Last week I wrote about blog monetization through the Amazon Affiliate Program. I needed a way to serve country-specific URLs depending on the location of the page visitor, so I wrote a geo-aware link shortener using Play Framework. This week I would like to introduce that application. The source code is available on GitHub. You may find that tool useful for your own purposes, or you may just want to read this as a tutorial on how to call backend services with Play Framework and the asynchronous WS client.
Let us define the purpose of this application: visitors (for example on a blog) are to be redirected to country-specific banners, slideshows of or plain links to the matching Amazon store front of the country of origin of the request or, if none exists in the visitor’s country, of the U.S. store. Links should be shortened as well. Let’s look at an example.
The link for
http://r.matthiasnehlsen.com/amazon-landing/link is requested. Then, depending on the visitor’s country, the following happens:
1) Request from the U.S.: the request is redirected to amazon.com
2) Request from the UK: the request is redirected to amazon.co.uk
3) Requests from countries that do not have an Amazon store (or where I have not created an account, such as India, Brazil, China, Japan) are redirected to amazon.com as well.
You can try this for yourself by following this link. When you click it, you should be directed to your country’s own store if you’re from the United States, Canada, the United Kingdom, France, Germany, Spain, or Italy, or otherwise to the U.S..
Now building this application was surprisingly simple with Play Framework. The relevant code easily fits on a single printed page. Before we get into the details, we will need a backend service for the actual lookup of the requesting IP address. One such open source service for that works well is already available: freegeoip. Freegeoip also runs as a free online service, but I would rather run this myself as that will give me a much faster and more predictable response time when there are only local requests. In fact, doing the GeoIP lookup locally only requires a consistent, single-digit number of milliseconds.
All you really need to do is follow the instructions here. That worked well for me both on my development Mac and on my Ubuntu server, with a slight change in the upstart script on Ubuntu, which I have committed to the project as a pull request.
Building the link shortener with Play Framework
We will need to use three building blocks of Play applications: the WS client, async controller actions and JSON parsing. A client requests a resource, which is handled by an async action. Inside this action, the WS client performs a GeoIP lookup by calling the local freegeoip service. The result of this async WS call, which is JSON, is then parsed for the country code matching the request. Then the model is asked for the URL matching the requested resource and country. We will look at the source code below, but here is a flowchart first:
I hope this flowchart helps a little in following through the source code below.
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That is really all, including all the imports and four calls to a request logger. The actual code is a mere 23 lines long, including error handling. Let’s go through this line by line. redirect is a controller method that takes two parameters, shortUrl and format, both of which are strings. They come from the route definition:
The above configuration means that the application will call controllers.Application.redirect with the two strings it parses out of the request’s path.
The controller action is built by Play’s ActionBuilder by calling Action.async with a block that takes a request of type play.api.mvc.Request and returns the future of a result. Next, we construct a fallback link to be delivered when the following GeoIP lookup fails in one way or another.
Next we fire up an asynchronous call to the local freegeoip instance for the IP address derived from the request. I only give this a timeout of 100 milliseconds as I do not want to hold up page loading for longer than that, no matter what. In reality, this is plenty for this local lookup if things are running smoothly and I usually only measure around 7 milliseconds to fulfill this request end to end.
First, we create the fallbackUrl which will either be the specific link to the U.S. store or a default URL if the map lookup with the shortUrl and format provided is unsuccessful. This is either the corresponding link to the shortUrl for the U.S. store, should that exist in the model, or a default URL (also specified in the model).
Next, we create a WsRequestHolder by specifying the URL, setting the timeout. Then we call get() on the request holder and map on it by providing a function to apply to a successful result of the future. In here, the result could either be a status code of 200 or something else, for example a 400 if the request failed. Such a failure can occur if freeogeoip is called locally from an IPv6 address, as is the case on my Mac. If the result is a 200, we expect the result body to contain valid JSON, so we parse it for the country code. This parsing step returns the Option of a string, depending on whether it has found the specified JSON property or not. We flatMap with a function that itself returns an Option of a URL string by taking the parsed country code string and looking that up in the Map of the model. If such an entry exists, the Map lookup will return Some(urlString), otherwise it will return None. Using flatMap now returns a single option instead of having to map on two options. Then we call getOrElse on the resulting Option, retrieving either the lookup result or the fallbackUrl. When encountering any result code other than 200, we redirect to the fallbackUrl.
Now there is a second failure scenario where not the result code indicates what went wrong, but instead the WS call fails altogether. That case would result in a failed future, which we can catch using recover, again redirecting to the fallbackUrl. To understand what is going on here, it is important to realize that both the map combinator on the WS result and the recover combinator return new futures, both of the SimpleResult type. This article, which I found in the Scala documentation, helped me to understand futures better. We could have used pattern matching on specific types of exceptions, but that wasn’t necessary in this case as we simply want to return a redirect to the fallbackUrl (plus log the exception).
Right now all the data lives in the model’s source code. Obviously it would be better to utilize a database for this purpose, but for a first version, this serves us fine. The model is really just a Map. Here’s a shortened version as an example:
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Building this geo-aware link shortener (in a very basic form) was really easy with Play Framework. I am currently using this tool for the lookup of Amazon store fronts to redirect affiliate links to the country of the visitor. But there is no reason why this can’t be used for all kinds of other scenarios where such a country-specific redirection of requests might be useful. Of course, it is not ideal to store the links in code. Instead, that data should live in a database of some kind, probably with the stored values cached inside the application, to avoid having to introduce additional round-trips for every lookup. Placing the data in a model object is already the first step towards building it. The redirecting controller would not have to change at all when this model object is replaced with one that uses a database. Then it would also be really useful to create new links from a user interface. That could, for example, easily be achieved with an AngularJS application for link maintenance. There’s surely stuff for me to do. Please let me know if you think this more elaborate version I just described (database, UI) is useful, either to use it yourself or to follow a tutorial in which we build this application. The more successful this application turns out to be in terms of contributing to this blog, the more time I will find to work on these improvements. You can literally vote with your wallet by clicking on links delivered by the described application.
Please let me know your thoughts on the above. I will be happy to clarify anything that is difficult to grasp right away. And please let me know if you encounter problems making any of this work for yourself.
That’s it for today - hope to see you back soon. And now that you have made it this far in the article, why don’t you follow me on Twitter @matthiasnehlsen so you’ll know when the next article is out.