Friday, 23 December 2016
For me, Christmas and Jabber/XMPP go together. I started being involved with the Jabber community around the end of 2000. One of the first things that I built was a bot that recorded the availability presence of my online friends, and show this on a Christmas tree. Every light in the tree represents one contact, and if the user is offline, the light is darkened.As we are nearing Christmas, I put the tree up on the frontpage again, as many years before.
Over the years, the tooltips gained insight in User Moods and Tunes, first over regular Publish-Subscribe, later enhanced with the Personal Eventing Protocol. A few years later, Jingle was born, and in 2009, stpeter wrote a great specification that solidifies the relationship between Christmas and Jabber/XMPP.
Many things have changed in those 16 years. I've changed jobs quite a few times, most recently switching from the Mailgun team at Rackspace, to an exciting new job at VimpelCom as Chat Expert last April, working on Veon (more on that later). The instant messaging landscape has changed quite a bit, too. While we, unfortunately, still have a lot of different incompatible systems, a lot of progress has been made as well.
XMPP's story is long from over, and as such I am happy and honored to serve as Chair of the XMPP Standards Foundation since last month. As every year, my current focus is making another success of the XMPP Summit and our presence with the Realtime Lounge and Devroom at FOSDEM in Brussels in February. This is always the highlight of the year, with many XMPP enthousiasts, as well as our friends of the wider Realtime Communications, showing and discussing everything they are working on, ranging from protocol discussions to WebRTC and IoT applications.
Like last year, one of the topics that really excite me is the specification known as Mediated Information eXchange (MIX). MIX takes the good parts of the Multi User Chat (MUC) protocol, that has been the basis of group chat in XMPP for quite a while, redesigned on top of XMPP Publish-Subscribe. Modern commercial messaging systems, for business use (e.g. Slack and HipChat), as well as for general use (e.g. WhatsApp, WeChat, Google's offerings), have tried various approaches on the ancient model of multi-part text exchange, adding multi-media and other information sources, e.g. using integrations, bots, and cards.
MIX is the community's attempt to provide a building block that goes beyond the tradional approach of a single stream of information (presence and messages) to a collection of orthogonal information streams in the same context. A room participant can select (manually or automatically by the user agent) which information streams are of interest at that time. E.g. for mobile use or with many participants, exchanging the presence information of all participants can be unneeded or even expensive (in terms of bandwidth or battery use). In MIX, presence is available as a separate stream of information that can be disabled.
Another example is Slack's integrations. You can add streams of information (Tweets, continuous integration build results, or pull requests) to any channel. However, all participants have no choice to receive the resulting messages, intermixed with discussion. The client's notification system doesn't make any distinction between the two, so you either suffer getting alerts for every build, or mute the channel and possibly miss interesting discussion. The way around it is to have separate channels for notifications and discussion, possibly muting the former.
Using MIX, however, a client can be smarter about this. It can offer the user different ways to consume these information streams. E.g. notifications on your builds could be in a side bar. Tweets can be disabled, or modeled as a ticker. And it can be different depending on which of the (concurrent) clients you are connected with. E.g. the desktop or browser-based client has more screen real-estate to show such orthogonal information streams at the same time, a mobile client might still show the discussion and notifications interleaved.
All-in-all MIX allows for much richer, multi-modal, and more scalable interactions. Some of the other improvements over MUC include persistent participation in channels (much like IRC bouncers, but integrated), better defined multi-device use (including individual addressing), reconnection, and message archiving. I expect the discussions at the XMPP Summit to tie the loose ends as a prelude to initial implementations.
I am sure that FOSDEM and the XMPP Summit will have many more exciting topics, so I hope to see you there. Until then, Jabber on!
Thursday, 10 October 2013
Logitech is my brand of choice for input devices. Unfortunately, though, Logitech seems to focus on their unifying receiver for most of their stuff, to the detriment of their Bluetooth offering. Every now and then, they do come out with a nice Bluetooth device, usually targetting ultrabooks or tablets. Last month I stumbled upon the new Logitech Ultrathin Touch Mouse (t630). As usual, it is marketed for Windows compatibility, with Linux officially not supported. They do have a second model targetted to Mac users with the t631, but I suspect the only difference is its color.
