Where to place logic for loading initial server data in React app?

Edit: Dan Abramov recently stated

In future versions of React we expect that componentWillMount will fire more than once in some cases, so you should use componentDidMount for network requests.

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In componentDidMount

Read here.

Fetch data in componentDidMount. When the response arrives, store the data in state, triggering a render to update your UI.

When fetching data asynchronously, use componentWillUnmount to cancel any outstanding requests before the component is unmounted.

Documentation is really scarce on "why in componentDidMount". I believe componentWillMount is not called if you use server-side render, so this could be a reason why componentDidMount is preferred.