I took the “always on application” and ran through some numbers this evening. I was curious – with the hosting options available today, what would it cost someone to run an “always on” application.

The way I’m thinking about it, GAE probably isn’t the right thing. They don’t easily let you run repeated cron jobs, making an agent style application (i.e. proactive, rather than reactive) not quite a good fit. So what would be? A VM. Sure, you could slice it down farther and get a slicehost style thing – they’re excellent, but many of them have restrictions about “long running processes” – and that is exactly what I want.

So what’s it take to run a VM? There’s an interesting question. The cheapest I found was Rackspace – that offered a 256MB Linux hosting solution for $10/mo. Linode starts at 512MB for $19.95. Of course you need to at least make a vague attempt to compare to an EC2 instance. The always on really bites you there – the cheapest I could get my numbers was around $6/mo (a single micro instance, 1 yr commitment – minimal to no bandwidth).

I have to wonder – how well would something like that be received? Say you could offer a private agent/proxy service for $10/mo. I suspect you could get it cheaper – the compute needed for this kind of service would seem to smaller than the minimum you can purchase as a slice of compute. But people aren’t generally buying like this – at least not today.

I think most folks who have a desire for something like this would be more likely to have a desktop computer at home they could dedicate to leaving on. That’s assuming they have an internet service to their house too – so the real cost is more than the hosted compute, but since people often have it anyway… I’m guessing they’d be more likely to want to just use it.

The closest to paying as you go for an application gets to the Platform as a Service – Google App Engine or Microsoft Azure. Frankly neither of which I particularly want because they both represent a sort of technology lock in. In development now is OpenStack – Rackspace & NASA doing some of that base line commoditization work to drive down the costs of running small VM slices. We’re already on the sweet side of Moore’s Law for the commoditization of compute resources – it’s just a few more years until it’s a complete no brainer.

I’ve been thinking about a problem I’m having with twitter – that I want to see more than I can easily watch unless I’m checking on it every hour or so. This gets especially problematic when I’m gone for a long weekend or *gasp* on vacation and completely off the grid for a week. I don’t want to see it while I’m gone – but I would like to be able to review through what happened after the fact.

I also like using the desktop and iOS applications for watching twitter. They’re generally really good – give me a nice sliding window of news. But they only “go back” so far. The essential problem is that twitter has a limit to the history they’ll let you easily interrogate for your feed. My feed is wide enough that I’m starting to miss some pieces, especially as more of it is coming from around the globe and different time zones.

In thinking about it, we I think I need architecturally is something that’s “always on” on the Internet that can grab and watch my feed. Or, more efficiently, accept a twitter stream as events if they have a pub/sub model that is user specific (I don’t think so, but I didn’t look).

To me, that means leaving a desktop or something on all the time – kind of a pain, really. It’s way more compute than this little task needs. And my head is into cloud computing and various infrastructures – so what would it take to have something running – application as a service if you will – that I could cobble to do this for me? I think it would be pretty easy to arrange this with Google App Engine – and that feels roughly about right. A platform as a service engine – I can write and lay down the code on top of it, and then access it from the web through a browser or specifically out of an iOS application. I suppose someone could do this on a slicehost or other thin virtualization platform as well. This could all go into a dedicated JEOS VM and run on Amazon too.

What we’re down to is what’s it cost to run this sort of thing on a regular basis. Always on applications – can you do it cheaper “in the cloud” than you can at home?

And this whole thing is getting darned close to the concept of a personal agent – dealing with (aggregating, filtering) higher speed feeds on the internet than I want to dedicate my attention to. If you go beyond twitter, there’s a whole pile of social networking feeds that I review and look at with a variety of tools today – Netnewswire, browser, other dedicated applications… but all about consuming, filtering, and aggregating information I’m interested in. I get a heady feeling looking forward at how we might use it all.