Fortunately, this device mostly works fine on my Ubuntu 13.04
laptops. Plural, because this tiny mouse can be set up to pair with two
devices, switchable with a switch on the bottom. The only problem is
reconnect with the device when it has been powered down or switched
back from the other channel. It turns out that Logitech might not be
following standards, and requires repairing every time. In my search
for similar cases, I found a bug report for another device that has had
similar issues, and the solution presented there also works for the
Ultrathin Touch Mouse.
The trick is to tell
gnome-bluetooth to always
send the pincode (
0000, as usual) upon
connecting. For this, it needs an entry in /usr/share/gnome-bluetooth/pin-code-database.xml like this:
<!-- Logitech Ultrathin Touch Mouse --> <device oui="00:1F:20:" name="Ultrathin Touch Mouse" pin="0000"/>
I filed a bug report to have this included by default. After adding the entry, add the mouse as a new input device and it should work as expected.
On to the mouse' features. Besides detecting motion with its bottom laser, the surface is a touch pad that can be depressed as a whole. Pressing in the top left and top right corner will trigger the left and right mouse button events (button 1 and 3). To do a middle-click, you have to press in the center of the touch pad, not at the top middle, as you'd expect. Vertical and horizontal scrolling can be done with swipe gestures, respectively up/down and left/right. This will trigger buttons 4 through 7.
On top of that, there are some additional gestures, which Logitech
has pictured in a handy
overview. First, there is a two-finger left or right swipe
for doing previous and next actions. In X11 this will trigger buttons
8 and 9, and Firefox, for example, will respond to move back and forth
in a tab's history. The other three gestures generate keyboard events,
instead of the usual mouse events. A double-finger double-tap yields a
press and release of the
Super_L key. In Unity this
brings up the dash home by default. Finally there are swipes from the
left edge and from the right edge. The former triggers
Super_L Tab, which switches between the two last used tabs
in Firefox, the latter
XF86TouchpadOff, which doesn't have a default action bound
to it, as far as I can tell. Logitech also mentions the single-finger
double tap, but that doesn't seem to register any event in the input
The mouse can be charged with via its micro-USB connector, also on
the bottom, with a convenient short USB cable in the box. The micro-USB
connector on that cable is also angled so the mouse doesn't have to be
upright when charging. The battery state is reported to the kernel, but
there is another bug in
upower that will make
batteries in bluetooth input devices show up as laptop
Having used the mouse for a few days now, I like it a lot. It is really tiny, but not in a bad way (for me). The two-finger swipe gestures are a bit tricky to get right, but I don't really use them anyway. I also tried hooking it up to my Nexus 7, and that works nicely. All-in-all a great little device, especially while travelling.
Kicking off the revival of this publication, I recently did a guest post on our use of Elasticsearch at Mailgun. Since I have joined the Mailgun team at Rackspace in May, my primary project was to reimplement the Mailgun customer logs so that we can serve billions of searchable events. Head over to the HackerNews page for some additional details.
Wednesday, 2 February 2011
Atom and RSS feeds are typically used to support syndication of
existing works, most commonly weblog entries. They are XML documents
that provide a common representation that can be consumed by feed
readers, unlike the HTML pages for such a work. ActivityStreams is
format for syndicating social activities around the web.
Based on the Atom Syndication Format, it tries to provide a feed for
activities, rather than existing works. This includes the
act of posting a blog entry, but can also express
activities typical for social networking sites, like adding friends,
liking something or affirming an RSVP for an
At Mediamatic Lab, we've recently gave notifications an overhaul. We had some code scattered around for sending notices to users, like when they received a friend request. We wanted to add a bunch of notifications so that people are aware of what happens in the network of sites, with their profile or works they've created. For example, when someone tags a person as being in a picture, it would be nice for that person to get a message about that. We also have a collection of RFID-enabled Interactive Installations that generate XMPP notifications for our backchannel system. I'll come back on this.
Whenever something happens that you want to send a notification for, there are a couple of things that you want to include in the notification: what happened, when it happened, and which persons and/or things are involved. The concepts of ActivityStreams turned out to coincide with how we wanted the notification to work. It abstracts activities in actors, objects and targets, along with a human-readable text to describe each activity.
A verb is an identifier for the kind of activity that has taken
place. A verb takes the form of a URI, much like
attributes in Atom link elements, or properties in RDF. The most basic
post, with the URI
An actor is the (usually) person that has performed the activity. Objects are the main persons or things that the activity was performed upon. For example, when I post a picture, I am the actor, and the picture is the object. A target is an object an action was done to. An example could be the photo album my uploaded picture was posted to. Actors, objects and targets usually have a URI, a title and an object type, similar to RDF classes.
