How distfiles mirrors know to fetch new files

[Ryan Schmidt]

We have three distfiles mirrors, in the US, Norway and Sweden:

US: http://distfiles.macports.org/
NO: http://trd.no.distfiles.macports.org/
SE: http://arn.se.distfiles.macports.org/

In /graphviz the US mirror has distfiles up to the current port  
version, graphviz-2.21.20090209.0545.tar.gz.

But Sweden only has files up to graphviz-2.21.20090106.0545.tar.gz.

Norway only has files up to graphviz-2.21.20081216.0545.tar.gz.

I believe the US mirror, which runs right next to the Subversion  
repository, gets updated in a post-commit hook, so as soon as someone  
commits a new version of a port, the mirror fetches the new file.

How do Norway and Sweden get informed that they should fetch new  
files? I fear they're not getting informed at all.