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+---
+layout: article
+section: Blog
+title: "SPAs, PWAs and SSR"
+author: "Marco Otte-Witte"
+github-handle: marcoow
+twitter-handle: marcoow
+topic: javascript
+---
+
+Single Page Apps, Progressive Web Apps and classic Server side rendered
+websites are often seen as orthogonal approaches to building web apps where
+only one is best suited for a particular project and one has to make a choice
+to go with one of them. In this post we'll explore why that doesn't have to be
+the case, since all 3 approaches can actually be combined in order to achieve
+the best result.
+
+<!--break-->
+
+## Desktop-grade apps
+
+Modern websites are in many cases not really websites anymore, but in fact full
+blown apps with desktop-grade feature sets and user experiences that happen to
+run in a browser as opposed to standalone apps. While the much-loved
+[Spacejam Website](https://www.spacejam.com/archive/spacejam/movie/jam.htm)
+was a pretty standard page in terms of interactivity and design only about 2
+decades ago
+
+![Screenshot of the Spacejam Website](/images/posts/2019-03-16-spas-pwas-and-ssr/spacejam.png)
+
+we can now go to [Google Maps](https://www.google.com/maps), zoom and rotate
+the earth in 3D space, measure distances between arbitrary points and have a
+look at our neighbor's backyard:
+
+![Video of Google Maps](/images/posts/2019-03-16-spas-pwas-and-ssr/maps.gif)
+
+All of this functionality, interactivity and visual appeal comes at a cost
+though, mainly in the the form of JavaScript code. The median size of
+JavaScript used on pages across the Internet is now
+[ca. 400KB on the desktop and ca. 360KB on mobile devices](https://httparchive.org/reports/state-of-javascript),
+an increase of ca. 36% and ca. 50% respectively over the past 3 years. All of
+this JavaScript not only has to be loaded via often spotty connections but also
+parsed, compiled and executed - often on mobile devices that are far less
+powerful than the average desktop or notebook computer (have a look at
+[Addy Osmani's in-depth post on the matter](https://medium.com/@addyosmani/the-cost-of-javascript-in-2018-7d8950fbb5d4)
+for more details).
+
+What all this leads to is that for many of these highly-interactive,
+feature-rich and shiny apps that are being built today, the first impression
+that users get is often this:
+
+![Video of a loading JS app](/images/posts/2019-03-16-spas-pwas-and-ssr/loading.gif)
+
+
+## SPAs
+
+While JavaScript-heavy apps can be slow to start initially, the big benefit of
+the Single Page App approach is that once the application has started up in the
+browser, it is running continuously and handles route changes in the browser so
+that no subsequent page loads are necessary &mdash; SPAs trade a slow initial
+load for fast or often immediate subsequent route changes.
+
+The initial response though that delivers the user's first impression will
+often be either empty or just a basic loading state as shown above. Only after
+the application's JavaScript has been loaded, parsed, compiled and executed,
+can the application start up and render anything meaningful on the screen. This
+results in relatively slow _time to first meaningful paint_ (TTFMP) and _time
+to interactive_ (TTI) metrics.
+
+#### TTFMP: Time to first meaningful paint
+
+This is the time when the browser can first paint any **meaningful** content on
+the screen. While the time to first paint metric simply measures the first time
+**anything** is painted (which would be when the loading spinner is painted in
+the above example), for an SPA the time to first **meaningful** paint only
+occurs once the app has started and the actual UI is painted on the screen.
+
+#### TTI: Time to interactive
+
+This is the time when the app is first usable and able to react to user inputs.
+In the above example of the SPA, time to interactive and time to first
+meaningful paint happen at the same time which is when the app has fully
+started up, has painted the UI on screen and is waiting for user input.
+
+## The App Shell Model
+
+One popular approach for improving the startup experience of JavaScript-heavy
+applications is called the
+[App Shell Model](https://developers.google.com/web/fundamentals/architecture/app-shell).
+The idea behind this concept is that instead of responding to the user's first
+request with an empty HTML document with only some script tags and maybe a
+loading spinner, the server would respond with the minimal set of code that is
+necessary for rendering the app's minimal UI and making it interactive. In most
+cases, that would be the visual framework of the app's main blocks and some
+barebones functionality associated to that (e.g. a working slideout menu
+without the individual menu items actually being functional).
+
+Although this does not improve the app's _TTFMP_ or _TTI_, at least it gives
+the user a first visual impression of what the app will look like once it has
+started up. Of course the app shell can be cached in the browser using a
+service worker so that for subsequent visits it can be served from that
+instantly.
+
+## Back to SSR
+
+The only really effective solution though for solving the problem of the
+meaningless initial UI - be it an empty page, a loading indicator or an app
+shell - is to leverage server-side rendering and respond with the full UI or
+something that's close to it for the initial request.
+
+Of course it wouldn't be advisable to go back to classic server-side rendered
+websites completely, dropping all of the benefits that Single Page Apps come
+with (instance page transitions once the app has started, rich user interfaces
+that would be almost impossible to build with server side rendering, etc.) A
+better approach is to run the same single page app that is shipped to the
+browser on the server side as well as follows:
+
+* the server responds to `GET` requests for all routes the single page app
+  supports
+* once a request comes in, the server constructs an application state from the
+  request path and potentially additional data like a session cookie and
+  injects that into the app
+* it then executes the app and renders the app's UI into a string, leveraging
+  libraries like [SimpleDOM](https://github.com/ember-fastboot/simple-dom) or
+  [jsdom](https://github.com/jsdom/jsdom)
+* that string is then served as the response to the browser's initial request
+* the pre-rendered response still contains all `<script>` tags so that the
+  browser would load and execute these scripts and start up the app in the
+  browser as usual
+
+![Diagram of the approach](/images/posts/2019-03-16-spas-pwas-and-ssr/diagram.png)
+
+With this setup, the _TTFMP_ metric is dramatically improved. Although users
+still have to wait for the app's JavaScript to load and the app to start before
+they are able to use it, instead of only being shown a meaningless UI while
+they wait for that to happen, they see the app's full UI immediately as that is
+served in the response to the initial request. The initial UI can even be
+partly interactive - elements like links or even functional elements like hover
+menus that can be implemented in CSS only will already be usable before the app
+has started up.
