When Google rolls out its mobile-first index, it will begin by indexing individual sites that are ready for it.
This was confirmed by John Mueller in a recent Google Webmaster Central office-hours hangout.
Mueller went on to say sites may or may not be informed the mobile-first switch has changed for them. This presents a challenge when it comes to tracking analytics and ranking positions.
On the positive side, with Google rolling out the mobile-first index on a per-site basis, those that are prepared have a good chance of being indexed before others.
Those that aren’t ready for the mobile-first switch will have to wait a bit longer to be indexed, Mueller says.
This can be seen a positive thing for both searchers and site owners. Searchers will see only the most mobile-friendly sites, as intended, while site owners who have put in work preparing for the mobile-first index will be rewarded with priority indexing.
Mueller kept his answer brief, while stating Google will have more to announce about this in the future.
There is no new news with respect to when the mobile-first index will roll out. The last we have heard from Google regarding the mobile-first switch is that it will not occur until 2018.
You can listen to Mueller’s full response in the video below, starting at the 3:35 mark.