Logical decoding on a standby node

Question

PostgreSQL 9.4 introduced Logical Decoding: http://www.postgresql.org/docs/9.4/static/logicaldecoding.html

It also has Cascade Streaming Replication: http://www.postgresql.org/docs/9.4/static/warm-standby.html#CASCADING-REPLICATION

Logical Decoding seems to be based on Streaming Replication. However a PostgreSQL server will refuse to start a Logical replication slot on a hot standby. It will only allow Logical Decoding on the master node.

What prevents a hot standby node to replicate itself in cascade using Logical Decoding?

Answer 1

A patch was submitted for PostgreSQL 10, but didn't get in.

Work continues for PostgreSQL 11.

Clients like pglogical will need modification to support streaming from a standby once the basic functionality is in postgres.

Answer 2

A bit late...

It's essentially just unsupported because nobody had the time to implement it. There's a bunch of complications around reserving the required resources on the primary, and about continuing to correctly work when the standby, or one in a cascade above, is promoted. Nothing impossible, just work.

That kind of question you're probably more likely to get an answer about on the postgresql lists themselves.