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A '''distributed lock manager''' ('''DLM''') runs in every machine in a cluster, with an identical copy of a cluster-wide lock database. [[Operating system]]s use lock managers to organise and serialise the access to resources. In this way a DLM provides [[software application]]s which are [[distributed programming|distributed]] across a cluster on multiple machines with a means to synchronize their accesses to [[shared resource]]s.
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, relies on the [[OpenVMS]] DLM in just this way.▼
▲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,
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:▼
▲===Resources===
* Database▼
* Table▼
▲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:
* Record▼
▲*Database
* Field▼
▲*Table
▲*Record
▲*Field
A [[process (computing)|process]] 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.
==
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 of been 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.▼
*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 [[#Lock value block|lock value block]] are preserved, even when no processes are locking it.▼
*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.▼
*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.▼
*Protected Read (PR). This is the traditional ''share lock'', which indicates a desire to read the resource but prevents other from updating it. Others can however also read the resource.▼
*Protected Write (PW). This is the traditional ''update lock'', 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.▼
▲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
*Exclusive (EX). This is the traditional ''exclusive lock'' which allows read and update access to the resource, and prevents others from having any access to it.▼
▲* 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 [[#Lock value block|lock value block]] are preserved, even when no processes are locking it.
▲* 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.
▲* 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.
▲* Protected Read (PR). This is the traditional ''share lock'', which indicates a desire to read the resource but prevents other from updating it. Others can however also read the resource.
▲* Protected Write (PW). This is the traditional ''update lock'', 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.
▲* Exclusive (EX). This is the traditional ''exclusive lock'' which allows read and update access to the resource, and prevents others from having any access to it.
The following [[truth table]] shows the compatibility of each lock mode with the others:
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==
A process can obtain a lock on a resource by ''enqueueing'' a lock request. This is similar to the [[QIO]] technique that is used to perform I/O. The enqueue lock request can either complete synchronously, in which case the process waits until the lock is granted, or asynchronously, in which case an [[Asynchronous System Trap|AST]] occurs when the lock has been obtained.
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It is also possible to establish a ''blocking AST'', which is triggered when a process has obtained a lock that is preventing access to the resource by another process. The original process can then optionally take action to allow the other access (e.g. by demoting or releasing the lock).
==
A lock value block is associated with each resource. This can be read by any process that has obtained a lock on the resource (other than a null lock) and can be updated by a process that has obtained a protected update or exclusive lock on it.
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It can be used to hold any information about the resource that the application designer chooses. A typical use is to hold a ''version number'' of the resource. Each time the associated entity (e.g. a database record) is updated, the holder of the lock increments the lock value block. When another process wishes to read the resource, it obtains the appropriate lock and compares the current lock value with the value it had last time the process locked the resource. If the value is the same, the process knows that the associated entity has not been updated since last time it read it, and therefore it is unnecessary to read it again. Hence, this technique can be used to implement various types of [[cache (computing)|cache]] in a database or similar application.
==
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=
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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Both [[Red Hat]] and [[Oracle Corporation|Oracle]] have developed clustering software for [[Linux]].
[[OCFS2]], the Oracle Cluster File System was added<ref>[
Red Hat's cluster software, including their DLM and [[GFS2]] was officially added to the Linux kernel <ref>[
Both systems use a DLM modeled on the venerable VMS DLM.<ref>[
==Other implementations==
[[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" />▼
Other DLM implementations include the following:
▲* [[Google]] has developed ''Chubby'', a lock service for loosely
* [[Apache ZooKeeper]], which was created at [[Yahoo]], is [[open-source software]] and can be used to perform distributed locks<ref name="zookeeper">[
* [[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 [[
==References==▼
▲[[Redis]] is an open source, BSD licensed, advanced key-value cache and store.<ref name="redis">http://redis.io/ Retrieved on 2015-04-14</ref> The [[Redlock Algorithm]] uses Redis for distributed lock management.<ref name="redlock">http://redis.io/topics/distlock Retrieved on 2015-04-14</ref>
▲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.
▲A DLM is also a key component of more ambitious [[Single-system image|single system image]] projects such as [[OpenSSI]].
▲==References==
{{Reflist}}
{{Refbegin}}
* [https://web.archive.org/web/20101219074254/http://h71000.www7.hp.com/doc/
* [
* [
{{Refend}}
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