Single-page application

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A single-page application (SPA), also known as single-page interface (SPI), is a web application or web site that fits on a single web page with the goal of providing a more fluid user experience akin to a desktop application.

In a SPA, either all necessary code – HTML, JavaScript, and CSS – is retrieved with a single page load,[1] or the appropriate resources are dynamically loaded and added to the page as necessary, usually in response to user actions. The page does not reload at any point in the process, nor does control transfer to another page, although modern web technologies (such as those included in HTML5) can provide the perception and navigability of separate logical pages in the application. Interaction with the single page application often involves dynamic communication with the web server behind the scenes.

The term single-page application was coined by Steve Yen in 2005, though the concept was discussed at least as early as 2003[2] and Stuart (stunix) Morris wrote the Self-Contained website at http://slashdotslash.com with the same goals and functions in 2002.[3]

Architectural characteristics

Technical approaches

There are various techniques available that enable the browser to retain a single page even when the application requires server communication.

AJAX

The most prominent technique currently being used is Ajax.[1] Predominantly using the XMLHttpRequest object from JavaScript, other AJAX approaches include using IFRAME or script HTML elements. Popular libraries like jQuery, that normalize AJAX behavior across browsers from different manufacturers, have further popularized the AJAX technique.

Browser plugins

Asynchronous calls to the server may also be achieved using browser plug-in technologies such as Silverlight, Flash, HTML5 Socket or Java applets.

Data transport (XML, JSON and AJAX)

Requests to the server typically result in either raw data (e.g. XML or JSON), or new HTML being returned. In the case where HTML is returned by the server, JavaScript on the client updates a partial area of the DOM (Document Object Model). When raw data is returned, often a client-side JavaScript XML / (XSL) process ( and in case of JSON a template ) is used to translate the raw data into HTML, which is then used to update a partial area of the DOM.

Thin server architecture

An SPA moves logic from the server to the client. This results in the role of the web server evolving into a pure data API or web service. This architectural shift has, in some circles, been coined "Thin Server Architecture" to highlight that complexity has been moved from the server to the client, with the argument that this ultimately reduces overall complexity of the system.[4]

Thick stateful server architecture

The server keeps the necessary state in memory of the client state of the page. In this way, when any request hits the server (usually user actions), the server sends the appropriate HTML and/or JavaScript with the concrete changes to bring the client to the new desired state (usually adding/deleting/updating a part of the client DOM). At the same time the state in server is updated. Most of the logic is executed in server and HTML is usually also rendered in server. In some ways the server simulates a web browser, receiving events and performing delta changes in server state which are automatically propagated to client. This approach needs more server memory and server processing, but the advantage is a simplified development model because a) the application is usually fully coded in server, b) data and UI state in server are shared in the same memory space with no need of custom client/server communication bridges.

Thick stateless server architecture

Is a variant of the stateful server approach, in this case the client page sends the necessary data of the current state to the server usually through AJAX requests, the server with this data is able to reconstruct in some way the client state of the page part going to be modified and generate the necessary data or code, for instance as JavaScript code, returned to the client to bring it to a new state usually modifying the page DOM tree according to the action which motivated the request. The stateless variant of the thick server architecture requires more data sent to server and may require more computational resources per request to reconstruct partially or fully the client page state in server, in the same time this approach is more easily scalable because there is no per client page data kept in server and therefore AJAX requests can be dispatched to different server nodes with no need of session data sharing or server affinity.

Running locally

Some SPAs may be executed from a local file using the file URI scheme. This gives users the ability to download the SPA from a server and run the file from a local storage device, without depending on server connectivity. If such an SPA wants to store and update data, it must be self-modifying. That is, the SPA must be capable of writing itself to disk, including a representation of the state that is to be persisted. These applications benefit from advances available with HTML5, particularly Web Storage.

Challenges with the SPA model

Because the SPA is an evolution away from the stateless page-redraw model that browsers were originally designed for, some new challenges have emerged. Each of these problems has an effective solution[5] with:

  • Client-side JavaScript libraries addressing various issues.
  • Server side web frameworks that specialize in the SPA model.[6][7][8][9]
  • The evolution of browsers and the HTML5 specification aimed at the SPA model.[10]

Search engine optimization

Because of the lack of JavaScript execution on crawlers of all popular Web search engines,[11][12] SEO (Search engine optimization) has historically presented a problem for public facing websites wishing to adopt the SPA model.[13]

Google currently crawls URLs containing hash fragments starting with #!,.[14] This allows the use of hash fragments within the single URL of an SPA. Special behavior must be implemented by the SPA site to allow extraction of relevant metadata by the search engine's crawler. For search engines that do not support this URL hash scheme, the hashed URLs of the SPA remain invisible.

Alternatively, applications may render the first page load on the server and subsequent page updates on the client. This is traditionally difficult, because the rendering code might need to be written in a different language or framework on the server and in the client. Using logic-less templates, cross-compiling from one language to another, or using the same language on the server and the client may help to increase the amount of code that can be shared.

