However, if you are following well-structured markup practices, the first place most engines will look after headings is text formatted as being a paragraph. So, when you are preparing your content for the Web, be sure that there are plenty of keywords worked into your content, again following a balanced approach where you are enhancing the keyword count but not diminishing the content’s importance.

Here’s a welcome paragraph on a client’sWeb site:

I specialize in luxury homes in the Tucson foothills and University of Arizona area. I am also very knowledgeable about Tucson golf course properties and retirement neighborhoods. My business has been built by personal referrals, so I’m looking forward to getting to work with you.

Note some of the powerful keywords and combinations in use:

• luxury
• luxury homes
• homes
• Tucson
• Tucson foothills
• foothills
• University
• University of Arizona
• Arizona
• Tucson golf
• Tucson golf course
• golf
• golf course
• golf course properties
• Tucson golf course properties
• retirement
• retirement neighborhoods.

Naturally, all these keywords and terms should be included in your keyword meta element, too.

Keeping URLs Short and Relevant

This is a topic that’spopped up throughout this series, and for good reason. Whether you’re approaching it from the perspective of information architecture, usability, or ranking and promotions, the shorter and more logical your Web site address is, the easier it’s going to be to promote and to catalog.

For example, www.indometric.com/articles/ is going to be a fairly easy URL to promote, whereas www.indometric.com/category/web-development/ is going to have numerous pitfalls. So, keep URLs as short and sweet as you possibly can.

It’s also highly recommended that when you have a www within your domain name that a visitor is able to resolve your site with or without the initiating “www”—www.indometric.com and indometric.com resolve to the same location.

Another problem with URLs and promotion is dynamically created URLs or those that display a session ID.

Consider the following URL, a real URL to an article of mine:

http://www.informit.com/content/index.asp?product_id={F41929EF-0DCC-444C-AE9C-EA20A98C3853}&session_id={6C98C46C-F571-4647-890DAB05E18223DD}

Anyone who takes this URL and copies it into their browser window may encounter problems when the browser tries to resolve to the correct page. The reason is due to the session ID being added to the URL. That ID is specific to my session, not yours, and will cause problems when another person tries to resolve the URL.

ExperiencedWeb developers and programmers will know that they should remove the session ID, but these issues can and should be dealt with on the server-side. URLs of this nature go against most usability guidelines and cause serious problems when it comes to trying to promote specific pages within Web sites.

Did you catch the other problem with the extended URL? If you drop it as is into an XHTML document, it will not validate unless you escape the ampersand within the URL.

Solicit Reciprocal Links

The more people link to your site, the more “important” that Web address will appear to many search engines, with the site ranking improving based on its popularity. This is called reciprocal linking. One scam you should be aware of is that some SOEs and marketing firms will try to convince you that you need to populate “entry” pages—pages that link to your Web site—on other Web servers to increase links to you, thereby boosting your rank by tricking search engines into thinking that relevant Web pages have links to your site.

The best way to increase linking to you is to solicit it—especially from relevant and related Web sites. For example, the more Web sites related to Web development, Web design, and computer book topics that I have linked to molly.com, the better my ranking is going to be.

You can increase linking a number of ways beyond frank solicitation. Today’s Weblog software can be applied to businesses too, adding features with rolling “blogrolls”—links to related sites. For topically related sites, there are often Webrings—groups of related sites with links from one to the next—and these can be a great way to drive traffic to your sites. I’ve added my Web site to a number of rings in the past few years and have been astonished at the resulting increase in visitors to my site.Webrings are grouped by like interest (or geography, such as “Tucson Web sites”), so the people who come to your site as a result are likely going to have a pre-existing interest or need that your site can accommodate.

Do a search for <yourtopic webring> on any search engine and you’ll probably find an assortment of Webrings related to your topic, too.

If for some reason you do decide, or are told, that you need to research SOEs or other groups that can assist you with linking strategies, be as careful as you can to check out who you are working with. Who knows, you might be surprised and actually find someone who will help you with resources and actually contribute to your goals, but again, the likelihood of that is very low.

