when data , such as tool data , is commonly accessible , there is a danger that concurrent user processes will make overlapping modifications and leave the data in an incomplete or incomprehensible state . the invention provides a locking mechanism to guard against this . the lock objects are maintained in pool or repository . two types of lock objects are provided ; shareable locks that may be shared by many user processes and permit only read access to the locked data , and exclusive locks , useable by only one user process at a time , that permit the owning process to modify the locked data .