A data document is constructed by attaching a set of child nodes, selected from nodes in an existing data document or a document fragment, at an update location, which is a node in the existing data document or the document fragment that is identified by a navigation path. When the update command is a merge, one or mode nodes ("merge nodes") in the document fragment are identified as update locations, and the nodes in the existing data document are merged as child nodes of the merge nodes in the document fragment. When the update command is a join, a node ("join node") in the existing data document is identified as the update location, and an ordered list specifies which nodes in the existing data document and which nodes in the document fragment are to be joined as the set of child nodes for the join node. The join and merge update commands, navigation paths, and document fragments may transmitted as a series of fragment update units to construct the data document.