2. Introducing Zope

Zope is family of related Python packages focussed on web technologies. The first version of Zope has originated from a company called Zope Corporation.

Today the Zope Foundation holds the copyright of the Zope source code and supervises a diverse community of open-source contributers working on a variety of related projects.

This book is about the original Zope project, today known as Zope2. When we refer to Zope in this book without a narrower specification we speak of Zope2.

Other projects include the Zope3 web application framework, many individual packages located in the Zope Subversion Repository and projects being based or related to these packages like Grok and Repoze. One of the more widely known applications based on top of Zope2 is a content management system called Plone.

Zope2 itself is a web framework that allows developers of varying skill levels to build web applications. This chapter explains Zope’s purpose, what problems it solves and what audience it targets in greater detail. It also describes what makes Zope different and more powerful than similar applications.

NOTE: The moniker “Zope” stands for the Z Object Publishing Environment (the “Z” doesn’t really mean anything in particular).

2.1. The Static Web Site Dilemma

When a company or organization goes through the process of developing and eventually deploying a website, one of its most important goals is to present timely and up-to-date information to its website visitors.

Let us consider two examples of such time-dependent sites:

• a stock market information site that needs to be updated with new information continually, maybe as often as every five or 10 minutes. It will also present information tailored to each visitor’s preferred settings (portfolios, stocks to follow, etc.)
• a commercial website that helps its visitors sell and buy used automobiles. It is usually required that such a site run advertisements only for cars that have not yet been sold. It is also important that new ads be posted immediately after they’ve been placed by a seller.

These two examples describe two very different sites that nevertheless have one basic requirement in common: automated and periodic updates of the information presented. If this single requirement is not met, these sites will likely be unsuccessful.

So, how does Zope work to fulfill such a requirement? To understand this, we need to consider how websites are perceived by their visitors and the basic ways in which websites can be constructed.

In general, many website visitors think about navigation in terms of moving “from page-to-page” within a website. When they click a hyperlink, their browser transports them to a new page. When they hit their browser’s back button, they are returned to the last page they visited, and so on.

Some websites are static. A static website stores its information in files on a web server. Each file then represents a complete page on the website. This may seem like a simple and efficient way of creating a website; however, updating the information within those pages becomes a problem when the site consists of more than a few pages, and the pages, or parts of the pages, need to be updated frequently.

The layout of text and images that are displayed in a user’s web browser when the user visits a website are commonly composed in a simple language known as Hyper Text Markup Language (HTML). When a user visits a typical website, a chunk of text that is “marked-up” with formatting in HTML is transferred between the website and the user’s browser. The browser interprets the chunk of text and displays text and images to the user. The chunk of text which is transferred is typically referred to as a page.

To achieve this, the static website requires a person with a privileged level of access (sometimes termed the webmaster) to manually create and update the site’s content.

Typically, this is done by editing a set of text-based files on the web server (the machine that runs the website), where each file represents a single page. In some cases, a site-wide change to the “look-and-feel” of a static website requires that the webmaster visit and update each and every file that comprises the website.

The webmaster responsible for our automobile advertising website has the additional responsibility of keeping the ads themselves fresh. If each page in the website represents an ad for a particular automobile, he needs to delete the pages representing ads that have expired and create new pages for ads that have been recently sold. He then needs to make sure that no hyperlinks on other pages point to any of these deleted pages.

Obviously, this quickly becomes a lot of work. With any more than a few pages to update each day, this type of repetitive work can become pretty dull. In addition, being a human being, the webmaster may also make mistakes, such as forgetting to update or remove critical pages. While updating a static website with only 10 to 20 pages might be dull, it’s perfectly manageable. However, websites can typically grow to encompass thousands of files, making the process of “timely updates” a non-trivial (and sometimes impossible) task.

Somewhere down the line, smart webmasters begin to think to themselves, “Wow, this is a lot of work. It’s tedious and complicated, and I seem to be making a lot of mistakes. Computers are really good at doing tedious and complicated tasks, and they don’t make very many mistakes. I bet my web server computer could automatically do a lot of the work I now do manually.” And he would be right.

At this point, the webmaster is ready to be introduced to web applications. It is in this area where Zope’s strength and power becomes clear.

2.2. What Is A Web Application?

A web application is a computer program that users invoke by using a web browser to contact a web server via the Internet. Users and browsers are typically unaware of the difference between a web server that fronts a statically-built website and one that fronts a web application. But unlike a static website, a web application creates its “pages” dynamically, or on-the-fly, upon request. A website that is dynamically- constructed uses an a computer program to provide its content. These kinds of dynamic applications can be written in any number of computer languages.

Web applications are everywhere. Common examples of web applications are those that let you search the web, like Google; collaborate on projects, like SourceForge; buy items at an auction, like eBay; communicate with other people over e-mail, like Gmail; or view the latest news ala CNN.com.

