This will be done by the coordinating shard fetching the collection validator from it’s collection catalog. The coordinator will then preform a simple aggregation command with limit:1 to find an invalid document. db.products.aggregate([ { $match: {$nor: [

] }, {$limit: 1} ], {hint: {$natural: 1}}). $natural is needed to force a collection scan to prevent an index scan skipping documents.