On Fri, Dec 16, 2011 at 3:01 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> archive_command and restore_command describe how to ship WAL files
> to/from an archive.
>
> When there is nothing to ship, we delay sending WAL files. When no WAL
> files, the standby has no information at all.
>
> To provide some form of keepalive on quiet systems the
> archive_keepalive_command provides a generic hook to implement
> keepalives. This is implemented as a separate command to avoid storing
> keepalive messages in the archive, or at least allow overwrites using
> a single filename like "keepalive".
>
> Examples
> archive_keepalive_command = 'arch_cmd keepalive' # sends a file
> called "keepalive" to archive, overwrites allowed
> archive_keepalive_command = 'arch_cmd %f.%t.keepalive #sends a file
> like 000000010000000AB00000000FE.20111216143517.keepalive
>
> If there is no WAL file to send, then we send a keepalive file
> instead. Keepalive is a small file that contains same contents as a
> streaming keepalive message (re: other patch on that).
>
> If no WAL file is available and we are attempting to restore in
> standby_mode, then we execute restore_keepalive_command to see if a
> keepalive file is available. Checks for a file in the specific
> keepalive format and then uses that to update last received info from
> master.
>
> e.g.
> restore_keepalive_command = 'restore_cmd keepalive' # gets a file
> called "keepalive" to archive, overwrites allowed
Patch.
--
Simon Riggs http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