+
+## SPA + SSR + PWA
+
+Combining SPAs with classic SSR, we get the best of both worlds - a fast
+_TTFMP_ plus the benefits of an SPA like immediate page transitions, vivid UX
+etc. On top of that, patterns of PWAs can be added, for example caching the
+initial pre-rendered response in a service worker so that it can be shown
+immediately on subsequent visits or showing the app shell from the service worker
+cache and then injecting the SSR response into that which is likely available
+before the app has started up.
+
+SPAs, SSR and PWAs are not orthogonal concepts that are exclusive of each other
+but are actually complementary. SSR can be leveraged for a fast _TTFMP_ and
+some very basic functionality like links etc. Once the JavaScript referenced
+in the SSR response has been loaded, the SPA starts up in the browser, takes
+over the DOM and intercepts all subsequent route changes and handles them
+immediately in the browser. And finally concepts of PWAs like effective
+client-side caching and offline functionality via service workers can be added
+on top for maximal performance and the optimum user experience.
+
+## Deployment
+
+When leveraging SSR for SPAs, it is important to get some aspects of the
+deployment right. Pre-rendering the application for the first request is only
+an improvement over the classic way of serving an SPA and should not be a
+requirement to use the app. Neither should it slow down delivery of the app. To
+make sure these requirements are met, it is important to make sure of two
+things:
+
+* The pre-rendering must run within a timeout so that if for some reason it
+  takes longer than x ms or whatever a reasonable threshold for a particular
+  app might be, the server cancels the pre-renderer and instead serves the SPA
+  the classic way (which is responding with the static, empty HTML file).
+* Likewise, errors in the pre-renderer should not be forwarded to the users and
+  thus block them from booting the app in the browser. Whenever the
+  pre-renderer encounters an error, it should fall back to serving the app the
+  classic SPA way just like if it runs into the timeout.
+
+## Breethe
+
+We implemented the patterns described in this post in
+[Breethe](https://breethe.app), a PWA for accessing air quality data for
+locations around the world that we built as a tech showcase. Breethe is
+completely open source and
+[available on github](https://github.com/simplabs/breethe-client) and we
+encourage everyone interested in the topic to check it out for reference.
+
+![Video of Breethe](/images/posts/2019-03-16-spas-pwas-and-ssr/breethe.gif)
+
+## Drawbacks
+
+Server-side-rendering an SPA comes with a drawback which is added latency. When
+serving an SPA as a static HTML file that contains only a loading state or an
+app shell, the HTML file can be served from a CDN which reduces latency. When
+serving the response for the initial request from a Node server, there will
+always be some additional latency. The Node server will likely not be running
+on an edge note in a CDN, thus connecting to it will take the user slightly
+longer than connecting to an edge node. Also, the server takes some time to run
+the app, capture what it renders and respond with that, adding further latency.
+Thus, the browser will know slightly later which scripts to load and thus start
+loading them slightly later which leads to a longer _TTI_.
+
+However, with average _TTI_ measurements in the range of several seconds for
+many sites in particular when requested from mobile devices, an added latency
+of a few hundred milliseconds might be well worth it in many cases. Without
+SSR, _TTFMP_ is generally equal to _TTI_ for SPAs and PWAs - with SSR, _TTFMP_
+occurs as soon as the initial response is received while _TTI_ is only slightly
+delayed. So while the user needs to wait slightly longer for the app to be
+fully started up, the app's UI (and content) is available pretty much
+immediately. Whether that is a valuable improvement is a case-by-case decision
+of course. In the example of Breethe, when looking at the result page for a
+particular location, it's a pretty obvious decision:
+
+![Breethe - results for Muncich](/images/posts/2019-03-16-spas-pwas-and-ssr/breethe-results.png)
+
+While the user might need to wait for a few seconds (e.g. on a low performance
+mobile device with a spotty network connection) for the app to have fully
+started up and be interactive, the air quality data that they are interested in
+is available immediately.
+
+## Progressive Enhancement
+
+The possibilities of combining SPAs with SSR and PWA mechanics do not end with
+the above described patterns though but we can go a step further. As it turns
+out, leveraging SSR for an SPA enables to also leverage
+**Progressive Enhancement** patterns so that the initial response can be
+functional beyond just links and interactive elements built in CSS. Many things
+we're all doing in JavaScript in SPAs these days have equivalents in pure HTML.
+These might not be as powerful and elaborate as what can be done with modern
+JavaScript but they can serve as a fallback for when the app's JavaScript is
+slow to load or fails to load completely. That way we're coming back full
+circle to a concept from 1 or 2 decades ago where JavaScript enhances the HTML
+document rather than generating it - and all that while we're using an SPA that
+is completely written in JavaScript but that we're also running in the server
+side so that users don't necessarily have to wait until the app runs in their
+browsers.
+
+I will elaborate on how exactly the approach works in the next post of this
+series so stay tuned!
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