On 12/19/2011 08:17 AM, Robert Haas wrote:
If you want keepalives, why use log shipping rather than SR? Implementing a really-high-latency method of passing protocol messages through the archive seems like a complex solution to a non-problem
The problem being addressed is "how can people using archiving compute time-based lag usefully?" Thinking about an answer to that question that made sense for SR drove us toward keepalive timestamp sharing. This is trying to introduce a mechanism good enough to do the same thing for regular archive recovery.
In the archiving case, the worst case waiting to trip you up is always the one where not enough activity happened to generate a new WAL file yet. If people want lag to move correctly in that case anyway, a message needs to be transferred from archiver to recovery system. Simon is suggesting that we do that via shipping a new small file in that case, rather than trying to muck with putting it into the WAL data or something like that. It's a bit hackish, but a) no more hackish than people are used to for PITR, and b) in a way that avoids touching database code in the critical path for SR.
This idea might eliminate the last of the reasons I was speculating on for adding more timestamps into the WAL stream.
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