<meta name=”description” content=”I’m Molly E. Holzschlag, and this Web site shares my Web development work and personal thoughts.” />

Now look at Figure below, which is an exact reproduction of the search results found on such search engines as Google, Bing, and Yahoo.

An example of how a wide number of major search engines implement the meta description:

As you can see, the search engine has catalogued my description. Without an effective description in place, you may end up with other components falling into the ranking first, as shown in Figure 14-3.

Figure below without a meta description in place, the navigation appears before any discernable content:

Follow these general guidelines when creating descriptions:

• Include a “call to action” that encourages users to select your site. This will vary depending on the product and services you want. My description might be more effective if I used this approach, which might read “Computer Application Articles, Tutorials and Ebook for Developers.”
• Be brief. Stick to 25 words or less.
• Be clear and meaningful. “Beautiful Web sites” doesn’t mean much, really. Follow that link and you might find a site designer, or someone’s personal list of favorite sites.

Including top-level meaningful keywords in your description is a helpful way of improving your document’s keyword optimization.

Using Targeted Words in Headers

Not too long ago, many major search engines began looking at content either right away, or after grabbing a meta description (should one exist). H1 level headers get a lot of attention from many engines, because from a structural standpoint they are meant to describe the most important topic on the page.

Once again, in the frenzy to increase ranking, suddenly pages were appearing with multiple H1 headers packed with keywords. This practice is not only considered bad form and just plain rude, but it doesn’t sit too well with those search engines using it as a legitimate means of addressing descriptions.

But the proper use of H1 elements can still help. That means one H1 element per page, tops. Add your keywords in an appropriate and clear manner, and you’re helping everyone, not just the site’s potential rank.

Bad example:

<h1>Welcome!</h1>

Good example:

<h1>Welcome to Tucson Homes, Tucson’s Luxury Real Estate Site</h1>

Sprinkling keywords liberally throughout all headers is helpful. Just be sure you aren’t going overboard to the point of losing the real meaning in your headings.

<meta name=”keywords” content=”indometric, indometric.com, html, xhtml, css, design, web design, development, web development, php” />

Unfortunately, this method, while easy to implement (just add as many keywords as you’d like, separated by commas, to the meta element), has been misused over time. Instead of sticking to those keywords relevant to their sites, some unscrupulous but imaginative folks decided to add keywords to a site that might have nothing to do with its relevance.

For example, say you’re working for a site that sells baby clothing and related items, and the person doing the meta keywords adds in words such as “sex” or “porn”—completely irrelevant and inappropriate for your site. Because sex-related keywords remain extremely popular search words, they often end up where they shouldn’t be.

Another unscrupulous bit is the addition of multiple instances of important words numerous times so that in a keyword value, the word “baby” might appear alone (rather than in various reasonable combinations) 100 times.

Search engines got savvy to this behavior and put their foot down by creating stricter rules and sometimes blocking any sites that were using poor practices.

This action has actually helped many working in Web development to be more aware of exactly the kind of keyword and keyword combinations they want. A good exercise is to sit down at your computer with a text editor at the ready,
and just begin to write out words and word combinations that might be things you would search for if looking for the given product or service your Web site represents.

You may also wish to have other members of your team add to the list. You can even ask people who useWeb sites but do not necessarily know anything about making them to come up with keywords and keyword combinations. Collect, review, and edit these down to the most descriptive and relevant options for use on your site.

While repeating a single keyword on its own within the meta element is considered a no-no, using the keyword in various combinations is not. For example, I might use “Web design” and follow it up with “Web development” and “Web usability.” I’ve used the word “Web” three times but in relevant, applicable ways. This helps with ranking and clarifying the types of work you do.

Another important means of refining keywords within your site is to match your keywords to actual data reported via the search engine in use on your site (if you have a search engine, that is). Many search applications provide reporting services so you can see your top requested searches and add terms to your meta element as necessary. Figure aboive shows a listing of search entries on KeywordCountry.com. I can see that there are several instances of suitable terms to add to my meta element, such as “laptop.”

