Solr's default response format is still XML, despite the fact that Solr has supported the JSON response format for over a decade, developer mindshare has clearly shifted toward JSON over the years, and most modern/competing systems also use JSON format now by default.
In fact, Solr's admin UI even explicitly adds wt=json to the request (by default in the UI) to override the default of wt=xml, so Solr's Admin UI effectively has a different default than the API.
We have now introduced things like the JSON faceting API, and the new more modern /V2 apis assume JSON for the areas of Solr they cover, so clearly we're moving in the direction of JSON anyway.
I'd like propose that we switch the default response writer to JSON (wt=json) instead of XML for Solr 7.0, as this seems to me like the right direction and a good time to make this change with the next major version.
Based upon feedback from the Lucene Dev's mailing list, we want to:
1) Change the default response writer type to "wt=json" and also change to "indent=on" by default
2) Make no changes on the update handler side; it already works as desired (it returns the response in the same content-type as the request unless the "wt" is passed in explicitly).
3) Keep the /query request handler around since people have already used it for years to do JSON queries
4) Add a commented-out "wt=xml" to the solrconfig.xml as a reminder for folks on how to change (back) the response format.
The default format change, plus the addition of "indent=on" are back compat changes, so we need to make sure we doc those clearly in the CHANGES.txt. There will also need to be significant adjustments to the Solr Ref Guide, Tutorial, etc.