One way to increase the amount of code that can be shared between servers and clients is to use a logic-less template language like Mustache or Handlebars. Such templates can be rendered from different host languages, such as Ruby on the server and JavaScript in the client. However, merely sharing templates typically requires duplication of business logic used to choose the correct templates and populate them with data. Rendering from templates may have negative performance effects when only updating a small portion of the page—such as the value of a text input within a large template. Replacing an entire template might also disturb a user's selection or cursor position, where updating only the changed value might not. To avoid these problems, applications can use UI data bindings or granular DOM manipulation to only update the appropriate parts of the page instead of re-rendering entire templates.

Another approach used by server centric web frameworks like the Java based ItsNat is to render any hypertext in server using the same language and templating technology. In this approach the server knows with precision the DOM state in client, any big or small page update required is performed generating in server, and transported by AJAX, the exact JavaScript code to bring the client page to the new state executing DOM methods. Developers can decide which page states must be crawlable by web spiders for SEO and be able to generate the required state in load time generating plain HTML instead of JavaScript. In case of ItsNat framework this is automatic because ItsNat keeps the client DOM tree in server as a Java W3C DOM tree, rendering of this DOM tree in server generates plain HTML in load time and JavaScript DOM actions for AJAX requests. This duality is very important for SEO because developers can build with the same Java code and pure HTML based templating the desired DOM state in server, on page load time conventional HTML is generated by ItsNat making this DOM state SEO compatible. As of version 1.3 [15] ItsNat provides a new stateless mode, client DOM is not kept in server because in stateless mode client DOM state is partially or fully reconstructed in server when processing any AJAX request based on required data sent by client informing of the current DOM state; the stateless mode may be also SEO compatible because SEO compatibility happens in load time of the initial page not affected by stateful or stateless modes.

Because SEO compatibility is not trivial in SPAs, it's worth noting that SPAs are commonly not used in a context where search engine indexing is either a requirement, or desirable. Use cases include applications that surface private data hidden behind an authentication system. In the cases where these applications are consumer products, often a classic "page redraw" model is used for the applications landing page and marketing site, which provides enough meta data for the application to appear as a hit in a search engine query. Blogs, support forums, and other traditional page redraw artifacts often sit around the SPA that can seed search engines with relevant terms.

Browser history

With an SPA being, by definition, "a single page", the model breaks the browser's design for page history navigation using the Forward/Back buttons. This presents a usability impediment when a user presses the back button, expecting the previous screen state within the SPA, but instead the application's single page unloads and the previous page in the browser's history is presented.

The traditional solution for SPA's has been to change the browser URL's hash fragment identifier in accord with the current screen state. This can be achieved with JavaScript, and causes URL history events to be built up within the browser. As long as the SPA is capable of resurrecting the same screen state from information contained within the URL hash, the expected back button behavior is retained.

To further address this issue, the HTML5 specification has introduced pushState and replaceState providing programmatic access to the actual URL and browser history.

Page lifecycle

An SPA is fully loaded in the initial page load and then page regions are replaced or updated with new page fragments loaded from the server on demand. To avoid excessive downloading of unused features, an SPA will often progressively download more features as they become required, either small fragments of the page, or complete screen modules.

In this way an analogy exists between "states" in an SPA and "pages" in a traditional web site. Because "state navigation" in the same page is analogous to page navigation, in theory any page based web site could be converted to single-page replacing in the same page only the changed parts result of comparing consecutive pages in a non-SPA.

The SPA approach on the web is similar to the Single Document Interface (SDI) presentation technique popular in native desktop applications.

References

  1. ↑ 1.0 1.1 Flanagan, David, "JavaScript - The Definitive Guide", 5th ed., O'Reilly, Sebastopol, CA, 2006, p.497
  2. ↑ "Inner-Browsing: Extending Web Browsing the Navigation Paradigm". Retrieved 2011-02-03. 
  3. ↑ "Slashdotslash.com: A self contained website using DHTML". Retrieved 2012-07-06. 
  4. ↑ "Thin Server Architecture Working Group". Retrieved 2011-12-11. 
  5. ↑ "The Single Page Interface Manifesto". Retrieved 2010-11-12. 
  6. ↑ "Derby". Retrieved 2011-12-11. 
  7. ↑ "Sails.js". Retrieved 2013-02-20. 
  8. ↑ "nCombo". Retrieved 2013-01-31. 
  9. ↑ "Tutorial: Single Page Interface Web Site With ItsNat". Retrieved 2011-01-13. 
  10. ↑ HTML5
  11. ↑ Michael Mikowski. "How to optimize single page sites for search engines". Retrieved 6 January 2014. "When Google and other search engines index websites, they don’t execute JavaScript" 
  12. ↑ "What the user sees, what the crawler sees". Retrieved January 6, 2014. "the browser can execute JavaScript and produce content on the fly - the crawler cannot" 
  13. ↑ "Making AJAX Applications Crawlable". Retrieved January 6, 2014. "Historically, AJAX applications have been difficult for search engines to process because AJAX content is produced" 
  14. ↑ "Making AJAX Applications Crawlable". Retrieved 2011-01-13. 
  15. ↑ "ItsNat v1.3 release Notes". Retrieved 2013-06-09. 

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