In a dynamically-constructed website, the webmaster is not required to visit the site “page-by-page” in order to update its content or style. Instead, he is able to instruct the web server to generate the site’s HTML pages dynamically, where each page is made up of different bits of content. While each bit of content is unique, each can nevertheless appear in several pages if so instructed by the web server. In this way, the webmaster is able to create a common “look and feel” for the set of pages that make up his site. The software on the web server that generates these pages is the web application.

If our auto-classifieds webmaster chose to construct a web application to maintain his classifieds system, he could maintain a list of “current” ads separate from the HTML pages, perhaps stored in a database of some kind. He could then instruct his web application to query this database and generate a particular chunk of HTML that represented an ad, or an index of ads, when a user visited a page in his website.

A framework that allows people to construct a web application is often called a web application server, or sometimes just an application server. Zope is a web application server, as are competing products like WebSphere, JBoss, and (to some extent) SAP NetWeaver.

Zope is a web application server, which is not a web application in itself; rather it is framework that allows people to construct web applications. Sometimes this framework is called an application server.

Using some common computer programming language, an application server typically allows a developer to create a web application, but it also provides services beyond the basic capabilities of the programming language used. Examples of such services are web page template creation facilities, a common security model, data persistence, sessions, and other features that people find useful when constructing a typical web application.

2.3. How You Can Benefit From Using An Application Server

If you are considering writing even a moderately-sized web application, it is typically a good idea to start your project using an application server framework, unless your application requirements are extremely specialized. By starting a web application project with an application server framework (as opposed to a “raw” computer language, such as Java, Perl, Python, or C), you are able to utilize the services of the framework that have already been written and proven to work, and you avoid the need to write the functionality yourself “from scratch” in a “raw” language.

Many application servers allow you to perform some of the following tasks:

Present Dynamic Content – You may tailor your web site’s presentation to its users and provide users with search features. Application servers allow you to serve dynamic content and typically come with facilities for personalization, database integration, content indexing, and searching.

Manage Your Web Site – A small web site is easy to manage, but a web site that serves thousands of documents, images, and files requires heavy-duty management tools. It is useful to be able to manage your site’s data, business logic, and presentation from a single place. An application server can typically help manage your content and presentation in this way.

Build a Content Management System – A content management system allows non-technical editors to create and manage content for your website. Application servers provide the tools with which you can build a content management system.

Build an E-Commerce Application – Application servers provide a framework in which sophisticated e-commerce applications can be created.

Securely Manage Contributor Responsibility – When you deal with more than a handful of web users, security becomes very important. You must be able to safely delegate tasks to different classes of system users. For example, folks in your engineering department may need to be able to manage their web pages and business logic, designers may need to update site templates, and database administrators need to manage database queries. Application servers typically provide a mechanism for access control and delegation.

Provide Network Services – You may want to produce or consume network services. A network service-enabled web site must to be able to accept requests from other computer programs. For example, if you’re building a news site, you may wish to share your news stories with another site; you can do this by making the news feed a network service. Or perhaps you want to make products for sale on your site automatically searchable from a product comparison site. Application servers offer methods for enabling these kinds of network services.

Integrate Diverse Systems – Your existing content may be contained in many places: relational databases, files, separate web sites, and so on. Application servers typically allow you to present a unified view of your existing data by integrating diverse, third-party systems.

Provide Scalability – Application servers allow your web applications to scale across as many systems as necessary to handle the load demands of your sites.

The Zope application server allows you to perform all of these tasks.

2.4. Why Use Zope Instead of Another Application Server

If you’re in the business of creating web applications, Zope can potentially help you create them at less cost and at a faster rate than you could by using another competing web application server. This claim is backed by a number of Zope features:

• Zope is free of cost and distributed under an open-source license. There are many non-free commercial application servers that are relatively expensive.
• Zope itself is an inclusive platform. It ships with all the necessary components to begin developing an application. You don’t need to license extra software to support Zope (e.g., a relational database) in order to develop your application. This also makes Zope very easy to install. Many other application servers have “hidden” costs by requiring that you license expensive software or configure complex, third-party infrastructure software before you can begin to develop your application.
• Zope allows and encourages third-party developers to package and distribute ready-made applications. Due to this, Zope has a wide variety of integrated services and add-on packages available for immediate use. Most of these components, like Zope itself, are free and open-source. Zope’s popularity has bred a large community of application developers.
• Applications created in Zope can scale almost linearly using Zope’s built-in “Zope Enterprise Objects” (ZEO) clustering solution. Using ZEO, you can deploy a Zope application across many physical computers without needing to change much (if any) of your application code. Many application servers don’t scale quite as transparently or as predictably.
• Zope provides a granular and extensible security framework. You can easily integrate Zope with diverse authentication and authorization systems, such as LDAP, Kerberos, and RADIUS, simultaneously and using pre-built modules. Many other application servers lack support for important authentication and authorization systems.
• Zope runs on most popular microcomputer operating system platforms: Linux, Windows, Solaris, FreeBSD, NetBSD, OpenBSD, and Mac OS X. Many other application server platforms require that you run an operating system of their licensor’s choosing.
• Zope can be extended using the interpreted Python scripting language. Python is popular and easy to learn, and it promotes rapid development. Many libraries are available for Python that can be used when creating your own application. Many other application servers must be extended using compiled languages, such as Java, which cuts down on development speed. Many other application servers use less popular languages for which there are not as many ready-to-use library features.

