December 12, 2009
Webhooks. User-defined callbacks on the web. Yes. YES! Wait, what does that even mean? That stupid website doesn’t seem to explain them at all!
Let’s explore this through an analogy. This analogy uses stockbrokers! Yay! (?)
Consider, before the web, before email, how people would interact with their stockbroker: the telephone. Whenever you wanted to manage your investments and find out what’s really going on in the market, you would call your stockbroker. It’s similar to how you might use your browser to visit Twitter to update your status and find out what’s going on with your friends, right? This is the basis of the analogy. Bear with me.
Imagine in this story your phone is a browser, a phone call is a web request, and the broker is a web application, like Twitter.
It’s 1989 and you like to think of yourself as a big time stock trader with an impressive portfolio. Good for you. You got to this point by staying in close touch with your stockbroker. You’d call her (!) to find out what was happening in the market, discuss whether buying certain stock was a good idea, and maybe put in an order. You were able to get information out of the system and put orders into the system by picking up your phone and calling your broker.
The problem is that you can only react to events in the market as quickly as you get the information. This means you’d have to call your broker quite a bit to stay on top of a fast moving, highly volatile market. A programmer might try to automate this with a script. You take a similar approach: you hire an assistant to deal with your stockbroker. Perhaps this is unheard of in reality, but stay with me.
Now an assistant is not unlike a computer program in theory. You can give them instructions and based on certain conditions and input, they’ll do as you say. For example, maybe you instruct your assistant to buy a certain type of stock after it behaves a certain way, and if there’s not enough money to buy the amount that you told them, they can transfer the money from your savings account after a quick confirmation phone call with you. In today’s world, that sort of thing is not terribly out of the question to automate with a script using the web APIs and programmatic infrastructure available to us now.
Again, in order to react to events in the market in a timely manner, it requires lots of phone calls to your stockbroker. Sure, your assistant can handle it, but it’s a lot of work and wastes a lot of time. In fact, it wastes you money because you pay for your assistant by the hour! If you were a programmer, your script would have to constantly poll the broker API requiring a touchy cron setup or a long running process that gets more inefficient the closer to real-time you want it to be. Not to mention it’s just more work than you should have to deal with. If only there was some way for your assistant or script to be notified when things happen so they could simply react.
One day, your stockbroker says they’re providing a new service they describe as callbacks. You give them a phone number, and they’ll call it whenever something interesting happens regarding stocks relevant to you. Wow! Well, you could give them your number, but there are two problems. First, it’s 1989 and you don’t happen to have a cell phone. You can’t always be reached. Second, even if you could, you’d still want your assistant to handle the events because they have been instructed with how to deal with them and will do a lot of the legwork for you. Luckily, your assistant has a cell phone and is always available, so you give the broker their number.
Now your assistant is working very efficiently making you lots of money. Their job is so much easier because they don’t have to do anything until they get a call from either you or, more importantly, the broker. They can finally react to events as they happen, without a lot of nonsense trying to stay on top of things. What’s more is they will automatically take care of whatever situations you’ve told them.
What a cool feature, this callbacks thing. Thanks, broker!
If you haven’t guessed, the callback feature in this story is the exact mechanics of webhooks. Webhooks would allow you to tell the web apps that you use to “callback” scripts you have online. These scripts will deal with whatever events that web app produces. These callbacks use the same protocol you use to talk to them and that you both know how to use: web requests. Putting your script online at a URL is analogous to your assistant that has a cell phone: it’s always available for direct connection at a persistent “phone number,” unlike you, with dynamic IPs and NATs and turning off your computer. And just like an assistant, the script can do a lot of interesting things for you that you wouldn’t necessarily want to do, most of which is a sort glue work of making different systems work together given some logic. For example, transferring money from your savings account to the broker account when there isn’t enough.
A concrete example of a story made possible from webhooks that might be a useful scenario for many of you involves Twitter. Let’s say Twitter supported webhook callbacks for when somebody follows you. Right now you get an email, and from there you can decide what to do manually: follow them back, block them, or do nothing. I used to go out of my way to block users that I knew were spam bots, but now there’s so many it’s not worth the time. And of course I also generally follow back people that I actually know. If Twitter would simply call a script of mine whenever somebody followed me passing along the user ID, I could very easily run this logic in a PHP script or a simple App Engine app. Or perhaps I’d use Scriptlets (ahem, which was made exactly for these kinds of web scripts). It would work like this:
First, use the Twitter API to look up the user from the ID, and grab their name. Then use the Facebook API to check if that name shows up in my list of friends on Facebook. If so, use the Twitter API to follow them back. Otherwise, if they’re following over 1000 users and that number is more than twice the number that’s following them (which is roughly the heuristic I use manually), use the Twitter API to block them. All automatic.
If you could do this, not only would you have glued together Facebook and Twitter in an interesting and useful way, but you’ve sort of extended the net functionality Twitter provides you. You could imagine perhaps someday Twitter releasing features that would do exactly what you just did. But they won’t. In fact, they’ll never release features that are so specific to you (who says my spam algorithm is the algorithm everybody should use?). This is how webhooks make the web more about you. You can start extending web applications and gluing them together just the way you want.
This is a win-win for web application users and developers. Users get more functionality. Developers can implement less.
Coming back to the stockbroker analogy, there is a type of order called a limit order where you say to buy or sell when it’s at a certain price. With webhooks (or the broker “callback” service), this is merely a convenience because you could easily set that up outside of their system. Speaking of that example, when I mentioned your assistant transferring money between accounts, that reminds me, you know that feature banks have called overdraft protection? If banks had webhooks, all banks could have overdraft protection. Wow, right? That would have been nice for me when I had Washington Mutual, a bank known for its lack of overdraft protection.
What else could you do in a world with webhooks? Basically everything would have a common event-driven infrastructure, allowing you, with just a little bit of scripting glue, to accomplish so much more and make the systems you use better, and more personal.