml. I will do the conversion once we start approaching committable state.

What it implements
- logical replication
- partial replication (replication sets)
- multiple sources for single subscriber
- origin filtering (so that if replication is setup both ways, there is no cyclic replication)

It currently doesn't do multi-master or automatic DDL. I think DDL should be relatively easy if somebody finishes the deparse extension as the infrastructure for replicating arbitrary commands is present in this patch.

It's rather large patch so I will just go very briefly over high level overview of how it works, the details need to be discussed separately IMHO:
 - node - stores information about "nodes" (postgresql databases)
- node_interface - represents connection string(s) to nodes, we separate interfaces to different catalog mainly to allow for setups where different subscribers see different address of the provider server - local_node - stores exactly one tuple which points to the nodes catalog tuple that represents the local node of the current database - subscription - represents subscription between two nodes, it includes configuration of the subscription like replication set and origin filters

- basically just implements the pglogical_output hooks according to the catalogs

 - several background workers
 - supervisor is worker which manages all the other workers
- manager is per database worker which manages individual database (node) and its subscriptions - apply does the actual replication, one apply process is started per subscription, connects to the walsender on the other side and applies the changes received from there

[1] https://commitfest.postgresql.org/8/418/

 Petr Jelinek                  http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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