The magic of ISR

Felipe,4 min read


ISR stands for incremental static regeneration. It's a technique that I first heard a while ago when reading about the framework Next.js (opens in a new tab) and that I'll talk more about in this post. But first some context.


More acronyms? Unfortunatelly, yes. I'm not a fan of acronyms but these ones appear to be wildely used these days on the frontend tech circles. But don't worry, we'll go through each one of them.

CSR: Client Side Rendering

Starting with the last, CSR, or client side rendering, means that the browser (the client) is responsible for rendering the content of a website. It is normally what you would expect from a React app by default when using create-react-app: when you access a webpage, the server sends javascript to the browser, then the browser parses that javascript, execute it and the get the html to display to you. One of the biggest drawbacks of this approach is that you have to wait a longer time to get to see some content on the page, when compared to the other approaches.

SSR: Server Side Rendering

This approach is older than client side rendering. It's what was done when using php, for instance. When you hit a webpage, the server is responsible for generating the html specific to your request and send it to the browser, so when it receives the response the html is ready to be displayed. Next.js is a framework that brings this approach to React. Interestingly enough, CSR was born to make for a faster experience for the user. I said earlier that CSR takes a longer time to show the first meaningful content to the user. But if it is expected that the user makes lots of page transitions, SSR can feel slower than CSR, because for each navigation between pages, a new request has to be made to the server to fetch the html for said page, so SSR also has its drawbacks.

SSG: Static Site Generation

When you have a website where the content of its pages don't need to change between requests, like this blog post(!), you could generate its html (and js and styles, which I forgot to mention on the other approaches) once and store them somewhere else, like a CDN, which could be closer to where the user visiting the website is, without the need to request the server. The massive advantage of this approach is that it is blazing fast 🔥. The biggest drawback is that, as the names says, it is not dynamic. If I had to make a change on this blog post, I would need to build it again (generate its html, js, css) and update the CDNs with the new version. Luckily, Next.js also supports this technique and does all that work for us.

With all these approaches introduced, we can finally talk about the magic!

The magic

As I mentioned before, this blog is a perfect fit for static site generation (just like many blogs 😬). I don't plan to change the contents of the pages very often and I want everybody who access the posts to see the same content. But yesterday I wanted to do something different. I wanted my photos page to show my photos uploaded to flickr. And I didn't want to rebuild the blog every time I uploaded a new photo to flickr. So I saw the perfect opportunity to use the magic of incremental static regeneration. Without going into too much details (opens in a new tab) (because I don't know much about it), you simply add a revalidate attribute on the return of a page's getStaticProps to turn on ISR.

On my photos page, inside getStaticProps, I made a call to flickr api to get my photos and embed them on the page. Adding the revalidate: 3600 to the return of the method makes Next.js trigger a regeneration of the previously built page on the background when 3600 seconds have passed since the first request to the page, which will fetch any new photo I took without me having to rebuild my blog!


With ISR, activated with a single line of code (opens in a new tab), I can have a piece of dynamic content inside my static website, having the benefits of both worlds. Arthur C. Clarke once said that any sufficiently advanced technology is indistinguishable from magic (opens in a new tab). I like to think that we can learn a new piece of technology and even fully understand it so it doesn't feel that much advanced anymore (which I hope to do with ISR, some day), but still keep that feeling of seeing something magical 🪄.

© Felipe Milani.RSS