Our new notification system does a couple of things whenever a an activity has taken place. It figures out the verb, actor, object and possibly the target and then creates a notification entry. It then calculates the parties that should get this notification in their inbox. This is usually the actor and the owner of the object. A person's profile is always owned by itself, so when the object is a person, that person would get a notification on things happening to them. When a party is not local (i.e. on another site in the federation), the notification is sent to the other site to be processed there. Each person's inbox is viewable as a stream of activities, much like Jaiku or Facebook, and is also published as an ActivityStreams feed (e.g. ralphm's activities. New notifications can then be processed by other modules.
One of them is the Message module, that sends out e-mails for notifications, according to personal preferences. For now, you can choose what kind of notifications you want to receive an e-mail about, by choosing the verbs you are interested in. Examples currently include: friend requests/confirmations, changes to things you own, people liking, linking to, RSVPing (for events), sharing (to Twitter, Facebook, etc) or commenting on things you own, and people tagging you in a picture.
Another module is the Publish-Subscribe module, that provides XMPP Publish-Subscribe nodes for each person, along with a node for all notifications for that site. This allows for applications that use the stream of activities for a person or the whole site, in near-real-time. An example could be a mobile app to track activity for you and/or your friends, or IM notifications much like Identi.ca or Jaiku.
Another possibility is a backchannel. We developed a backchannel system for events we deploy our RFID-enabled Interactive Installations at. A backchannel feed is aggregated from a configurable set of sources, of which the incoming items are formatted into notifications to be put up on a live stream. Every time someone takes a (group) picture with our ikCam, the image is posted on the backchannel, along with a text listing the people in the picture.
On top of that, we also can include tweets by tracking particular keywords and people. We use (and improved) twitty-twister to interact with Twitter's Streaming API from Twisted. I've recently changed the streaming code of twisty-twitter to consume JSON instead of the deprecated XML (with a bunch of tests), and a way to detect unresponsive connections.
With activities now also available as XMPP notifications, the logical step was to consume these for the backchannel as well. We have an office backchannel on a big screen that tracks Twitter for keywords related to Mediamatic and its events, and the notifications from our interactive installations. It now also includes activity on our sites, and this turns out to be a great way to see everything happening in our sites.
So, did it all go smoothly? No. We found quite some things in the ActivityStreams' concepts in combination with anyMeta and our interactive installations that we didn't expect when we started the project.
One of the big ones was Agents. Our interactive installations have
their own user accounts to take pictures, process votes, etc. These
accounts have special privileges to perform actions like making all
people in an ikCam picture contacts in the network. We also have a
Physical I-like-it button, which is an RFID reader placed next to a
physical object (e.g. a painting) that has a representation in one of
the sites. When reading a tag, it creates a like
relationship between the holder of the RFID tag and the object. When we
just enabled the first enabled the new notifications functionality, a
message popped up on the backchannel:
ikPoll Agent likes
That was quite unexpected but quite logical when we thought about it. ikPoll Agent is the user account for the I-like-it button, that is powered by the same software as our more generic ikPoll installations. We defaulted the actor of an activity to the user account performing the action. Although the agent creates a link from the person to the object, the link was not created on behalf of the physical user. So we needed to introduce the concept of Agents, and have that also stored and communicated along with activities. The same action would now yield an entry with the title 'ralphm likes iTea (via ikPoll Agent).
Another was pictures taken with the ikCam. Besides posting the
image, all actors are tagged in the picture, the picture is optionally
linked to an event and a location. This yields a bunch of
notifications, where we would like to have only one:
a self-portrait. We have started work on compound activities
that would have the enclosed activities linked to it and back, a bit
like the Atom
Threading Extension.. This would allow aggregators like the
backchannel only show the umbrella notification.
A final one was our verb
link. This was supposed
to be a catch-all verb for the activity of creating a semantic link
between two things, of which the predicate didn't already have its own
verb (like friending, liking, etc.). It now looks like having a
notification like 'person A linked to thing B' might need some more
information. An e-mail notification at least has the links to
respective pages, but that doesn't quite work on a backchannel beamed
on a big screen. For now we ignore such notifications for the
backchannel, until we have a better solution. It might be that we need
to include the link's predicate in the notification, or make links
themselves first-class citizens (with their own URI).