2.5. Zope Audiences and What Zope Isn’t

Managing the development process of a large-scale site can be a difficult task. It often takes many people working together to create, deploy, and manage a web application.

Information Architects

make platform decisions and keep track of the “big picture”.

Component Developers

create software intended for reuse and distribution.

Integrators

integrate the software written by component developers and native application server services, building an application in the process.

Web Designers

create the site’s look and feel.

Content Managers

create and manage the site’s content.

Administrators

keep the software and environment running.

Consumers

use the site to locate and work with useful content.

Of the parties listed above, Zope is most useful for component developers, integrators, and web designers. These three groups can collaborate to produce an application using Zope’s native services and third-party Zope Plugins. They typically produce applications useful to content managers and consumers under the guide of the information architect. Administrators deploy the application and tend to the application after it is has been created.

Note that Zope is a web application construction framework that programmers of varying skill levels may use to create web-based applications. It is not itself an application that is ready to use “out of the box” for any given application. For example, Zope itself is not a blog, a content management system, or a “e-shop-in-a-box” application.

However, freely available Plugins built on top of Zope offer these kinds of services. At the time of this writing, the Python Package Index lists roughly 400 Plugins that you can browse and even reuse in your own applications. These include Plugins for blogging, content management, internationalization, and e-commerce.

Zope is not a visual design tool. Tools like Macromedia Dreamweaver and Adobe GoLive allow designers to create “look and feel”. You may use these tools to successfully manage Zope-based web sites, but Zope itself does not replace them. You can edit content “through the web” using Zope, but it does not try to replace the features offered by these kind of tools.

2.6. Introduction to Zope Maintenance and The Zope Community

A community of developers is responsible for maintaining and extending the Zope application server. Many community members are professional consultants, developers, and webmasters who develop applications using Zope for their own gain. Others are students and curious amateur site developers. Zope Corporation is a member of this community.

The Zope Foundation controls the distribution of the defacto, “canonical”, official Zope version, and permits its developers, as well as other selected developers, to modify the distribution’s source code.

The Zope community gets together occasionally at conferences, but it commonly discusses all things Zope on the many Zope mailing lists and web sites. You can find out more about Zope-related mailing lists at Zope.org’s mailing list page.

Zope Corporation makes its revenue by using Zope to create web applications for its paying customers, by training prospective Zope developers, by selling support contracts to companies who use Zope, and by hosting Zope-powered websites; it does not make any direct revenues from the distribution of the Zope application server itself.

2.7. Zope’s Terms of Use and License

Zope is free of cost. You are permitted to use Zope to create and run your web applications without paying licensing or usage fees. You may also include Zope in your own products and applications without paying royalty fees to Zope’s licensor, Zope Foundation.

Zope is distributed under an open source license, the Zope Public License or ‘ZPL’. The terms of the ZPL license stipulate that you will be able to obtain and modify the source code for Zope.

The ZPL is different than another popular open source license, the GNU Public License. The licensing terms of the GPL require that if you intend to redistribute a GPL-licensed application, and you modify or extend the application in a meaningful way, when you redistribute a GPL-licensed application, you must distribute it under the terms of the GPL, including licensing any modifications or extensions you make under the GPL. You must also provide the full source code, including source for your modifications.

However, this is not required for ZPL-licensed applications. You may modify and redistribute Zope without contributing your modifications back to Zope Corporation, as long as you follow the other terms of the license faithfully.

Note that the ZPL has been certified as OSD compliant by the Open Source Initiative and is listed as GPL compliant by the Free Software Foundation.

2.8. Zope History

In 1996, Jim Fulton (the current CTO of Zope Corporation, the orginators of Zope) was drafted to teach a class on CGI programming, despite not knowing very much about the subject. CGI, or common gateway interface, programming is a commonly-used web development model that allows developers to construct dynamic websites. Jim studied all of the existing documentation on CGI on his way to the class. On the way back from the class, Jim considered what he didn’t like about traditional, CGI-based programming environments. From these initial musings, the core of Zope was written on the plane flight back from the class.

Zope Corporation (then known as Digital Creations) went on to release three open-source software packages to support web publishing: Bobo, Document Template, and BoboPOS. These packages were written in a language called Python, and respectively provided a web publishing facility, text templating, and an object database. Digital Creations developed a commercial application server based on their three open-source components. This product was called Principia. In November of 1998, investor Hadar Pedhazur convinced Digital Creations to open source Principia. These packages have evolved into what today are the core components of Zope.

Most of Zope is written in the Python scripting language, with performance-critical pieces written in C.