LinkedIn is the career-oriented social network that prides itself on professional excellence. But the company's original mobile offering was anything but—it left much to be desired. There was an iPhone application, but no support for Android or tablets. The backend was a rickety Ruby on Rails contraption; afflicted with seemingly insurmountable scalability problems. And despite serving only seven or eight percent of the LinkedIn population, the company's original mobile build required approximately 30 servers in order to operate. This was clearly not made to sustain a growing mobile user base.

Now, a little over a year has passed since LinkedIn relaunched its mobile applications and website. And the company recently marked the anniversary by debuting a number of new mobile features, including real-time notifications and support for accessing company pages from mobile apps.

Mobile is gradually becoming a central part of the LinkedIn landscape. The company says roughly 23 percent of its users access the site through one of its mobile applications, up from ten percent last year. As our friends at Wired reported last month, the underlying design language and development philosophy behind the company’s mobile experience is playing an influential role as the company works to revamp its website.

No one knows more about this paradigm shift than Kiran Prasad, LinkedIn’s director of mobile engineering. Prasad was one of the key players in LinkedIn's efforts to resurrect its mobile offerings, and he was kind enough to walk Ars through the process. LinkedIn's mobile man provided a behind-the-scenes look at how the company built its mobile applications and the associated backend infrastructure, while also describing the design strategy the company used to build its mobile interfaces. As you'd probably expect from the brand, it was all a professional level effort.

Initiating an overhaul

LinkedIn decided it was time for a massive overhaul as the company began recognizing the increasingly important role that mobile access would play for the social network’s users. Jet-setting professionals rely on their smartphones to stay in touch while they travel, right?

Mobile soon became a major strategic focus for LinkedIn. In August of 2011, roughly five months after the mobile overhaul began, the company launched an all-new set of mobile apps powered by a completely new backend.

When the effort began, the mobile engineers at LinkedIn had several major goals. They wanted cross-platform compatibility, with support at launch for Android, iOS, and the mobile Web. They also wanted to simplify the user interface, taking the number of icons on the main screen down from 12 to three or four. Finally, they wanted a holistic rewrite—rebuilding both the frontend and backend together with an eye for boosting scalability.

Prasad previously worked on WebOS at Palm before joining LinkedIn. He brought a wealth of knowledge about building native-like user interfaces with modern Web standards. He regards the mobile Web as a platform in its own right, one that LinkedIn had to support well alongside Android and iOS. So Prasad decided that making HTML5 a central part of the company’s mobile strategy would make it easier to reach all of those environments.

HTML offers a useful way to reach more screens, but Prasad believes there are still many places where native user interfaces and native code are needed in order to deliver the best possible user experience. LinkedIn set out to use a hybrid model; blending the two to theoretically offer advantages of both.

This approach made it easier for a small team to support multiple platforms. And crucially, it allowed other LinkedIn developers outside of the mobile team to contribute to the effort using their existing skills.

Everything must be simple

Prasad told me that simplicity is at the heart of LinkedIn’s mobile vision. The company’s internal definition of simplicity, he said, holds that a good solution must be fast, easy, and reliable—in that specific order. Each of those characteristics is considered ten times more important than the next, making performance the chief concern. That philosophy motivated his team’s decisions in almost every area, ranging from visual design to backend engineering.

The thing people value most, according to Prasad, is their time. When users encounter flaws in software, they tend to be more forgiving if the software is extremely fast. A crash is less disruptive, for example, if the application is quick to restart and offers a quick path back to where the user was.

Speed is also especially important for mobile experiences in his eyes, because smartphone users tend to have shorter sessions (often less than three minutes long). Users expect applications to deliver relevant information as quickly as possible.

Ease of use was his next major priority, and Prasad said LinkedIn measures this by counting the number of clicks that it takes for a user to complete a given operation. If it takes more than three, he said, users are going to lose patience and easily be drawn away by push notifications or other things happening on their phone.

Reliability is the final of the three priorities. Basic robustness and stability are important, Prasad told me, but reliability also encompasses other ideas like consistency and predictability. To him, a reliable application is one the user can depend on to behave the same way every time. When the user taps a button and it takes them to another screen, for example, going back and tapping on the same button should take them there again. A surprising number of applications ignore that seemingly obvious design principle, Prasad said.

During the design process, LinkedIn used a simple metaphor to encourage predictability and ease of use. The idea is that an application is like a house—there are rooms with specific functions, and there are corridors that connect those rooms in a practical way. When you are putting together a room, you don’t fill it with an incongruous range of functions. You may end up with a living room, a bedroom, a kitchen, and a bathroom. But you don’t cook in the living room and you don’t sleep in the bathroom.

Continuing this house analogy, when you begin designing an application, you start by defining the structure. You determine what rooms you want, what their purposes will be, and how those rooms will be connected. You don’t start with the visuals (in the house analogy, these are like the carpeting or the paintings on the walls). When you introduce a feature, you think about the room in which it belongs.

For the mobile application, LinkedIn decided that it didn’t want more than four “rooms” of functionality. You can clearly see the house metaphor in action when you open the LinkedIn iPhone application. The main screen limits itself to four icons: your updates, profile, messages, and groups. It serves as the hallway, allowing the user to tap an item in order to enter a given room.