The distributed lock manager (DLM) provides functions that allow cooperating processes in a cluster to synchronize access to a shared resource, such as a raw disk device or a program. For the DLM to effectively synchronize access to a shared resource, all processes in the cluster that share the resource must use DLM functions to control access to the resource. For example, a distributed database application might use lock manager services to coordinate access to the shared disks participating in a database.
An application secures a lock on a named shared resource. Resource names can be single-dimensional or tree-structured. A resource tree allows you to create a hierarchy of locks and sublocks that reflect the structure of a shared resource. The DLM supplies functions that:
Provide mutual exclusion, restricted sharing, and full sharing of data access
Notify a process holding a lock when its lock is blocking another process's access to a resource
Notify a process that has queued a lock request to a resource when its request has been granted
Convert a lock's mode between less restrictive and more restrictive lock modes
Return information about locks
The DLM employs a distributed, centralized tree design. It does not replicate lock information on each cluster member. Rather, the cluster member that manages a lock tree maintains all information about that tree. The member that holds a given lock is aware of only its contribution of that lock to the resource. Any member system can serve as the master for any lock tree, which distributes the overall lock management load.
The DLM uses a distributed directory service to quickly locate the directory node for a resource tree. A directory table associates a root resource name with the cluster member that is the manager of the resource. This directory table is identical on all cluster members.
The DLM is designed to handle member failures. If a lock holder fails, its locks are released. If a member system fails, a new lock master for locks previously mastered on that member is chosen and provided with all pertinent lock information.