Learn more about KeywordCountry here:

• How to Build Traffic
• How to Optimize Internal Webpages
• External Optimization Tricks
• Content that Search Engines Love
• Get Ideal content for websites

This article lets you in on the insider’s view of why so many Web marketing methods are problematic, and shows you the right way to use common techniques such as keyword and description tagging, how to expand and extend keywords within your content, how to get submitted to search engines in a timely fashion, how to get the word out to interested parties, and how to increase interest via sponsored links and offline promotional opportunities.

I will not teach you how to spam, use annoying ads, or compromise the integrity of the products and services you represent. Just apply a balance between great Web site design and customer service and you will never have to rely on gimmicks.

About Web Site Marketing

A couple of marketers decided to take their “guerrilla marketing” tactics to the Internet, persuading a lot of newcomers that aggressive marketing would make them profitable. In 1995 the Web itself was very young, and we were all trying to sort out the best way to do things. Some of these guerrilla ideas caught on in a big way, but almost always in detriment to the comfort and prosperity of all.

Guerrilla marketing tactics encouraged (and still encourage) e-mail spam, browser hijacking, overuse of ads (especially popups), and a range of adware that’s become a scourge to the good users of the Web. From a management and team perspective, marketing is often considered the most difficult aspect of a successfulWeb site team, as they tend to have conflicting goals. Marketing goals often conflict with technology goals, and there also tends to be a different focus in marketing. Their job is the sale, and while designers and developers seek to support that, sometimes the two conflict in their goals.

On the Web, no practice can work without the technology necessary to make it work. Technology and how well we use it makes up a significant portion of a site’s success. Of course, marketing and promotional methods are not all bad, and certainly many methods, such as regular specials or Web-specific coupons, can end up being extremely useful in successfully promoting your site. The best of marketing and promotion savvy, in my opinion, are those that relate to successful business strategy, great management, and workflow—all topics found elsewhere in this series article. Now Focus Is on Structure and Content While you will see how to use long-existing techniques such as meta keyword and description in this post, the real focus for today’sWeb sites is to ensure that they are structured and written so that search engine computers can successfully rank them.

By getting rid of deeply nested tables for layout, ensuring that style is handled with CSS, and making your sites accessible for the disabled, you open up your site to be quickly and easily catalogued by a search engine. Add to that effective titling of your documents, clear language use, and many other cues related to readability and usability, and the rest of the job is pretty straightforward.

A lot of information will try to steer you otherwise—toward paying money for additional software or services to help you get ranked. This is only one of the many myths that exist regarding most search engines and how they rank you.
Other myths include the following:

• A competitor can ruin your ranking. A competitor cannot purposely sabotage your ranking or remove you from a search engine under normal circumstances. However, if they have done a better job than you with the design of their site, or they have a very large amount of relevant links to their site, or they have a domain name that is very close to the main concept of the site, they might get a higher ranking (molly.com, for example, will rank higher on most search engines when people search for “molly”).
• You can pay your way to the top of Google. No, you can’t. You can buy sponsored links and advertising that will appear alongside or above searches of relevance to your site, but there are numerous factors that get sites “to the top.”
• Paid inclusion services are the way to go. No, they’re not. With very few exceptions, inclusion services are scams, and finding those exceptions is very difficult (see the next section).
• 35% off Yahoo! Merchant Solutions packages + no setup fee (save $50)

No matter what your site’spromotion goals, in the immortal words of rapper Chuck D, “Don’t believe the hype.”

Avoid Unscrupulous Marketing Techniques

Here’s a fact: No one can promise you top rankings on all major engines by doing anything to your site. I take that back! They can promise, and they do promise, but they don’t deliver. In the past few years, a rash of Search Engine Optimization (SEO) companies has emerged to “help” us optimize the code on our site and fix problems related to search engine rankings. Many SEOs are straight-out frauds, doing nothing for their clients or doing very little and calling it a day.

Of course, some of these companies aren’t fraudulent, but the proliferation of the bad guys is such that you as a consumer must be hyper-vigilant if you decide to use any of their services.

Google has an excellent overview of SEOs that provides guidance in how to choose one, and how to report complaints about any SEO in the United States or abroad. See www.google.com/webmasters/seo.html.

