Source.scala

def source(client: MongoClient): Source[Document, NotUsed] = {
  val observable = client.getDatabase("local")
                        .getCollection("oplog.rs")
                        .find(and(
                              in("op", "i", "d", "u"),
                              exists("fromMigrate", false)))
                        .cursorType(CursorType.TailableAwait)
                        .noCursorTimeout(true)

  Source.fromPublisher(observable)
}

val sources = shards.map({ shard =>
  val client = MongoClient(shard.uri)
  source(client)
})