On 09/20/2013 06:33 AM, Andres Freund wrote:
Hi,
The points I find daunting are the semantics, like: * How do we control whether a standby is allowed prevent WAL file removal. What if archiving is configured? * How do we control whether a standby is allowed to peg xmin? * How long do we peg an xmin/wal file removal if the standby is gone * How does the userinterface look to remove a slot if a standby is gone * How do we decide/control which commands use a slot in which cases?
I think we are going to want to be flexible enough to support users with a couple of different points of use-cases. * Some people will want to keep xmin pegged and prevent WAL removal so a standby with a slot can always catch up, and wi * Most people will want to say keep X megabytes of WA (if needed by a behind slot) and keep xmin pegged so that the WAL can be consumed by a logical plugin.
I can see us also implementing a restore_command that the walsender could use to get archived segments but for logical replication xmin would still need to be low enough
I don't think the current patch set is incompatible with us later implementing any of the above. I'd rather see us focus on getting the core functionality committed and worry about a good interface for managing slots later.
Greetings, Andres Freund
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