Web template system

The basic process for a server-side web templating system: content (from a database), and "presentation specifications" (in a web template), are combined (through the template engine) to mass-produce web documents.

A web template system uses a template processor to combine web templates to form finished web pages, possibly using some data source to customize the pages or present a large amount of content on similar-looking pages. It is a web publishing tool present in content management systems, web application frameworks, and HTML editors.

Web templates can be used like the template of a form letter to either generate a large number of "static" (unchanging) web pages in advance, or to produce "dynamic" web pages on demand.

Overview

A web template system is composed of:

• A template engine: the primary processing element of the system;^[1]
• Content resource: any of various kinds of input data streams, such as from a relational database, XML files, LDAP directory, and other kinds of local or networked data;
• Template resource: web templates specified according to a template language;

The template and content resources are processed and combined by the template engine to mass-produce web documents. For purposes of this article, web documents include any of various output formats for transmission over the web via HTTP, or another Internet protocol.

Motivations and typical uses

Applications

Web templates can be used by any individual or organization to set up their website. Once a template is purchased or downloaded, the user will replace all generic information included in the web template with their own personal, organizational or product information. Examples of common uses of Templates are listed below:

• Display personal information or daily activities as in a blog.
• Sell products on-line.
• Display information about a company or organization.
• Display family history.
• Display a gallery of photos.
• Place music files such as MP3 files on-line for play through a web browser.
• Place videos on-line for public viewing.
• To set up a private login area on-line.

Mass-production

Various agencies and organizations use web template systems for mass-production of content when slower production alternatives prove unfeasible.

For an introductory overview, take a news website as an example. Consider a "static website", where all web pages are static, built by a web designer. It would be very repetitive work to change individual pages as often as the news changes. A typical strategy to automate the web designer's "repetitive work" using Templates could be as follows:

1. choose a web template system to maintain the website;
2. group news items with different presentation needs;
3. specify the "presentation standards" through web templates, for each group of news;
4. specify a content resource to generate or update the content of each news item.

Separation of concerns

A common goal among experienced web developers is to develop and deploy applications that are flexible and easily maintainable. An important consideration in reaching this goal is the separation of business logic from presentation logic.^[2] Developers use web template systems (with varying degrees of success) to maintain this separation.^[2]

For the web designer, when each web page comes from a web template, they can think about a modular web page structured with components that can be modified independently of each other. These components may include a header, footer, global navigation bar (GNB), local navigation bar and content such as articles, images, videos etc.

For programmers the template language offers a more restricted logic, only for presentation adaptations and decisions, not for complex (business model) algorithms.

For other members of the "site team", a template system frees webmasters to focus on technical maintenance, content suppliers to focus on content, and gives all of them more reliability.

Moreover, it has the following advantages to its use:

• Ease of design change: presentation variations on templates are "content invariant", meaning a web designer can update the presentation without wider infrastructural preoccupations.
• Ease of interface localization: menus and other presentation standards are easy to make uniform, for users browsing on the site.
• Possibility to work separately on design and code by different people at the same time.

One difficulty in evaluating separation of concerns is the lack of well-defined formalisms to measure when and how well it is actually met.^[2] There are, however, fairly standard heuristics that have been borrowed from the domain of software engineering. These include 'inheritance' (based on principles of object-oriented programming); and 'templating and generative programming', (consistent with the principles of MVC separation).^[3] The precise difference between the various guidelines is subject to some debate, and some aspects of the different guidelines share a degree of similarity.^[4]

Flexible presentation

One major rationale behind "effective separation" is the need for maximum flexibility in the code and resources dedicated to the presentation logic.^[3] Client demands, changing customer preferences and desire to present a "fresh face" for pre-existing content often result in the need to dramatically modify the public appearance of web content while disrupting the underlying infrastructure as little as possible.

The distinction between "presentation" (front end) and "business logic" (infrastructure) is usually an important one, because:

• the presentation source code language may differ from other code assets
• the production process for the application may require the work to be done at separate times and locations
• different workers have different skill sets, and presentation skills do not always coincide with skills for coding business logic
• code assets are easier to maintain and more readable when disparate components are kept separate and loosely coupled^[3]

Reusability

Not all potential users of web templates have the willingness and ability to hire developers to design a system for their needs. Additionally, some may wish to use the web but have limited or no technical proficiency. For these reasons, a number of developers and vendors have released web templates specifically for reuse by non-technical people. Although web template reusability is also important for even highly skilled and technically experienced developers, it is especially critical to those who rely on simplicity and "ready-made" web solutions.

Such "ready-made" web templates are sometimes free, and easily made by an individual domestically. However, specialized web templates are sometimes sold online. Although there are numerous commercial sites that offer web templates for a licensing fee, there are also free and "open-source" sources as well.

Example

With the model typically held in a relational database, the remaining components of the MVC architecture are the control and view. In the simplest of systems these two are not separated. However, adapting the separation of concerns principle one can completely decouple the relationship.