Thankfully, the techniques in this article, along with your well-designed and structured Web site will alleviate all of those woes. In fact, in just a few short pages, you’ll learn enough to open your own SEO should you desire a little time on the guerrilla-marketing front.

Martin and Emiko created a Web site (www.martin-emiko.net) to help keep their distant families informed about their lives and growing family. He’s from Switzerland and she’s from Japan, so they are always far away from someone in the family. In addition to a great collection of stories and photos, the use of a long image across the top of each page, shown in Figure, is a simple but dramatic design trick and an ideal way to showcase the many landscapes they’ve photographed in their travels.

Here are a few other family Web sites you may want to visit for ideas:

• The Demar Family Web site at members.cox.net/botiff features an animation on the front page that includes a recording of the voices of some family members. The combination of the mother’s voice welcoming you to the site and the kids making snide comments in the background as she talks provides a humorous introduction that draws you into the site and gives you a good idea about what to expect when you get there
• The Miller Family Web site at www.millfam.org boasts several sophisticated features, put together by using a combination of services. Check out the bulletin board for neighborhood and family events, and visit their content-managed site, as they call it, to see a family blog linked to a custom family Web site.
• The Bratz family Web site, at webpages.charter.net/bratz, features a fall sports schedule with a full calendar of games and other sporting events of interest to family and friends.
• Vicki and Tony’s Web site at www.geocities.com/vickips/ is an example of how much you can do with a good sense of humor, even on a free Web site like Geocities.

Travel sites around the world

The Mansz and McKerral family Web site at www.mansz.com, shown in Figure, is more than just a place to share photos of friends and family. They’ve created a site that’s a showcase of bird photos from their travels and a great resource for other birders. Whatever your hobby or passion, a family Web site can provide a great place so share your expertise and even make new friends.

Tribute sites to make you proud

Creating a Web site for friends or family members is a beautiful way to preserve their memory and pay tribute to their life. Figure features a simple Web page that my Uncle Tom created for my grandmother, Carol McCain. We all have fond memories of grandma’s house on the lake, where I learned to water ski and enjoyed sitting on the long deck staring out at the water and waiting for a chance to wave at the Dixie boat.

Millions of families have created Web sites and enjoy sharing their photos and stories. Rather than expressing concern about making their stories and photos public on the Web, many families report that they have made new friends and reconnected with old ones thanks to their Web sites. Here are some other things you can do to protect your personal information:

• Don’t include your home address or phone number on your Web site.
• Don’t include your work address or phone number.
• Be careful when writing about the times when you won’t be at home, such as the dates you’ll be on vacation.

If you want more control over your site and no ads, a Web site built with the templates included in this blog and hosted on a commercial service provider is a great option. With just a little knowledge of a program such as Adobe Dreamweaver, you can customize predesigned Web page templates. The beauty of creating your own site design with a program like Dreamweaver is that you can add as much text and as many images as you want, and you can change the design as much or as little as your talent and time allow.

When you use a program such as Dreamweaver (whether you use templates or not), you create and edit your Web pages on your own computer and then transfer them to the server when you’re ready to publish your site to the Web. The transfer process is easy and the benefit of building the site on your computer first is that you can test your work and experiment with different options before you publish your site on the server and make it available on the Internet.

No matter what option you choose, you can register your own domain name and point it to your Web site. This means you can direct people to your own special Web address, such as www.topweddingsites.com, whether you host your site at AngelFire.com, at www.myevent.com, or on your own server.

Best Practices and Models

A great Web site combines beautiful images and well-written text in a design that makes it easy to find your way to the most important information. But if you’re staring at a blank computer screen, creating a Web site can seem daunting. To help inspire you, this section presents a few well-designed or innovative Web sites. So sit back, relax, and get your browser ready to go for a ride — after reading the descriptions of these sites, you may want to see them for yourself.

Wedding sites that inspire

When Jonathon and Joanne decided to get married on a Caribbean island, they knew that creating a Web site would make it easier to coordinate the travel arrangements and accommodations for their guests. On the site, shown in Figure, they posted helpful information, such as the “Things to Bring” section, which includes passports, suntan lotion, and snorkel gear.

