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DLMs have been used as the foundation for several successful [[clustered file system]]s, in which the machines in a [[computer cluster|cluster]] can use each other's storage via a unified [[file system]], with significant advantages for performance and [[high-availability|availability]]. The main performance benefit comes from solving the problem of [[cache coherency|disk cache coherency]] between participating computers. The DLM is used not only for [[file locking]] but also for coordination of all [[disk (storage device)|disk]] access. [[VMScluster]], the first clustering system to come into widespread use, relied on the [[OpenVMS]] DLM in just this way.
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== Resources ==
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 software|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:
* Database
* Table
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== Deadlock detection ==
When one or more processes have obtained locks on resources, it is possible to produce a situation where each is preventing another from obtaining a lock, and none of them can proceed. This is known as a [[Deadlock (computer science)|deadlock]] ([[E. W. Dijkstra]] originally called it a [[Deadlock (computer science)|deadly embrace]]).<ref>{{cite book|page=105|url=https://books.google.com/books?id=E6wOK8OYBG4C&pg=PA105|title=Ada: Concurrent Programming|isbn=9780929306087|
A simple example is when Process 1 has obtained an exclusive lock on Resource A, and Process 2 has obtained an exclusive lock on Resource B. If Process 1 then tries to lock Resource B, it will have to wait for Process 2 to release it. But if Process 2 then tries to lock Resource A, both processes will wait forever for each other.
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[[OCFS2]], the Oracle Cluster File System was added<ref>[https://www.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=29552b1462799afbe02af035b243e97579d63350 kernel/git/torvalds/linux.git - Linux kernel source tree]{{Dead link|date=July 2019 |bot=InternetArchiveBot |fix-attempted=yes }}. Kernel.org. Retrieved on 2013-09-18.</ref> to the official [[Linux kernel]] with version 2.6.16, in January 2006. The alpha-quality code warning on OCFS2 was removed in 2.6.19.
Red Hat's cluster software, including their DLM and [[GFS2]] was officially added to the Linux kernel <ref>[https://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=1c1afa3c053d4ccdf44e5a4e159005cdfd48bfc6 kernel/git/torvalds/linux.git - Linux kernel source tree] {{Webarchive|url=https://archive.
Both systems use a DLM modeled on the venerable VMS DLM.<ref>[https://lwn.net/Articles/137278/ The OCFS2 filesystem]. Lwn.net (2005-05-24). Retrieved on 2013-09-18.</ref> Oracle's DLM has a simpler API. (the core function, <code>dlmlock()</code>, has eight parameters, whereas the VMS <small><code>SYS$ENQ</code></small> service and Red Hat's <code>dlm_lock</code> both have 11.)
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* [[Google]] has developed ''Chubby'', a lock service for loosely coupled distributed systems.<ref name="chubby">[http://research.google.com/archive/chubby.html Google Research Publication: Chubby Distributed Lock Service]. Research.google.com. Retrieved on 2013-09-18.</ref> It is designed for coarse-grained locking and also provides a limited but reliable distributed file system. Key parts of Google's infrastructure, including [[Google File System]], [[Bigtable]], and [[MapReduce]], use Chubby to synchronize accesses to shared resources. Though Chubby was designed as a lock service, it is now heavily used inside Google as a [[name server]], supplanting [[Domain name system|DNS]].<ref name="chubby" />
* [[Apache ZooKeeper]], which was created at [[Yahoo]], is [[open-source software]] and can be used to perform distributed locks<ref name="zookeeper">[https://zookeeper.apache.org/doc/r3.1.2/recipes.html]. Zookeeper.apache.org. Retrieved on 2013-09-18.</ref> as well.
* [[Etcd]] is open-source software, developed at [[CoreOS]] under the [[Apache License]].<ref name="CoreOS">{{cite web|url=https://coreos.com/|title=CoreOS|website=coreos.com}}</ref> It can be used to perform distributed locks as well.<ref name="etcd">{{Citation|title=etcd: Distributed reliable key-value store for the most critical data of a distributed system|date=2018-01-16|url=https://github.com/coreos/etcd|publisher=CoreOS|accessdate=2016-09-20}}</ref>
* [[Redis]] is an open source,
* HashiCorp's [[Consul (software)|Consul]],<ref name="consul">[https://consul.io/ Consul Overview]. Retrieved on 2015-02-19.</ref> which was created by [[HashiCorp]], is open-source software and can be used to perform distributed locks as well.
* Taooka distributed lock manager<ref name="taooka">[http://taooka.com/ Taooka Description] {{Webarchive|url=https://web.archive.org/web/20170503112902/http://taooka.com/ |date=2017-05-03 }} Retrieved on 2017-05-04.</ref> uses the "try lock" methods to avoid [[Deadlock (computer science)|deadlock]]s. It can also specify a TTL for each lock with nanosecond precision.
* A DLM is also a key component of more ambitious [[single system image]] (SSI) projects such as [[OpenSSI]].
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* [https://web.archive.org/web/20101219074254/http://h71000.www7.hp.com/doc/82final/4527/4527pro_044.html#jun_227 HP OpenVMS Systems Services Reference Manual – $ENQ]
* [https://github.com/chadrem/officer Officer - A simple distributed lock manager written in Ruby]
* [
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