A method for modifying key-value pairs of a B+ tree is provided. The method receives a request to modify a particular key-value pair. Each node of the tree has a modification number. The method traverses a path on the tree from the root node toward the particular node. The traversing includes upon reaching a parent node of the path, acquiring a shared lock on both the parent node and a child node one level below the parent node. Upon determining that the child node is the particular node, the method stores the modification number of the particular node, releases the shared lock on the particular node, compares a current modification number of the node with its stored number, and acquires an exclusive lock on the node if the numbers are the same. The method increments the current modification number of the node and modifies it while in the exclusive lock.