Going to FOSDEM and/or the 10th XMPP Summit in Brussels? I'll be talking about this and other topics in my talk on Federating Social Networks on Saturday 5 February.
Thursday, 14 January 2010
Last week, Blaine Cook congratulated me on Idavoll being in Apple Mac OS 10.6 Server, as its Notification Server. I did have contact with Apple's server team ages ago, about them using Idavoll and having added some customizatons, but never knew where it ended up. The list of Open Source projects used in Apple's products confirms the use of Idavoll, and Wokkel, too, as a dependency of Idavoll. Cool!
Idavoll, and thus Notification Server, is a generic XMPP publish-subscribe service, in Python with Twisted. Upon inspection of the code and the differences against the mentioned versions, most of the customizations match those I was already aware of: an SQLite backend, the whitelist node access model and associated member affiliations. The link to Notification Server at the open source list goes nowhere (yet), so I am unsure about the actual license of their additions. I contacted the server team, and will write again if I have more news on this.
At the nice post by Jack Moffitt on Apple's use of XMPP, Kael mentions the presence of more Publish-Subscribe goodness in Calendar Server. This is actually the stuff that uses Notification Server for push notification in iCal. As Jack says, it is truly great to see large corporations like Apple to embrace XMPP like this. I really wish Google Calendar had a similar feature. Now I only get meeting invites through e-mail. Apple's particular use of Publish-Subscribe reminds me of Joe Hildebrand's effort on WebDAV notifications, and I think that there are a lot of applications that could benefit from such push features.
As I touched upon earlier, at Mediamatic Lab, we use XMPP Publish-Subscribe for exchanging things for federation. But we've also built a bunch of interactive installations, most of them dealing with RFID tags we call ikTags. To name two examples, the ikCam takes a (group) picture, uploads it and friends the depicted persons by reading their tags. The ikPoll is a polling station where people can 'vote' on questions with the tag. Typically, there are also publish-subscribe notifications coming out of those interactions, so you can create a live stream of things happening at an event like PICNIC. Combined with the Twitter Streaming API and our own status messages, this creates an entertaining back channel, coincidently powered by Idavoll.
Thursday, 17 December 2009
Two exciting projects I've been recently working on at Mediamatic Lab are two highly connected sites around the Jewish Community in the Netherlands during World War Ⅱ. The first is one of the oldest sites we have made, the Digital Monument. This site contains verified information on all of the Dutch jews that have died during WWⅡ along with their households, documented posessions and known documents and pictures. It is maintained by a team of editors of the Jewish Historical Museum.
The second is a brand new community site, to complement the Monument by allowing anyone to add new information, pictures and stories on people at the Monument.
The Monument is very impressive, as I learned back at the first BarCamp Amsterdam, hosted by Mediamatic. You will know what I mean if you spend a little as five minutes paging through the site. Today, however, I want to talk about the technology behind both sites.
Data Model Changes
The data in the Monument is highly semantic in nature. People are part of households, as head-of-family, spouse, son or daughter. Or some other relation. Households have a location and lists of possessions. Tied to all of these are supporting documents and pictures. In anyMeta, all of these are modeled as things with edges between them with a certain predicate. A typical household would be modelled like this:
For the community site, however, we wanted to have more direct relationships between people: parent-child relations, sibling relations, partner relations and a more generic (extended) family relationship. As the community also has most things of the monument imported, this meant a change in the data model and a subsequent conversion in the monument.
In anyMeta, (almost) everything is a thing. As such, the predicate on an edge between two things is also represented by a thing. This has traditionally been named role. Like all things in an anyMeta site have a resource URI, the resource URI of a role is the predicate's URI. We try to use existing (RDF) vocabularies as much as possible for this.
For relationships between people, we've used the
used in FOAF. So this was the first place to look for the desired new
predicates. However, this vocabulary does not have a property for
expressing a generic extended family relationship. Fortunately,
XFN has the
kin relationship type, along with
Richard Cyganiak described how to express XFN relations in
RDF, so we used that to base our predicates on.
