I have some more info on ttl usage now. I haven't tested every aggregator in
my collection, but I've at least done all the ones currently listed in the
profile (except for MyYahoo! which can't subscribe to anything at my
domain). I figure this should be enough to clarify the situation.
For anyone that wants to double check my results, these are the test feeds I
The first one is just a baseline test with no ttl to check the default
refresh time. The second has a ttl of 15 to test whether aggregators would
treat the ttl as the minimum refresh frequency. The last one has a ttl of
150 to test whether aggregators would treat the ttl as the maximum refresh
For the tests to work the aggregator's default refresh frequency needs to be
greater than 15 and less than 150. For most this isn't a problem, but
Internet Explorer uses 24 hours by default, Opera and Googler Reader use 3
hours, and JetBrains Omea uses 4 hours. For IE and Omea I set the default
refresh time to 60 before testing. For Opera I set the individual refresh
times for each feed as I subscribed. For Google Reader I used a special test
feed with a ttl of 210 (same as the urls above - just change the number on
All of the following aggregators appeared to ignore ttl completely. In other
words, all feeds refreshed at the same default frequency.
Google Reader 2007-05-31
JetBrains Omea Reader 2.2
NewsGator Online 2007-05-31
The next lot of aggregators appeared to treat ttl as the maximum refresh
frequency. The ttl150 feed would refresh every 150 minutes (180 in some
cases, but never more frequently than 150). The ttl15 feed refreshed at the
CITA RSS Aggregator 2.6
Opera 9.02 (8585)
The odd one out was FeedDemon (both version 126.96.36.199 and 188.8.131.52), which
basically treated ttl as a recommended rate. When you subscribe to a feed,
FeedDemon will set the refresh rate for that feed to the current ttl. So the
ttl150 feed's initial refresh rate was set to 150 minutes; the ttl15 rate
was set to 15. Those values could be overridden though - they didn't
actually limit the refresh rate in any way.
It's also worth noting that the ttl15 feed only actually refreshed every 30
minutes, despite being set to 15. I'm not sure if that is a limit of the
FeedDemon trial version, a bug, or there's some other reason for it.
Some other things you should know if you want to run these tests yourself.
GreatNews will sometimes force a refresh just by clicking on a feed to view
the results, so it's best to leave it alone for a day while the results
accumulate. Some aggregators (especially the online ones) will behave
unpredictably for the first refresh or two - sometimes they'll refresh
sooner than expected, sometimes later. Once again, it's best to leave them
for a day and let the results accumulate.
The tests rely on an aggregator supporting etags, or archiving results so
you can see a history of all the refreshes that have happened. There are
some that can do neither. These tests won't work very well for such
Bottom line: it seems most aggregators either ignore ttl or treat it as the
maximum refresh frequency (i.e. the aggregator won't refresh more often than
specified). We have one aggregator that is known to treat ttl as a
recommended refresh frequency. We still do not know of any that treat it as
a minimum refresh frequency.