Yesterday I wrote about Web Hooks and how powerful it could be if one web service sends HTTP requests to another web service. Today I want to take that concept one step further. What if you tell that service that you would like it to send a POST request back to you, whenever an event happens? This slight modification makes for a very powerful tool.
Let's take the example of popular real-time web applications like Facebook's instant messenger or FriendFeed's "Real-time" view. Both of these services make use of a technique called long polling, where the client sends an HTTP request and the server does not respond until it has some event to deliver. The client can only keep the request open for so long, so it periodically times out and re-sends the request. (It also re-sends the request if it does receive some data).
The problem with this technique is that it's really trying to turn a client into a server. It's really fighting against the way that HTTP wants to work. So why fight it? Imagine that all of our browsers have simple, lightweight, HTTP servers installed. The client could request to upgrade to reverse HTTP, and then the server could initiate a connection with the client. Now, as events come in to the web service, the service could directly send those updates to the client.
Going back to the example of Facebook IM, here's how that would work: When I open a Facebook page, my client sends a request to Facebook's IM server. Facebook's IM server sends a response with the HTTP/1.1 Upgrade header reading "PTTH/0.9" (funny, huh?). Then, the client knows to accept an HTTP connection from Facebook's IM server. Facebook's IM server then opens that connection with the client, and sends HTTP POSTs every time it receives a new instant message that the client should receive. The client's web browser would have some JavaScript hooks to parse the body of those requests, so that it could update the content of the instant message window on the page.
Isn't this brilliant? It directly meshes with the HTTP protocol, and makes this system which seems like a hack right now, instantly become an elegant solution. I really wish I could take credit for thinking this up, but I did not. My coworker Donovan Preston blew my mind with this a few weeks back. If you're looking for a more visual example of how this might work, or a reference implementation of the protocol in action, check out this wiki page.
A few months back GitHub rolled out its implementation of something that they call "Service Hooks". The idea behind these hooks is that when you commit some new piece of code to GitHub, they want to be able to alert other services that you have committed that code. For example, one of the service hooks is the ability to send a tweet to Twitter, and another of those hooks updates the Lighthouse ticket tracker.
I thought this was a really good idea when they rolled it out, so I did a bit of searching and found out that there is a larger body of work surrounding this idea, and that body of work is called Web Hooks. The central idea behind web hooks is that a user supplies a service that they use with a URL. Then, when that user performs an action on that service, the service agrees to send an HTTP POST request to the user's specified URL, with some information about the action that the user took on the service.
SlideShare has an excellent presentation deck about this idea, which likens it to Unix pipes. That analogy makes a lot of sense if you think about it. With the standard model that most websites follow today, a client can only send requests. This means repeated polling until the client receives the information that it is interested in. With web hooks, however, the service is responsible for passing that action along to the next service. This simple yet powerful mechanism can allow for very advanced systems which interoperate very simply through chaining.
Let's expore a concrete example of what this might look like. A few months back I signed up for a pro account on Flickr, so that I could upload some of the pictures that I had stored on my computer. What I did was to upload some pictures with descriptions, and then I went and posted on Twitter some of the links to those pictures. I also went and added that new Flickr account to FriendFeed so that others could see my pictures as well.
This was all a manual process. If both Flickr and Twitter supported web hooks, I could have simply set up their respective URLs and uploaded my pictures. The process might have happened like this: First, the pictures are uploaded. Then Flickr sends a POST request to Twitter, with the description of the picture and a link to the picture. Twitter sends a POST request to FriendFeed, adding the new item to my FriendFeed lifestream.
You could even write custom scripts to handle the web hooks. For example let's say that I want any tweet with the name 'Kevin' to be sent to my brother's email address. I could add a URL to Twitter linking to a script on my computer which scans the contents of the tweet. If the tweet has the name 'Kevin' in it, it would send an email. If not, it might do nothing.
I think that this concept is very powerful not only in terms of rendering trivial the interoperability between disparate services, but also in terms of simply saving on bandwidth and computing power. Technologies which constantly poll resources hoping for updated content seem silly in comparison to the powerful simplicity that web hooks provide.
There are definitely some drawbacks to a system like this. Firstly, the name: I actually can't think of a worse name for this concept. Web hooks?! Let's come up with something better. All joking aside though, this type of system does face a serious problem when it comes to the question of reliability. If a script receives no POST, it could mean that either no event happened, or that the internet connection went down for a bit, or that the service is down, or any number of other possible things. I think the solution for this is a hybrid model of sparse polling in conjunction with web hooks.
Most of all, this technology just seems so underused. There are ridiculously few people who implement something like this, yet it seems like an undeniably useful service--especially given its relative simplicity to implement. Let's all try to encourage the services that we use on a daily basis to support web hooks, because by doing just that, we can make the web a lot better.
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