A method, apparatus, and computer readable article of manufacture for tracing, on the basis of a key, an index created using a tree structure to access data. The method includes: searching the tree structure to reach a target node that can be associated with the key without acquiring a lock on the root node and on an internal node; setting the target node as a current node when the target node is a leaf node; moving the search to a node at a lower layer when the target node is an internal node by tracing an edge in the tree structure to reach a leaf node and setting the leaf node as the current node; and associating the key with the leaf node specified as the current node by acquiring a lock on the leaf node specified as the current node, thereby accessing the data.