What an App Shell is About and How Your Project Might Benefit From It - MPS Guide


application shell

The majority of mobile developers can agree most likely that web applications start competing with native ones these days. The arguments in favor of this vision are various: technologies keep going forward, the demand for custom mobile solutions is not in decline, and the web giant Google is persistent in the promotion of progressive web applications (PWA).

Whatever reasons we can find, nothing changes the fact that the PWA paradigm is here to stay. One of the pillars of the PWA strategy is the Application Shell model of web development. The present post is aimed at revealing a basic knowledge of the app shell to help you figure out how this model could contribute to your project. 

What is an App Shell?

The app shell model is one of the ways of building a progressive web application that provides fast and reliable downloading of apps by users. An app shell is a skeleton of the GUI in the form of a basic set of static elements in HTML, CSS, and JavaScript. Assume you have a single-page website with some standard elements such as a header, a footer, and a content section. Remove your dynamic content away and all remaining static elements will give you an idea of what an app shell is in essence.

In other words, an app shell is similar to a bundle of a native application published in an app store. It contains only components necessary to launch an application but content data is absent. It is cached once on the client side to provide an almost immediate launch during follow-up visits. Thus, the preliminary cached HTML appears even without an internet connection while JavaScript-driven content data has to be downloaded from the web. 

PWA and App Shells

In general, PWA is a web application created with certain technologies providing certain effects. Even though the word “progressive” from its definition has quite a relative meaning (progressive in comparison with what?), the idea behind both app shells and progressive web apps is clear and powerful. Neither refactoring nor remodeling is needed to start using the PWA architecture in your projects.

Reliable, fast, and engaging are the three major properties inherent in PWA while its basic structural components are the following:

Thus, it is legitimate to claim that any PWA is reliable, fast, and engaging due to its skeleton - the app shell wrapped with other components. Specs from Google insist on the difference between how a native application looks and feels in comparison with an ordinary web app. And namely, PWA with its properties is aimed at mitigating such a difference.

Let’s see how the app shell helps with meeting this challenge.

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Application Shell Architecture

Almost all major browsers have been supporting PWA since 2018. This is not a big deal from the users’ perspective. But in terms of the inner structure of applications, PWA is a quiet revolution. In fact, browsers start working as some virtual machine that contains and launches progressive web applications, so does Android which is a virtual machine for Android-native applications.

The app shell as a central technology of any PWA is cached on a local device for a reason. Otherwise, the PWA properties would barely be achievable. What do the properties mean in the context of the app shell architecture? 

  1. Reliable. A loaded application launches almost immediately without regard to the status of the internet connection.
  2. Fast. Data transfer over the network goes fast, the UI is smooth and responsive.
  3. Engaging. The user experience makes you feel comfortable to be encouraged to use an app again and again.

In other words, the app shell architecture offers developers the means of achieving such a goal as a rapidly downloadable web app that does not fail even with a poor connection and is capable of running offline. Ideally, an app shell should be able to:

  • download quickly;
  • request as fewer data as possible;
  • use static resources from a local cache;
  • separate content from navigation;
  • extract and display the page content;
  • cache dynamic content (if possible).

Service Workers and App Shells     

If an app shell is the skeleton of a progressive web application, then a Service Worker is its heart. The Service Worker is a proxy layer between frontend and backend through which all browser’s requests run. It has access to a caсhe storage for web resources as well as to an IndexDB for data. Both Server Workers and app shells provide free rein to develop a business logic of any kind.

For example, it is possible to get a query from the browser, check the status of the network, take some data from the storage to process, and send some results back to the browser making the latter “think” that the response comes from a server. The two browser layers - the client’s frontend and a Service Worker enable developers to create full-fledged applications.

In the context of programming, the Service Worker is a JavaScript file available in the HTML page code. It allows developers to determine how to work with frontend queries as well as to arrange any other functionality.

How to Use the App Shell

The very availability of the app shell model does not necessarily mean that everyone needs app shells. It may sound questionable, especially in the light of the clear advantages the technology offers. However, the deeper the understanding of various app-shell advantages you possess the wider the vision of your programming opportunities appears. Let’s recognize some options in the algorithm of why and how to use the app shell.

  1. Performance. Follow-up visits happen immediately. Both static resources and user interface are cached locally not to be downloaded each time. Content can be cached during the first visit as well. That’s great for either the news-feed apps or Instagram-like ones. But it can make no sense for some special business models where the difference between the downloading speed of static elements and dynamic ones can be especially offending to the eye.
  2. Engagement. Fast native-like navigation facilitates user engagement. However, the navigation differs from project to project. It means that the app shell model can be inappropriate for some heavy application in which the navigation options should be as changeable as the dynamic content is.
  3. Data usage. Both programmers and users from well-developed countries are accustomed to seeing data as quite cheap info, if not the free one. This is not so in the entire world, however. The app shell helps with reducing costs in the areas where both data and internet connection are still expensive. But developers should stay prudent with what they are going to cache. No cache storage is bottomless and not every data is worth caching.

So to sum up, the developers who appreciate the app shell architecture should not indulge in their attempts to apply it to every project they meet. Making a wise choice implies an understanding of how the app shell can contribute to one or another project. 

Why App Shell is Worth Implementing into Your Project 

It’s no secret that the speed with which your website is downloaded has a direct impact on the behavior of your users. Such an indicator as a retention rate reflects how satisfied your users are with your software product. They will leave your website most likely if it is too slow. Different studies show different minimum time periods affordable for your main page to render. But in any case, this is a matter of seconds. 

Due to the ability to separate navigation from content, the app shell model can give you those few seconds that make your visitors stay on your page. Moreover, being cached only once the main elements of your home page will appear immediately over a single tap during all subsequent visits. This is exactly what Google talks about when mentioning how native apps look and feel. Namely, speed is worth your time and effort if you want your web app working like a native one. 

Also, the way you develop your web project matters when the app shell model is to be chosen. If the lion share of your code is written manually for whatever reason (there is nothing weird in a JavaScript-shell application) the app shell remains quite optional. But if your project implies using frameworks the app shell architecture is strongly recommended. Besides, documentation of many web-development frameworks contains tutorials on how to use app shells. For example, in the case of an Angular PWA, the app shell model is quite a common practice. 

Hence, there might be a ton of reasons for using the app shell model in your project as well as not fewer arguments against it. But it is clear that an app shell can hardly be a fly in your project’s ointment if you are looking for native-like speed.

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There is no universal formula of success for the diversity of contemporary web projects. But the properties that reflect a truly successful web application remain unchanged: reliable, fast, engaging. The paradigm of progressive web applications with its constituent app shell model offers quite an easy way how to achieve those properties. 

Both the end-users and the owners of a web project can have no idea how the app shell architecture works. But what they all enjoy is a native-like web app that makes them feel comfortable. That’s why prior to developing a web project of any sort it is worth consulting an expert in both PWA and app shells. Contact us today to be certain whether your web project needs to follow the PWA paradigm with app shells. 

PWADecember 27, 2020
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