Of course there needs to be some common core, at least conceptually, for such a distributed search to make sense, but a lot of it can be handled without changing the end point indexes: There is no real need for globally unique identifiers, as the addition and removal of site-specific prefixes can be handled in the aggregator. Likewise, variations in field names or even terms can be handled by mapping. One big challenge is relevance ranking, but SOLR-1632 should help there, coupled with aggregator-dictated weighting of the different sites to compensate for local boosting. We do grouping across one Solr instance which has the grouping field and one which does not, compensating by presenting the document results from the missing-field-Solr as single-entry groups.

Most important parts of the Solr API are fairly stable so the major missing piece seems to be a Solr-bridge, exposed as a plain shard but internally taking care of the mapping and the calls to a remote Solr.

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