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Mobile archiving service Evernote is throttling access to third-party applications which use its application programming interface (API) - and admits that apps that sync Evernote data will fall foul of the new rules.

The restriction takes the form of a ceiling on the number of API calls an hour, rather than a bandwidth cap. It comes into effect immediately for new developers, and will affect all existing production apps by November.

“Third-party applications that fully synchronize user information will almost certainly exceed the rate limits,” says the company in a blog post. “If your integration fully syncs a user's account, get in touch with us to discuss an increased rate limit for your API key.”

Evernote has been a stealth success story, with over 50 million users storing their unstructured data on the Evernote service.

The software and archive suite's rich API is used by dozens of third-party applications.

Not only is the API flexible and feature-rich, but Evernote has also been fairly generous. Generous enough to allow third-party clients to be developed for platforms where it does not provide a native client (e.g. Everpad for Linux, and the excellent Notekeeper on Symbian Nokia Belle).

And even where Evernote does provide a branded native app, many users prefer a third-party alternative such as Clearly for the iPad. These will need to be rejigged. So, is Evernote “pulling a Twitter” and clamping down on development?

It seems unlikely. Evernote wholesales a service as well as providing its own retail application, and it gives every indication that it knows its runaway success owes a lot to the popularity of these third-party apps and hardware support.

Evernote data is largely accessed solely by the account-holder, in contrast to data on Twitter, which is a public-facing communication service where the account-holder's data might be accessed by millions, and therefore a rogue app could spam some 200 million users. So panicking may be premature.

There are more technical details on the developer blog here. ®

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