Like RELATIONSHIP, most of the XFN properties are subproperties
foaf:knows property, and have some
hierarchy themselves, too. In anyMeta, we didn't have the concept of
subproperties, yet, so we added a new role for expressing subproperty
relationships between roles, and introduced the concept of
implicit edges. These are edges with a
superpredicate of the explicit edge that is being created. For
xfn:child property is a subproperty
foaf:knows. Whenever an edge between two people
gets created with the child role, another implicit one with the knows
role is added, too.
After conversion and with the implicit edges present, the new data model of the example above looks like this:
The blue arrows are the new, derived edges. A spouse edge is made between those people that respectively have a head-of-family and partner relation to the same household (this can be assumed to be correct for this dataset). For person that have a son or daugther edge to a household, a child edge is made from the head-of-family and partner persons (if any) in that household to this person. We haven't (yet) added derived sibling edges, as this relation depends on the parents of both persons, too.
You can also see gray, dashed edges. These are the implicit edges that follow from the property hierarchy. Another thing to notice, is that the biographies are gone. We put the texts in there directly on the persons and households, instead.
Besides the regular pages of all people, households and other things, you can also use our semantic browser to look at the relationships between things. For example, Mozes and his family can be browsed from here.
Sunday, 19 July 2009
Even before I got to work for Mediamatic Lab, Mediamatic was using Twisted. My friend Andy Smith used it for a bunch of projects around physical objects, usually involving some kind of RF tags. Examples include the Symbolic Table and the Friend Drinking Station. From this grew fizzjik, a Twisted based library that implements support for several kinds of RFID readers, network monitoring and access to online services like Flickr and of course anyMeta.
On the other hand, I have dabbled in Twisted for quite a while now, mostly contributing XMPP support in Twisted Words and through the playground that is known as Wokkel. But why go through all that effort, while there are a several different Python-based XMPP implementations out there? And why does Mediamatic use Twisted? Why do I believe Twisted is awesome?
First of all, we like Python. It is a great little language with
extensive library support (
batteries included), where
everything is an object. Much like in anyMeta. It is a language for
learning to program, to code small utility scripts, but also for entire
But going beyond that, building applications that interact with different network protocols and many connections all at the same time is a different story. Many approach such a challenge by using preemtive threading. Threads are hard. Really hard. And Python has the GIL, allowing the interpreter to only execute byte codes in one thread at a time.
So in comes Twisted. Twisted is a framework for building networked applications in Python, through a concept known as cooperative multitasking. It uses an event loop that hands off processing of events (like incoming data on a socket or a timer going off) to non-blocking functions. Events loops are mostly known from GUI toolkits like GTK, and so Twisted goes even beyond networking by working with such toolkits' event loops, too. As most network protocol implementations only have a synchronous interface (i.e. one that blocks), Twisted includes asynchronous implementations of a long list of network protocols. For the blocking interfaces that come from C libraries, like databases, Twisted provides a way to work with their threads, while keeping all your controlling code in the main thread. Asynchronous programming does take some getting used to, hence Twisted's name.
So how do we use Twisted? Well, a recent application is our recent RFID polling system. It allows people to use their ikTag (or any card or other object with a Mifare tag), tied to their user account on an anyMeta site, to take part in a poll by having their tag read at an RFID reader corresponding to a possible answer. The implementation involves:
Communication with one or more RFID readers (that also have output capabilities for hooking up lamps, for example).
Communication over HTTP to access anyMeta's API to store and process votes.
Network availability monitoring (can we access the network and specifically our anyMeta site?)
Power management monitoring (do we still have power?)
Device monitoring. Are RFID readers plugged in or not? Which device handle are they tied to? Also watch coming or leaving devices.
A (GTK-based) diagnostic GUI
Additionally, we also want to show polling results, so we have a browser talking to a local HTTP server and a listener for XMPP publish-subscribe notifications.
This is quite a list of tasks for something as seemingly simple as a polling stations. But wait: there can be multiple readers tied to a particular poll answer, likely physically apart, a polling question can have maybe 50 answers (depending on the type of poll, like choosing from a collection of keywords) or there could be a lot of questions at one event.
So, back to Twisted. Twisted has HTTP and XMPP protocol support (both client and server-side), can talk to serial devices (like your Arduino board) and DBus (for watching NetworkManager and device events) and provides event loop integration with GTK to also process GUI events and manipulate widgets based on events. Together with Wokkel, it powers the exchange of information in our (and your?) federating social networking sites. In Python. No threads and associated locking. In rediculously small amounts of code. That's why.
