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Edge Functions

What is an Edge Function?

An Edge Function is a piece of code that is serverlessly deployed on the Edge

Okay, that doesn't mean anything to me. What do you mean 'serverlessly?' What is the 'Edge?' I'm hungry!

The edge is a term we software engineers like to use to make a simple subject sound mysterious

It just means that when you visit a webpage, the information is sent to your browser from somewhere geographically close to you. That place that the information is sent from, the closest possible place to you where the information is stored, is what we call the edge

The real reason we give it a special name is that it's kind of a complicated concept when we look behind the curtain

Let's think of the World Wide Web as, well, a big web that covers the whole world. At various points on this web there are places that store information. At these places there are little helpers that bring information to you, the user (similar to how a server at a restaurant brings food to you). We call these helpers 'servers'

Now, a 'serverless' website doesn't actually mean that there's no server involved. The information (or food) still has to get to you somehow. But as the builder of a serverless website, I don't put the server in my own home and maintain it myself. Instead, I pay a company like Netlify or Vercel to do the serving for me

A food delivery analogy

This serverless concept is similar to a situation in which I open a restaurant chain without public dining areas and hire out a delivery service to bring the food to my customers' homes

This restaurant and delivery service partnership works well for me because I don't have to manage payroll and health insurance for my servers or try to predict at which times I'll need more or less staff. Instead, the delivery service takes care of their servers and I pay them only when I need to deliver food

The difficulty comes when the delivery service only has a few depots which they can dispatch servers from and they don't know which depot to use. If the server drives all the way from Seattle to my restaurant in Los Angeles only to deliver to someone a few blocks away, that leaves a lot of time for a delicious dinner to grow cold

But what if the delivery company had a depot in Los Angeles and said, "Hey, this restaurant is in LA, why don't we send someone from our LA depot to deliver it?" This is essentially the idea behind edge functions

With edge functions, I still let another company manage my servers, but that company makes sure that the server they're sending the information from is the closest one to the user visiting my website. This may sound like the obvious thing to do, but it's harder to pull off than it sounds, which is why it's exciting that edge functions are now available to developers like us!

Why would I want to use Edge Functions?

Edge Functions are Fast

Since data doesn't have to travel as far to get to users, edge functions can deliver content in the blink of an eye!


Edge functions can be used as middleware. Simply put, middleware receives a signal then makes some changes to the data in that signal before passing it along to its next destination

You might be familiar with middleware through working with Express.js in server-side code. With Express, a server receives a request, performs some operations, and then returns a response. All the code that performs the operations in the middle of that process, between when a request is received and when a response is returned, is called middleware

You can use middleware edge functions to break up a large application into smaller modules that work together. One popular use case here is authentication. If you can move your authentication logic to an edge function, authorizing your users will be quicker, your main servers will do less work, and your code may also get more organized. The same can be said for caching when using an edge-enabled key-value data store like Deno KV or Cloudflare Workers KV

Location-Based Personalization

Since edge functions can be used as middleware at a location geographically close to your users, they can do quite a lot to personalize a user's experience!

One obvious example is language localization. Suppose I have a website based in France, but I have users all over Europe so my website is simultaneously provided in multiple languages. Although my users can request a specific language manually or via browser/OS settings, if no specific language is requested the site is rendered in French. However, I can write an edge function that serves my website in a different language depending on which country it is accessed from

This way, users in Greece get my website in Greek, users in Finland get my site in Finnish, users in Malta get it in Maltese, and so on

Furthermore, if I have caching set up as described above, Turkish-language assets would be cached for users in Turkey, allowing them to access it very quickly

Let's take another example: sales tax. Suppose I have an eCommerce site hosted in the United States. Sales tax varies widely across the US and it can be a pain to calculate. With edge functions I can automatically show the correct sales tax for my customers, whether they're in Washington or Virginia, and calculate a final price without breaking the flow that leads a potential customer to a purchase

How can I use Edge Functions?

Luckily, using edge functions is pretty simple, because most of the setup is done by a hosting provider like Vercel or AWS. The tricky part here is making sure that your program will run on the edge. There are limitations to what you can run, and each hosting providers' limitations are slightly different

One thing you'll have to consider when using edge functions is the size of your program. Each hosting provider sets a limit on the maximum size of code they will host in an edge function. A common maximum is 50MB, but in some cases it may be as small as 10KB!

Now, 50MB may seem huge, but keep in mind that this includes all your program's dependencies, including the famously massive node_modules folder. There are of course ways to mitigate this limitation, such as breaking up your project into smaller parts that can each be hosted on their own edge function, but it's still an important factor to be aware of

In addition, there will be a limit on how long your function can run before being timed out. Again, this will be set by the hosting provider - it could be as long as 30 seconds or less than a millisecond

Last but not least, most providers use a custom runtime environment (instead of Node.js) to execute your code. Common consequences of this include requiring the use of ES modules and not providing access to some browser and/or Node APIs

Keep in mind that these are only a selection of the limitations placed on edge functions. For full info on relevant limitations, please contact your hosting provider

Why are Edge Functions exciting?

One cool thing about edge functions is that they help make the web accessible to users all over the world!

Living in North America, we get used to webpages loading quickly and not waiting on long response times. After just a few seconds we often get restless and start complaining or switching to a different tab

We're allowed this luxury because over 35% of all websites are hosted in the United States. Internet users in South Africa, for example, are more used to longer wait times since only about 0.5% of websites are hosted in their country and the nearest country to them with a larger piece of the web hosting pie (Brazil, with a 1.1% share) is thousands of miles away across the Atlantic Ocean!

Since edge functions bring data from a source physically close to the user, developers can now put faster speeds and more relevant content into the hands of users in South Africa and all over the world!

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