The subject of Web 2.0 has become profoundly important over the last year. Web 2.0 describes the next generation of the Web as an application platform where most of a user's software experience resides. The subject is somewhat controversial, but it's becoming ever more apparent as the successor to monolithic system architecture, prepackaged software, and traditional Web applications.

Software as a Service (SAAS) and Web as Platform are only two of the larger mantras of Web 2.0 that most of the major software vendors have begun to embrace recently. Yet not only is Web 2.0 still very misunderstood, it's actually part of an even larger way of thinking about software in a fully service-oriented manner. This includes building composite applications, remixing data, building ad hoc supply chains, harnessing user involvement, aggregating knowledge, and more. Web 2.0 is becoming embodied in best practice sets such as service-oriented architecture (SOA).

The term Web 2.0 was originally coined by O'Reilly's Dale Dougherty to describe the forces behind the huge post-dot-com success of Internet companies like Google, eBay, Amazon, and iTunes, as well as noncommercial, emergent Web phenoms such as Wikipedia and BitTorrent. Web 2.0 describes Web experiences that fundamentally engage users by: 1) allowing them to participate in sharing information and enriching data freely, 2) readily offering their core functionality as open services to be composited or "mashed up" into new services and sites, and 3) placing the Web at the center of the software experience both in terms of data location as well as where the software is.

Applications in which the Web app is primarily an online catalog are changing the most. Instead of being just a way to browse for products or information, the Web 2.0 app is itself the tip of an iceberg that integrates services and data from multiple sources and then makes the results available to users and other Web 2.0 apps. At the end of the day, the integration achieved by one Web 2.0 app will likely get rolled up into someone else's Web 2.0 app.

Now evangelized by Tim O'Reilly and others as Web 2.0, the concepts themselves are not really new, but they are beginning to dominate the IT industry's collective consciousness. The powerful force of architecture of participation, which is the combined network effects of pervasive two-way participation (blogging, wikis, and media sharing), is having a huge effect and is creating a single, communal service architecture on the Web. In the end, users want access to information anywhere, from multiple sources, without synchronization, delay, or maintenance (software upgrades, data backups, etc.). Users want to be able to share knowledge and collaborate with peers. To do this they need to be using the same underlying set of technologies and paradigms, and this is what Web 2.0 promotes.

A Closer Look at Web 2.0
While large traditional software organizations such as Microsoft, IBM, and Oracle are struggling mightily to deliver traditional prepackaged software on long-term, aperiodic cycles, a newer generation of companies (Google, Yahoo!, Amazon, eBay, and many other smaller companies like 37signals and del.icio.us) are delivering capable software entirely online. This new agile way of providing functionality as a service over the Web provides a nimble, continuous experience with no software upgrades, and no synchronization of data or programs among work, home, or mobile locations. Equally important is that it provides a way to build new capabilities on top of existing functionality that becomes larger than the pieces (see Figure 1).

Tim O'Reilly, one of the leading evangelists of the Web 2.0 approach, generally provides seven classic characteristics of Web 2.0 software. These are described in the subsections below.

Web as Platform
Software and services are now the same thing, and the Web has become a computing platform in its own right. The Web is where most software is moving for cost, convenience, agility, and increased overall value.

Harnessing Collective Intelligence
The network effects of massive amounts of users make the collaborative Web a much more potent force than stand-alone software. Wikipedia says network effects "cause a good or service to have a value to a potential customer dependent on the number of customers already owning that good or using that service." Put another way, online collaborative entities such as Wikipedia are a network effect of the combined contributions of their users. This is a classic example of the high-value emergent properties of Web 2.0 forces.

Data Is the Next Intel Inside
The core functionality of many modern information systems is not software; more accurately, it's the valuable data within it. Look at Google's search database, Amazon's products and associated reviews, or eBay's auctions. While the services these sites provide are also important and integral, the data they possess are just as important, perhaps even more so.

End of the Software Release Cycle
When software is on the Web, upgrading becomes a different experience. Discrete changes become less obvious while continuous improvement becomes the norm. Because services are always available 24 hours a day to anyone connected to the global Internet, upgrades and improvements to service are instantly available and encouraged to be as nondisruptive as possible.

Lightweight Programming Models
When the clients of Web software are numerous and diverse, complex standards can get in the way, reduce interoperability, and stifle connectivity. Web 2.0 realizes that demand for services will route around unnecessary impedance and leverage the easiest methods that work well. This has led to simpler services such as REST and RSS instead of SOAP and WS-* standards. Remixing and compositing of services is also much easier with clean, clear, simple models, and this has also promoted loose coupling and suppler services, especially in the large. Dynamic programming languages that support rapid change are becoming more popular too.

Software Above the Level of a Single Device
PCs are an increasingly smaller aspect of the Web. With so many different devices such as mobile phones, PDAs, and even digital video recorders and personal media servers becoming connected to the Web and both providing and consuming functionality and content, the Software as a Service landscape of the Web now includes these in the picture.

Rich User Experiences The Web has ceased to be about static Web pages. They still exist, but they are much less important. More central to the Web are rich user experiences that immerse the user in the functionality of the services available on the Web without getting in the way. The AJAX browser application model is famously a Web 2.0 technique that uses the raw ingredients of modern browsers to provide the full interactive experience of native applications to the user while leveraging XML Web services on the back end to provide access to data and services.

There are many interlocking, reinforcing details that are vital to appreciating the best practices in the Web 2.0 toolset, however, the intent of this article isn't to explain every nuance of Web 2.0. Instead the article should convey a general mental model of it and describe Web 2.0's striking similarity to the SOA model. The point is that Web 2.0 describes the Web as a galactic collection of high-value Web services to be used, reused, and leveraged to meet users' needs. The Web itself provides the universal fabric upon which all of this rests and this includes the standards, the users, and the data. The premise of this article is that Web 2.0 actually describes the Web as the convergence of software services into a global service-oriented architecture.