An application may need to perform small irregular changes in an XML document. It can be time-consuming to create a modified copy of a whole document for making each small change. It can be also hard to express condition for making the change in terms of XPath in order to use XSLT or XQuery.

Virtuoso offers a way to modify an existing XML entity by local operations like "insert a subtree before current node" or "replace one subtree with a given one". These operations are called DOM operations because they are similar to JavaScript operations with the Document Object Model.

DOM operations are the only operations that change an existing XML document instead of creating a new modified document. It is very important to understand that one document can be shared between many XML entities so changes made via one entity may affect data accessible via other entities that share the same document. Every XML document has special flags to "lock" its content and thus prevent the content from being changed. E.g. XSLT and XQuery processors optimize access to the source document in hope that every access to any entity will produce the same result during the whole run of the processor. If an extension function will modify the source document in the middle then the result of the processing is probably corrupted and this error may be very hard to detect and debug. To prevent this sort of problems, the XSLT processor locks the source at start and unlock at end so any attempt to modify the source during the run will signal an error. The document can be locked in the following cases:

One additional issue of using DOM operations is that any DOM operation invalidates DTD and schema validation information that was associated with the document before the operation.