Jonathon and Joanne’s Web site was created using blogging software, which makes it easy to add updates (even from a tropical island). Studying other people’s wedding sites is a great way to see what you can do with your own site. You’ll find many more wedding sites online at www.topweddingsites.com.

To create a photo album, you just upload your images; they become instantly available on the Web to anyone with your user ID and password. (Passwords are required to ensure privacy so you can restrict who views your personal
images.) You don’t even have to optimize your images (optimizing involves reducing the file size of images so they download faster), so you won’t need an image program such as Photoshop. Indeed, sites such as Ofoto and Shutterfly encourage you to upload high-resolution images because the better the resolution, the better the print quality.

The following sites offer online photo album and printing services:

• Ofoto (www.0foto.com): Kodak’s online photo site makes it easy to upload images for free and share your photo album with friends and family. Like other photo sites, Ofoto makes money by charging for prints. At 29 cents per print, Ofoto is not the cheapest, but they boast that the archival quality of their pictures means they’ll last as long as conventional film prints. The site offers basic image editing and cropping tools and enables you or anyone with access to your site to create and order printed calendars and photo books from your pictures.
• Shutterfly (www.shutterfly.com): Shutterfly offers simple editing tools, enables you to post and share photos for free, and sells printing services. Shutterfly has an intuitive interface, and their specialized printing options enable you to turn your pictures into greeting cards, bound photo albums, personalized calendars, coffee mugs, T-shirts, or tote bags. Prints at Shutterfly cost 29 cents each.

• Yahoo Photos (photos.yahoo.com) offers online photo album services similar to Ofoto and Shutterfly, but their digital prints are priced at only 19 cents each.

Different types of sites

Here’s a brief list of some of the different kinds of Web sites you can create and the technical options that go with them:

• Online photo album sites: Even if you plan to create a more complex Web site later, you may want to set up an online photo album site because it’s one of the fastest and easiest ways to put photos online. Best of all, these photo sites are free, and they don’t bombard your pages with lots of advertising like the free Web site services at Geocities or Tripod. Photo album sites make their money by charging for prints, which they are happy to send to you or your loved ones for about 20 or 30 cents each. The most popular free online photo album sites, including Ofoto.com, shown in Figure 1-1, are featured in the “Photo album and printing services” sidebar.
• Free online services: If price is your biggest concern and you want to create a Web site, you may be pleased to find that several companies will let you publish your site for free. The catch is that these companies then sell advertising on your pages, and you have no control over what ads will run next to your words and pictures. Yahoo! features a free Web site creation tool at Geocities.com, and you’ll find similar solutions at Angelfire.com and Tripod.com. All three enable you to create template-based sites or to upload your own pages to their system (although they offer limited space and visits to your site). You find more about these and other Web server options in next article.
• Specialty Web-based solutions: The easiest way to create a professionallooking Web site for your family is to use an online service, such as family.myevent.com, as shown in Figure. You’ll find similar services at www.eweddings.com and www.babyjellybeans.com, but I chose to feature family.myevent.com because I think it’s one of the best in terms of price and ease of use and because you can create a variety of Web sites in one place (for weddings, babies, bar mitzvahs, or any other event). The family.myevent.com site offers a complete package of features, including planning tools, maps, and hotel reservation systems. Simply choose the template you want for your design, fill in a few forms, and upload your photos. You can create and publish a complete Web site in a matter of minutes.

• Blogging software: Millions of people now have blogs, or online journals, on the Internet. Their popularity has spawned a variety of software programs designed to facilitate easy updates, such as Blogger.com, as shown in Figure. A blog may be your best option if you want to make frequent updates to your Web site. One of the more famous family blogs is The Trixie Update, which Trixie’s stay-at-home dad updates many times a day to keep her mom informed while she’s at work. That site and many other blogging sites are featured in next article.
• Predesigned templates: I included with this book a collection of templates to make it easy for you to create a variety of common Web sites, such as a wedding site and a baby site. To use these templates, you need a Web design program, such as Dreamweaver, and an image program, such as Adobe Photoshop Elements 3. Although you have to know the basics of these programs to create a template-based site, the process is still much easier than creating a site from scratch. You also have more design control than you do if you use templates at an online service such as Tripod, where you can change only the content. When you edit templates in a program such as Dreamweaver, you have the option of altering the design as much or as little as you choose. Many Web design programs include templates you can use, and you can buy or download free templates from a variety of Web sites. To use the templates included on the CD, check out next article through 14.

