Locking and Unlocking Resources

Sun Java System Web Server allows the server administrator to lock a resource so as to serialize access to that resource. Using a lock, a user accessing a particular resource is reassured that another user will not modify the same resource. In this way, the "lost updates" problem is resolved as multiple users share resources on the server. The lock database maintained by the server keeps track of the lock tokens issued and in use by clients.

Sun Java System Web Server supports the opaquelocktoken URI scheme, which is designed to be unique across all resources for all time. This uses the Universal Unique Identifier (UUID) mechanism, as described in ISO-1157.08.

Sun Java System Web Server recognizes two types of locking mechanisms:

• Exclusive Locks.

• Shared Locks.

Exclusive Locks

An exclusive lock is a lock that grants access to a resource to only a single user. Another user can access the same resource only after the exclusive lock on the resource is removed.

Exclusive locking sometimes proves to be too rigid and expensive a mechanism for locking resources. For example, in the event of a program crash or the lock owner forgetting to unlock the resource, a lock timeout or the administrator’s intervention will be required to remove the exclusive lock.

Shared Locks

A shared lock allows multiple users to receive a lock to a resource. Hence any user with appropriate access can get the lock.

When using shared locks, lock owners may use any other communication channel to coordinate their work. The intent of a shared lock is to let collaborators know who else may be working on a resource.