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<title>Distributed lock manager</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Distributed lock manager</span></span>

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<p>A <b>distributed lock manager</b> (<b>DLM</b>) runs in every machine in a cluster, with an identical copy of a cluster-wide lock database. <a href="Operating_system" title="Operating system">Operating systems</a> use lock managers to organise and serialise the access to resources. In this way a DLM provides <a href="Software_application" class="mw-redirect" title="Software application">software applications</a> which are <a href="Distributed_programming" class="mw-redirect" title="Distributed programming">distributed</a> across a cluster on multiple machines with a means to synchronize their accesses to <a href="Shared_resource" title="Shared resource">shared resources</a>.

</p><p>DLMs have been used as the foundation for several successful <a href="Clustered_file_system" title="Clustered file system">clustered file systems</a>, in which the machines in a <a href="Computer_cluster" title="Computer cluster">cluster</a> can use each other's storage via a unified <a href="File_system" title="File system">file system</a>, with significant advantages for performance and <a href="High-availability" class="mw-redirect" title="High-availability">availability</a>. The main performance benefit comes from solving the problem of <a href="Cache_coherency" class="mw-redirect" title="Cache coherency">disk cache coherency</a> between participating computers. The DLM is used not only for <a href="File_locking" title="File locking">file locking</a> but also for coordination of all <a href="Disk_(storage_device)" class="mw-redirect" title="Disk (storage device)">disk</a> access. <a href="VMScluster" title="VMScluster">VMScluster</a>, the first clustering system to come into widespread use, relied on the <a href="OpenVMS" title="OpenVMS">OpenVMS</a> DLM in just this way.

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<div class="mw-heading mw-heading2"><h2 id="Resources">Resources</h2></div>

<p>The DLM uses a generalized concept of a <b>resource</b>, which is some entity to which shared access must be controlled. This can relate to a file, a record, an area of <a href="Shared_memory" title="Shared memory">shared memory</a>, or anything else that the <a href="Application_software" title="Application software">application</a> designer chooses. A hierarchy of resources may be defined, so that a number of levels of locking can be implemented. For instance, a hypothetical <a href="Database" title="Database">database</a> might define a resource hierarchy as follows:

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<ul><li>Database</li>

<li>Table</li>

<li>Record</li>

<li>Field</li></ul>

<p>A <a href="Process_(computing)" title="Process (computing)">process</a> can then acquire locks on the database as a whole, and then on particular parts of the database. A lock must be obtained on a parent resource before a subordinate resource can be locked.

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<div class="mw-heading mw-heading2"><h2 id="Lock_modes">Lock modes</h2></div>

<p>A process running within a VMSCluster may obtain a lock on a resource. There are six lock modes that can be granted, and these determine the level of exclusivity being granted, it is possible to convert the lock to a higher or lower level of lock mode. When all processes have unlocked a resource, the system's information about the resource is destroyed.

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<ul><li>Null (NL). Indicates interest in the resource, but does not prevent other processes from locking it. It has the advantage that the resource and its <a href="#Lock_value_block">lock value block</a> are preserved, even when no processes are locking it.</li>

<li>Concurrent Read (CR). Indicates a desire to read (but not update) the resource. It allows other processes to read or update the resource, but prevents others from having exclusive access to it. This is usually employed on high-level resources, in order that more restrictive locks can be obtained on subordinate resources.</li>

<li>Concurrent Write (CW). Indicates a desire to read and update the resource. It also allows other processes to read or update the resource, but prevents others from having exclusive access to it. This is also usually employed on high-level resources, in order that more restrictive locks can be obtained on subordinate resources.</li>

<li>Protected Read (PR). This is the traditional <i>share lock</i>, which indicates a desire to read the resource but prevents other from updating it. Others can however also read the resource.</li>

<li>Protected Write (PW). This is the traditional <i>update lock</i>, which indicates a desire to read and update the resource and prevents others from updating it. Others with Concurrent Read access can however read the resource.</li>

<li>Exclusive (EX). This is the traditional <i>exclusive lock</i> which allows read and update access to the resource, and prevents others from having any access to it.</li></ul>

<p>The following <a href="Truth_table" title="Truth table">truth table</a> shows the compatibility of each lock mode with the others:

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<th>EX

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<th>NL

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<td style="background:#9EFF9E;color:black;vertical-align:middle;text-align:center;" class="table-yes">Yes

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<th>CR

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