For example, the view template may look like this:

<!DOCTYPE html>
   <html xmlns="http://www.w3.org/1999/xhtml">
   <head><title>Sites</title></head>
   <body><h1 data-xp="title"><!-- placeholder --></h1></body>
</html>

Then the control template will load the view and then can use xpath addressing to insert components from a database, for instance:

 <?php
$doc = new DOMDocument;
$doc->preserveWhiteSpace = false;
$doc->Load('view.html');
$titlenode = $doc->createTextNode("Like this");
$xpath = new DOMXPath($doc);
$xpath->registerNamespace("h","http://www.w3.org/1999/xhtml"); 
$query="//h:*[@data-xp='title']/comment()";
$entries = $xpath->query($query);
foreach ($entries as $entry) { $entry->parentNode->replaceChild($titlenode, $entry); }
echo $doc->saveXML();
?>

Kinds of template systems

A web browser and web server are a client–server architecture; often a web cache is also used to improve performance. Overall, there are five types of templating systems, classified based on when assembly happens - placeholders are substituted for variable information (such as names, addresses, and product info) and sub-templates:

• Server-side - run-time substitution happens on the web server
• Client-side - run-time substitution happens in the web browser
• Edge-side - run-time substitution happens on a proxy between web server and browser
• Outside server - static web pages are produced offline and uploaded to the web server; no run-time substitution
• Distributed - run-time substitution happens on multiple servers

Template languages may be:

• Embedded or event-driven.
• Simple, iterable, programmable, or complex.
• Defined by a consortium, privately defined, or de facto defined by an open implementation. Ownership influences the stability and credibility of a specification. However, in most jurisdictions, language specification cannot be copyrighted, so control is seldom absolute.

The source code of the template engine can be proprietary or open source.

Many template systems are a component of a larger programming platform or framework. They are referred to as the "platform's template system". Some template systems have the option of substituting a different template language or engine.

Programming languages such as Perl, Ruby, C, and Java support template processing either natively, or through add-on libraries and modules. JavaServer Pages (JSP), PHP, and Active Server Pages (ASP with VBScript, JScript or other languages) are examples, themselves, of web template engines. These technologies are typically used in server-side templating systems, but could be adapted for use on a "edge-side" proxy or for static page generation.

Static page generators

Outside server template system architecture.

HTML editors often use web template systems to produce only static web pages. These can be viewed as a ready-made web design, used to mass-produce "cookie-cutter" websites for rapid deployment. They also commonly include themes in place of CSS styles. In general, the template language is to be used only with the editor's software.^[5]

FrontPage and Dreamweaver are the most popular editors with template sub-system. A Flash web template uses Macromedia Flash to create visually appealing sites.

Many server-side template systems have the option to publish the output pages on the server, where the published pages will be static. It is a common feature on content management systems, like Vignette. But this does not have to be considered an out-server generation.

In the majority of the cases, this "publish option" doesn't interfere with the template system, and it can be made by external software, as Wget.

Server-side systems

Server-side template system

Server-side dynamic pages began to be generated by templates with pre-existent software adapted for this task. This early software was the preprocessors and macro languages, adapted for the web use, running on CGI. Next, a simple but relevant technology was the direct execution made on extension modules, started with SSI.

Many template systems are typically used as server-side template systems:

Technically, the methodology of embedding programming languages within HTML (or XML, etc.), used in many "server-side included script languages" are also templates. All of them are Embedded complex languages.

There are also preprocessors used as server-side template engines. Examples:

Edge-side systems

Edge-Side template and inclusion systems. “Edge-side” refers to web servers that reside in the space between the client (browser) and the originating server. They are often referred to as “reverse-proxy” servers. These servers are generally tasked with reducing the load and traffic on originating servers by caching content such as images and page fragments, and delivering this to the browser in an efficient manner.

Basic Edge Side Includes (ESI) is an SSI-like language. ESI has been implemented for content delivery networks. The ESI template language may also be implemented in web browsers using JavaScript and Ajax, or via a browser "plug-in".

Client-side systems

Client-side and distributed (decentralized) template system.

Many web browsers can apply an XSLT stylesheet to XML data that transforms the data into an XHTML document, thereby providing template functionality in the browser itself.
Other systems implement template functionality in the browser using JavaScript or another client-side scripting language, including:

• Closure Templates
• EJS
• Jenner
• jQote2
• jTemplates
• Mjt.Template
• Mustache
• Pure
• Tempo JSON templates
• webJS
• Yeast templates
• Merge On Browse

Distributed systems

The most simple form is transclusions (HTML frames). In other cases dynamic web pages are needed.

Examples:

• Ajax
• Rich Internet application

See also

• Comparison of web template engines
• Template processor
• Boilerplate code
• Bytecode
• CodeCharge Studio
• Layout engine
• Macro (computer science) - section Text substitution macros
• Preprocessor
• Template (file format)
• UIML (User Interface Markup Language)
• XSLT
• Virtual machine

References

External links

• JavaScript template libraries comparison from 2009
• Enforcing Strict Model-View Separation in Template Engines
• A Double-Model Approach to Achieve Effective Model-View Separation in Template Based Web Applications
• A PHP template engine comparison with graphic charts
• Comparisons/benchmarks of some Python template-engines and some generic thoughts about template-engines
• web-mode.el is an emacs major for editing web templates