As far as I understand this subthread the goal is to have a pg_basebackup that internally creates a slot, so it can guarantee that all the required WAL is present till streamed out by -X stream/fetch. The problem with just creating a slot is that it'd "leak" if pg_basebackup is killed, or the connection breaks. The idea with the ephemeral slot is that it'd automatically kept alive until the base backup is complete, but would also automatically be dropped if the base backup fails.
Makes sense?
So this would be needed when creating a standalone backup that would not be persistently connected to the master, yet we want to bring it up as a live/writable server in a single command, and we want to make it easy to script in case our script is killed?
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Simon Riggs http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services