An online service like this one makes it easy to create a family oriented Web site without special software or technical
expertise.

• Fully customized sites: If you use a program such as Macromedia Dreamweaver MX (which is what I used to create my own Web site, featured in Figure) or Dreamweaver, you can create a fully customized Web site. You’ll need more technical expertise to create a customized Web site than you need to use one of the template options, but you’ll have the greatest design control over your pages. Even if you start by using a template to create your site, you can always go back later and further customize your pages in one of these programs. However, just mastering the technical features of a Web design program doesn’t make you a great designer. Templates can help you avoid common design mistakes and create a professionallooking site, even if you can’t draw a stick figure.
• Database-driven sites: The most sophisticated Web sites on the Internet, such as the online store at Amazon.com or the news site at CNN.com, were created by using complex programming and databases. Combining a database that records information about users with the ability to generate pages automatically is what enables Amazon to greet you by name when you return to their site, track your orders as you buy books, and even make recommendations based on your previous purchases. I definitely don’t suggest that you begin with a custom database. You don’t need anything this sophisticated anyway, unless you want to sell lots of products or publish dozens of articles and photos to your site every day.

You don’t need advanced computer skills anymore to create a family Web site. Today, sharing photos online is as easy as pressing the Enter key to upload images to a photo site. And you can create a wedding or baby site by simply filling out a form. Even if you want to create a customized Web site, the software programs and resources available now make it easier than ever to personalize the way you share family stories and memories over the Internet.

In this article, I introduce you to the many ways you can build a Web site so you can better appreciate your options and decide which method is best for you. Then I take you on a tour of family Web sites so you can see what’s possible. Finally I compare the most popular Web design and image editing programs so you can find the best tools for your project.

Connecting Families Over the Web

My goal in this article is not just to help families create Web sites but to help people around the world get closer by sharing their stories and photos with loved ones over the Internet. Here are a few ways people like you are using the Internet to keep in touch:

• Baby sites: Many parents are using Web sites and blogs (online journals) to chronicle every milestone of their child’s life, from the first tooth to the first day of school. Next article features templates and tips for creating a baby Web site, and next article covers blogs, which are becoming increasingly popular on the Web.
• Sites for younger kids: Better than a bumper sticker, Web sites are a great way to showcase your favorite student’s work, clubs, after-school activities, hobbies, and more. Next article features templates and instructions for creating a sports or hobby Web site.
• Sites for older kids: Tech-savvy high-school kids are showing off with graduation sites, prom sites, and team sites. Creating a Web site is like having your own yearbook, only better. If you want to create your own completely customized site, see the introduction to Dreamweaver in next article.
• Travel sites: The days of vacation slide shows in the living room are fading fast. Today, families are creating vacation sites with online photo albums to share snapshots from their latest adventures.
• Wedding sites: Before they say ‘I do,’ more and more couples are building wedding Web sites that feature invitations, directions, guest registries, and more.

If you’re anxious to put up a Web site right away, skip ahead to one of the article listed in the preceding list. You find everything you need to get a site up quickly. If you have a little more time to think about your project or you want to create a more general family site, continue reading this blog to discover more about the range of options for creating a Web site.

Comparing Web Site Options

If you’ve never created a Web site before, you may not realize how many ways you can publish photos and other information on the Web, or how many software programs and service providers there are to choose from. You’ve probably seen Web sites in many styles, but you may not know that some of the differences in how those sites look depends on the type of technology used to create them.

Choosing how to create a Web site is an important first step. You can start simply, with an online photo album site or with an online service that does nearly everything for you. Or you can create a site based on templates, like the ones featured on the CD for next article, or you can create a completely customized Web site. This section provides a brief review of your options.