Not yet convinced? Add a Manhole to your application server, SSH into it, and get an interactive, syntax highlighted Python prompt with live objects from your application. Yes, really.
I am attending XMPP Summit #7 and part of OSCON 2009, with which it is co-located due the kind folks at O'Reilly. Much like last year, only this time in San José, California. Unlike the European version of the summit last February, we hope to focus more on doing than talking, although there will be plenty of that, of course.
Suggestions were made to do some interoperability testing, along with general hacking sessions. I am bringing my implementation of server-to-server dialback, and a bunch of other protocol implementations in Wokkel to the table. While there are a bunch of other protocol implementations in Python, I think the Twisted approach is so different that I want people to know about the ideas behind it. By introducting them to Twisted through Wokkel should give them at least a glimpse of why I believe Twisted is awesome.
So, nearing the summit I prepared a bunch of examples around the XMPP Ping protocol, as I mentioned before. Additionally I prepared an example echo bot on steroids, which is basically a stand-alone XMPP server that connects to other servers using the server-to-server protocol. It will accept presence subscriptions to any potential account at the configured domain, sending presence and echoing all incoming messages.
Besides the hacking sessions, I planning to discuss publish-subscribe delete-with-redirect, node collections, publish-subscribe in multi-user chats and service discovery meta data. Oh, and we might go on a field trip to discuss Google Wave XMPP-based federation protocols. Then, after the summit, I will hanging out at OSCON until Thursday, for hallway meet-ups on federating social networks with protocols like OpenID, OAuth and technologies like webfinger and pubsubhubbub. I also brought an RFID reader to play with.
Wednesday, 8 July 2009
Today's Wokkel 0.6.2 release is to show case some of the features in the previous 0.6.0 release. Most of the work was part of the things we have been building at Mediamatic Lab as part of a restructuring of how we federate our social networking sites using publish-subscribe.
First of all, I added a preliminary, but functional, implementation of server-to-server support, using the dialback protocol. This complements the router code that went into 0.5.0 and Twisted Words 8.2.0 to make a fully stand-alone XMPP server. Note that it does not implement any client-to-server functionality yet, but this can be added as separate server-side components now.
To show this off, I have created a bunch of examples around the XMPP Ping protocol, for which the protocol implementation itself is also a nice example of how to write XMPP protocol implementations using Twisted Words and Wokkel. Be sure to check out these examples.
The other feature I want to mention is publish-subscribe Resources. They provide an abstraction of (part of) a publish-subscribe service. The protocol parts are handled by Wokkel. This should make it easier to do node-as-code scenarios, by just filling in the blanks of the various methods that are called upon receiving requests from pubsub clients. I'll create some examples for this shortly.
Sunday, 31 May 2009
PubSubHubbub is a protocol and reference implementation for doing publish-subscribe using web hooks, polling in feeds triggered by a ping from the publisher, and POSTing Atom entries to notify subscribers. The notification part is similar to what I've been working on for the publish-subscribe stuff at Mediamatic Lab, where we spiced up Idavoll with an HTTP interface to bridge the gap between XMPP Publish-Subscribe and HTTP speaking entities.
Although I spend a lot of time working on XMPP based publish-subscribe, I understand the reasons for going for a full HTTP-based approach. XMPP can be intimidating for developers of web applications. While the differences between XMPP and HTTP are important (stateful connections, asynchronous processing, etc), the fact that it is different is reason often enough. Hosting facilities don't always offer ways to do XMPP, and there is not nearly enough running code out there to make it easier for people to play with these technologies to spice up their web application with non-IM XMPP functionality. Having platforms like Google App Engine provide sending and handling raw XMPP stanzas as part of the API would surely help.
That said, PubSubHubbub has two separate sides to it, the publishing part and the notification part. There's nothing that prevents a hub to do the publishing part using regular XMPP publish-subscribe. Instead of fetching the Atom Feed over HTTP every time, it could use autodiscovery to find out the publish-subscribe node and upgrade by subscribing to it instead. Similarly, the notification part could send out XMPP notifications. Combined with existing HTTP aggregator, that combination is very similar to how the aggregator for Mimír works.
I'm still not convinced that PubSubHubbub is the answer to the efficient exchange of updates on social objects, but I do think it is a good way to make smaller entities be part of a federation of social networking sites. Likely, we'll see a hybrid approach, to begin with.