Logical Replication - Type messages?

Hello psql-general,

I have an application which processes logical replication output from a
PostgreSQL server. We create publications and read data from a replication
slot with the pgoutput plugin.

Postgres has various message formats for logical replication, as defined
here:
https://www.postgresql.org/docs/10/protocol-logicalrep-message-formats.html

Currently, I process Relation messages to get most of the information I
need to work with Inserts, Updates, and Deletes. However, for type
information I'm currently using a SQL query to get the column type
information. I wanted to use the Type messages, but I never see them. I see
Begin, Commit, etc. but never Type.

- When are the Type messages generated and sent by the server?
- Is there a way to cause them to be generated?
- Are these type id > type mappings set in stone, or does PostgreSQL
allow for a server to have custom types (I notice that namespace is one of
the properties sent with the types)?

Any help is greatly appreciated - thank you!

Best regards,

Stefen Hillman