I have a test application running at test.rsscloud.io on ports 80 and 9876.
The following code works:
curl --location --request POST 'https://brokenriverbooks.com/?rsscloud=notify' \
--header 'Content-Type: application/x-www-form-urlencoded' \
--data-urlencode 'domain=test.rsscloud.io' \
--data-urlencode 'port=80' \
--data-urlencode 'path=/feedupdated-s8759' \
--data-urlencode 'registerProcedure=' \
--data-urlencode 'protocol=http-post' \
--data-urlencode 'url1=https://brokenriverbooks.com/feed/'
However if you change the port to 9876 it fails. For some reason the rssCloud plugin seems unable to hit ports that are not 80.
@josephscott I am not sure what you mean by "RSS blog/widget". My understanding is that WordPress uses SimplePie to generate the RSS feed of a WordPress website/blog. It appears to be common knowledge that WordPress itself caches the RSS feed of the site (see https://amandagiles.com/blog/code-snippets/updating-wordpress-rss-feed-cache-time/). To me, this is a problem for using rssCloud with WordPress, since most feed readers will not display new updates from the site if the feed it retrieves has not changed, even if it receives a response from an rssCloud server that the site has updated. In my opinion, for rssCloud use, the caching should be turned off or reduced to 5-10 seconds.
Here is my anecdotal evidence supporting this: I made posts on my WordPress.org site using version 0.5.0 of the RSS Cloud plugin. I made 3 posts and received three responses from the plugin (see this gist). However, the posts did not appear in FeedLand. I checked the URL of the feed in my browser (https://rsscloud.andysylvester.com/feed/), it was updated. I then did a curl command of the feed, and it had not updated. I then went to the Dashboard for my site, went to Settings/Readings, and changed the value of "Syndication feeds show the most recent x items" from 10 to 5. After doing that, the curl command returned the feed I was seeing in my browser, and eventually the posts showed up in FeedLand.
Following up on your last sentence, I think that the RSS Cloud plugin should trigger an invalidation of the cached feed so that feed readers will get the most recent version of the feed. Otherwise, the RSS Cloud plugin notification does not provide any benefit.