Google has announced pricing for its App Engine service for web application hosting, maintaining a free tier, then charging for storage of more than 500MB of data.

The company at its Google I/O conference in San Francisco also will expand access to the service and add APIs for caching and image manipulation. The service has been in a limited preview period since early April.

The pricing plan has developers still getting around 500MB of data and 5 million page views per month for free. After that, developers will pay 15 US cents (8p) to 18 cents (9p) per GB of data stored monthly.

Also, developers will be charged 10 cents to 12 cents per CPU core-hour consumed. On the bandwidth side, developers will pay 11 cents to 13 cents per month per GB of data transferred out of App Engine and 9 cents to 11 cents per month per GB transferred into App Engine, Google said. The pricing schedule is to be effective later this year.

Through its App Engine programme, Google is looking to address the issue of it being too difficult to develop web applications. "With App Engine, we hope to reduce that difficulty," by making the development experience easier and removing the startup costs, said Pete Koomen, product manager for App Engine at Google.

App Engine also will allow anyone to use the service, expanding beyond a list of 10,000 developers that gradually had grown to 60,000 developers. "We've decided to open the floodgates," Koomen said.

More than 150,000 developers have been on the product's waiting list.

With App Engine, developers do not need to concern themselves with such functions as provisioning of machines. "There's a lot there, and it's often very time-consuming," as well as costing money, Koomen said.

Google's caching API for App Engine will make it faster for developers to render applications. An image manipulation API allows developers to transform images like JPEG images in their applications, Koomen said. These images can be resized or rotated.

With App Engine, Google seeks to make the network the centre of power in computing, shifting it away from the desktop, said analyst Al Hilwa, program director for application development programmes at IDC. "That's their strategy, that they're moving the centre of power," he said.

Google's price tags for its App Engine services sound reasonable, Hilwa said. Smaller companies and departments in established companies could be the primary benefactors of App Engine, but risks need to be assessed in farming out web operations to another party.

"There's always risk, and I think the biggest risk is you've got infrastructure that is being run - something strategic for you as a company - that's being run by someone else," said Hilwa. Google's interest in the service might change over time, Hilwa said.

App Engine is hosted on the same infrastructure as Google's own applications. The only language supported on App Engine currently is Python, but the company is set to expand to other languages, Koomen said. "We think [Python is] a very natural language to write web apps in," said Koomen.

Google cites competitors like Amazon Elastic Compute Cloud service but said the differentiator is App Engine's focus on web development. "It's a platform aimed specifically at making web development easier," Koomen said. Developers have the option of using Google's AdSense service, an advertising platform for web publishers.

Also at the conference, Google is unveiling GWT (Google Web Toolkit) Release Candidate 1.5. GWT enables developers to develop and debug web applications in Java and then deploy them as JavaScript. This sidesteps common AJAX problems like browser compatibility and provides performance and productivity gains, Google said.

Version 1.5 includes Java 5 language capabilities, including Java 5 syntax support. Java generics, enumerated types, annotations, auto-boxing, and variable parameter lists are enabled as part of Java 5 backing.

Also, the compiler in version 1.5 produces faster code. This is done by performing deep inlining, better dead code elimination, and other forms of enhanced static analysis, Google said.

GWT features libraries to build AJAX, including reusable libraries for implementing user interfaces, data structures, client/server communication, internationalisation, and accessibility.