This project is a simple WebSub (a.k.a. PubSubHubbub) subscriber that sents off webmentions when it gets an update from the hub.

Automatic sending of webmentions is awkward to integrate with a static site generator like Jekyll for a number of reasons:

• Webmentions have to be sent out after the URL is established and accessible, or else the receiver will drop them. You probably don’t want to be generating your Jekyll site straight into your /var/www/ directory, in case of errors during site generation, so we have a bit of a paradox in when the webmention can be sent out.

• Webmentions should be sent out for every new and every updated post. Unfortunately, there is no easy way to tell which posts are new or updated at site generation (save perhaps by incremental builds). The easiest solution at site generation is to send out webmentions on every post after every site generation. Webmentions are idempotent, so this shouldn’t break receivers, but it’s neither good for site generation times nor neighborly towards other sites.

• Jekyll site generation should work in a clean environment, so caching which webmentions have been sent already won’t necessarily help.

So we need to send webmentions out after site generation, but the only shot we have at knowing which posts we should send webmentions based on is at site generation—and even that’s not guaranteed.

Static sites, though, probably are generating RSS feeds. Along with a WebSub hub, which already has to do minimal caching of old results, we can be pinged whenever there is an update on the site—either a new or an updated post. A really nice hub will only send those items that are new or have updated, but even if not, we are limiting the number of webmentions we are trying to send, and the frequency with which we do it.

This has the nice effect of decoupling webmention sending from site-generation. If something goes wrong with one of them, the other is not affected. The two services can live on separate hosts. Site generation can happen anywhere (even on a localhost), and does not necessarily cause webmentions to be sent (useful for testing).

This is a simple Sinatra web server that acts as a WebSub subscriber. Whenever it receives an HTTP POST to /callback, it parses the body as an RSS or Atom feed, and gets a list of all the posts in the update. For each post in the update, the server makes a request to its permalink, parses the page for absolute links within an h-entry, and then tries to send webmentions to each of those links.

Note: At the moment, the server does not handle any WebSub subscription renewal, nor does it require an HMAC from the hub. It also does webmentions synchronously, which needs to be changed before the code is ready to be deployed.

Copyright © 2017, Patrick M. Niedzielski.

Licensed under the Apache License, Version 2.0 (the “License”); you may not use this file except in compliance with the License. You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an “AS IS” BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.