The days when an API was an unusable developer tie-in to a boxed software product have given way to a new age of Web-based services build upon RESTful principles, and many enterprises now consider them to be must-have products.

Sam Ramji, vice president of strategy at Apigee, said that many companies are asking for services related to API implementation. He said requests range from “How can we look at this API as a business channel?” to “Can you write our API for us?” But no matter what the questions, Ramji said API requests aren’t just coming from developers anymore.

“The language of APIs is reaching the boardroom. We’re working with one customer, a multi-billion-dollar financial services company, and their CEO is the person asking, ‘What’s the API strategy?’ ” he said.

Oren Michels, CEO and cofounder of Mashery, said that APIs are now seen as a way to make money, even if they aren’t monetized directly. “The biggest question we get is, ‘How is this going to make me money?’ We’ve gone past the era of ‘I want to do an API because the cool kids are doing it,’ ” he said.

“It really is now at a point where the people who are generating revenue for their companies through the Internet are figuring out how that happens in the post-website era. What the enterprises are starting to figure out is how the usage patterns of the Internet are changing, and how do I make sure I am able to serve customers, users, and suppliers in the way they need to be served?”

Of course, implementing APIs is only part of the battle. Ramji said that there are a number of considerations to take into account when building and maintaining an API. Perhaps the most important of these is the stability of how an API works.

“There’s a set of best practices around versioning,” he said. “You will introduce instability if you don’t have a way to isolate old clients from new clients. Include the API version number in the API call, and people won’t have applications break when you update your API. It’s really smart to use a reverse proxy, because the current generation of load balancers doesn’t do a great job with API requests. API requests are small. They’re not HMTL. They’re mostly in JSON or XML, and there are different security rules around what you’re willing to cache: whether you can cache the whole thing or whether different parts of your API are cached differently.

“Some are closer to real time, and your response will change every few seconds, whereas others are cachable for 30 minutes. Social network data is changing constantly, while weather information can be updated every hour.”

The future of API development is bright, said Michels and Ramji. Ramji predicted that the tools will continue to expand and become more widely applicable to developing and testing APIs.

“A couple weeks ago, we launched a product called Apigee-To-Go, which is taking the API console we’ve built and making that embeddable like a YouTube video, or a SlideShare document,” he said. “APIs are social objects, and they’re best rendered through interactive media techniques.”

Michels, on the other hand, advocated the tracking of API users as they move through multiple channels. “We’ll see the evolution from APIs being mostly around content to where APIs are actually about transactions,” he said.

“It’s one thing to be Best Buy, and through this content API, we’re able to get some interesting stuff to happen. It’s something else for the API to power an application where the purchase can be made in-app, and the API delivers not traffic but orders. In the next few months, you’re going to see more companies emerging with the capabilities of actually handling legitimate transactions through the API. And then at that point the question of value and monetization becomes even more obvious.”