This article takes a modeling approach to the design of concurrent programs. Our models represent the behavior of real concurrent programs written in Java. The models abstract much of the detail of real programs concerned with data representation, resource allocation and user interaction. They let us focus on concurrency. We can animate these models to investigate the concurrent behavior of the intended program. More importantly, we can mechanically verify that a model satisfies particular safety and progress properties, which are required of the program when it is implemented. This mechanical or algorithmic verification is made possible by a model-checking tool LTSA (Labeled Transition System Analyzer). Exhaustive model checking using LTSA allows us to check for both desirable and undesirable properties for all possible sequences of events and actions. LTSA is available from the World Wide Web (http://www.wileyeurope.com/college/magee). As it has been implemented in Java, it runs on a wide range of platforms, either as an applet or as an application program.

The models introduced in previous article are based on finite state machines. Finite state machines are familiar to many programmers and engineers. They are used to specify the dynamic behavior of objects in well-known object-oriented design methods such as Booch (1986), OMT (Object Modeling Technique) (Rumbaugh, Blaha, Premerlani, et al., 1991) and, more recently, the all-encompassing UML (Unified Modeling Language) (Booch, Rumbaugh and Jacobson, 1998). They are also extensively used in the design of digital circuits – the original engineering use. For those not yet familiar with state machines, they have an intuitive, easily grasped semantics and a simple graphical representation. The state machines used in this article (technically, Labeled Transition Systems) have well-defined mathematical propertiepreviouss, which facilitate formal analysis and mechanical checking, thus avoiding the tedium (and error introduction) inherent in manual formal methods.

For instance, for the cruise control system described in section 1.1, we can model the various processes of the system as state machines. A state machine for the process responsible for obtaining the current speed is given in Figure 1.3. Starting from state(0), it indicates that once the engine is switched on, it transits to state(1) and can then repeatedly obtain a speed reading until the engine is switched off, when it returns to state(0). Other processes can be modeled similarly. We can compose the system from the constituent processes according to the proposed design structure, indicating the interactions between the processes. The advantage is that such models can be used to animate and check the behavior of the overall system before it is implemented. Figure 1.4 shows an animation of the model for the cruise control system. It clearly shows the problem encountered in our simulation: if the engine is switched off and on again when cruise control is enabled, the previous speed setting is resumed. Exhaustive analysis can be used to identify the problem under all possible situations. Furthermore, to help understand and correct the problem, the model checker produces the particular sequence of actions that led to it!

Figure 1.3: Speed input process.

Later article describe and illustrate how to provide and use such models to gain confidence in the correctness and validity of a proposed design. We illustrate how premature and erroneous implementations can be avoided by careful modeling and analysis. Further, we indicate how such models can be systematically transformed into Java programs. The cruise control system is fully described, modeled and implemented in next article.

Figure 1.4: Animation of the cruise control system.

Note that representing state machines graphically severely limits the complexity of problems that can be addressed. Consequently, we use a textual notation (Finite State Processes, FSP) to describe our models. The LTSA tool associated with the article translates FSP descriptions to the equivalent graphical description.

The article itself presents the initial models in both textual and graphical forms to enable the reader to become familiar with the meaning of FSP descriptions. Technically, FSP is a process calculus – one of a family of notations pioneered by Milner (1989), Calculus of Communicating Systems (CCS), and Hoare (1985), Communicating Sequential Processes (CSP), for concisely describing and reasoning about concurrent programs. The difference from these notations is largely syntactic: FSP is designed to be easily machine readable. Like CCS and CSP, FSP has algebraic properties; however, it is used in this article primarily as a concise way of describing Labeled Transition Systems.

The wide acceptance of Java with its in-built concurrency constructs means that concurrent programming is no longer restricted to the minority of programmers involved in operating systems and embedded real-time applications. Concurrency is useful in a wide range of applications where responsiveness and throughput are issues. While most programmers are not engaged in the implementation of safety critical systems such as Therac-25, increasing numbers are using concurrent programming constructs in less esoteric applications. Errors in these applications and systems may not be directly life-threatening but they adversely affect our quality of life and may have severe financial implications. An understanding of the principles of concurrent programming and an appreciation of how it is practiced are an essential part of the education of computing science undergraduates and of the background of software engineering professionals. The pervasive nature of computing and the Internet makes it also an important topic for those whose primary activity may not be computing but who write programs none the less.

