Distributed lock manager

Operating Systems use lock managers to organise and serialise the access to resources. A distributed lock manager (DLM) runs in every machine in a cluster, with an identical copy of a cluster-wide lock database. In this way a DLM provides software applications which are distributed across a cluster on multiple machines with a means to synchronize their accesses to shared resources.

DLMs have been used as the foundation for several successful clustered file systems, in which the machines in a cluster can use each other's storage via a unified file system, with significant advantages for performance and availability. The main performance benefit comes from solving the problem of disk cache coherency between participating computers. The DLM is used not only for file locking but also for coordination of all disk access. VMScluster, the first clustering system to come into widespread use, relies on the OpenVMS DLM in just this way.

DEC's VMS (virtual memory system) was the first widely available operating system to implement a DLM. This became available in Version 4, although the user interface was the same as the single-processor lock manager that was first implemented in Version 3.

The DLM uses a generalized concept of a resource, which is some entity to which shared access must be controlled. This can relate to a file, a record, an area of shared memory, or anything else that the application designer chooses. A hierarchy of resources may be defined, so that a number of levels of locking can be implemented. For instance, a hypothetical database might define a resource hierarchy as follows:

...
Wikipedia