Concurrent Programs

Most complex systems and tasks that occur in the physical world can be broken down into a set of simpler activities. For example, the activities involved in building a house include bricklaying, carpentry, plumbing, electrical installation and roofing. These activities do not always occur strictly sequentially, one after the other, but can overlap and take place concurrently. For example, the plumbing and wiring in a new house can be installed at the same time. The activity described by a computer program can also be subdivided into simpler activities, each described by a subprogram. In traditional sequential programs, these subprograms or procedures are executed one after the other in a fixed order determined by the program and its input. The execution of one procedure does not overlap in time with another. In concurrent programs, computational activities are permitted to overlap in time and the subprogram executions describing these activities proceed concurrently.

The execution of a program (or subprogram) is termed a process and the execution of a concurrent program thus consists of multiple processes. As we see later, concurrent execution does not require multiple processors. Interleaving the instructions from multiple processes on a single processor can be used to simulate concurrency, giving the illusion of parallel execution. Of course, if a computer has multiple processors then the instructions of a concurrent program can actually be executed in parallel rather than being interleaved.

Structuring a program as a set of concurrent activities or processes has many advantages. For programs that interact with the environment to control some physical system, the parallelism and concurrency in that system can be reflected in the control program structure. Concurrency can be used to speed up response to user interaction by offloading time-consuming tasks to separate processes. Throughput can be improved by using multiple processes to manage communication and device latencies. These advantages are illustrated in detail in next article. However, the advantages of concurrency may be offset by the increased complexity of concurrent programs. Managing this complexity and the principles and techniques necessary for the construction of well-behaved concurrent programs is the main subject matter of this blog.

In order to illustrate the need for a rigorous approach to concurrent program design and implementation, let us consider an example.

Consider an automobile cruise control system that has the following requirements. It is controlled by three buttons: resume, on and off (Figure 1.1). When the engine is running and on is pressed, the cruise control system records the current speed and maintains the car at this speed. When the accelerator, brake or off is pressed, the cruise control system disengages but retains the speed setting. If resume is pressed, the system accelerates or de-accelerates the car back to the previously recorded speed.

Figure 1.1: Cruise control system.

Our task is to provide a Java program that satisfies the specified requirements and behaves in a safe manner. How should we design such a program? What software processes should we construct and how should we structure them to form a program? How can we ensure that our program provides the behavior that we require while avoiding unsafe or undesirable behavior?

Given no guidance, we may be tempted simply to use previous design experience and construct the program as best as we can, using the appropriate Java concurrency constructs. To test the cruise control software, we could construct a simulation environment such as that illustrated in Figure 1.2. The website that accompanies this blog contains this environment as an interactive Java applet for use and experimentation (http://www.wileyeurope.com/college/magee). The buttons at the bottom of the display can be used to control the simulation: to switch the engine on or off; to resume or turn the cruise control system on or off ; and to press the accelerator or brake (simulated by repeatedly pressing the relevant button).

Figure 1.2: Simulation of the cruise control system.

The behavior of the system can be checked using particular scenarios such as the following:

• Is the cruise control system enabled after the engine is switched on and the on button is pressed?
• Is the cruise control system disabled when the brake is pressed?
• Is the cruise control system enabled when resume is then pressed?

However, testing such software is difficult, as there are many possible scenarios. How do we know when we have conducted a sufficient number of test scenarios?

For instance, what happens in the unlikely event that the engine is switched off while the cruise control system is still enabled? The system behaves as follows. It retains the cruise control setting, and, when the ignition is again switched on, the car accelerates so as to resume the previous speed setting!

Would testing have discovered that this dangerous behavior is present in the system? Perhaps, but in general testing is extremely difficult for concurrent programs as it relies on executing the particular sequence of events and actions that cause a problem. Since concurrent events may occur in any order, the problem sequences may never occur in the test environment, but may only show up in the deployed system, as with the Therac-25 machine.

There must be a better way to design, check and construct concurrent programs!