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{{Short description|Operating system designed to operate on multiple systems over a network computer}}
A '''distributed operating system''' is system software over a collection of independent software, [[Computer network|networked]], [[Inter-process communication|communicating]], and physically separate computational nodes. They handle jobs which are serviced by multiple CPUs.<ref name="Tanenbaum1993">{{cite journal |last=Tanenbaum |first=Andrew S |date=September 1993 |title=Distributed operating systems anno 1992. What have we learned so far? |journal=Distributed Systems Engineering |volume=1 |issue=1 |pages=3–10 |doi=10.1088/0967-1846/1/1/001|bibcode=1993DSE.....1....3T |doi-access=free }}</ref> Each individual node holds a specific software subset of the global aggregate operating system. Each subset is a composite of two distinct service provisioners.<ref name="Nutt1992">{{cite book|last=Nutt|first=Gary J.|title=Centralized and Distributed Operating Systems|url=https://archive.org/details/centralizeddistr0000nutt |url-access=registration|year=1992|publisher=Prentice Hall|isbn=978-0-13-122326-4}}</ref> The first is a ubiquitous minimal [[kernel (operating system)|kernel]], or [[microkernel]], that directly controls that node's hardware. Second is a higher-level collection of ''system management components'' that coordinate the node's individual and collaborative activities. These components abstract microkernel functions and support user applications.<ref name="Gościński1991">{{cite book|last=Gościński|first=Andrzej|title=Distributed Operating Systems: The Logical Design|url=https://books.google.com/books?id=ZnYhAQAAIAAJ|year=1991|publisher=Addison-Wesley Pub. Co.|isbn=978-0-201-41704-3}}</ref>
The microkernel and the management components collection work together. They support the system's goal of integrating multiple resources and processing functionality into an efficient and stable system.<ref name="Fortier1986">{{cite book|last=Fortier|first=Paul J.|title=Design of Distributed Operating Systems: Concepts and Technology|url=https://books.google.com/books?id=F7QmAAAAMAAJ|year=1986|publisher=Intertext Publications|isbn=9780070216211}}</ref> This seamless integration of individual nodes into a global system is referred to as ''transparency'', or ''[[single system image]]''; describing the illusion provided to users of the global system's appearance as a single computational entity.<!-- is transparency required for membership in the "dos" group?-->
{{TOC limit|3}}
==Description==
[[File:OS-structure2.svg|thumb|right|400px|Structure of monolithic kernel, microkernel and hybrid kernel-based operating systems]]
A distributed OS provides the essential services and functionality required of an OS but adds attributes and particular [[Computer configuration|configurations]] to allow it to support additional requirements such as increased scale and availability. To a user, a distributed OS works in a manner similar to a single-node, [[Monolithic kernel|monolithic operating system]]. That is, although it consists of multiple nodes, it appears to users and applications as a single-node.
Separating minimal system-level functionality from additional user-level modular services provides a "[[separation of mechanism and policy]]". Mechanism and policy can be simply interpreted as "what something is done" versus "how something is done," respectively. This separation increases flexibility and scalability.<!-- is this separation part of the definition or a desirable feature? isn't the reason for separating the kernel from services about hardware independent services rather than anything to do with scalability and flexibility? e.g., monolithic software can still support distributed requirements. and it's not clear what policy has to do with this topic. -->
==Overview==
===The kernel===
At each [[Locale (computer hardware)|locale]] (typically a node), the kernel provides a minimally complete set of node-level utilities necessary for operating a node's underlying hardware and resources. These mechanisms include allocation, management, and disposition of a node's resources, processes, communication, and [[input/output]] management support functions.<ref name="Hansen2001">{{cite book|editor=Hansen, Per Brinch|title=Classic Operating Systems: From Batch Processing to Distributed Systems|url=https://books.google.com/books?id=-PDPBvIPYBkC|year=2001|publisher=Springer|isbn=978-0-387-95113-3}}</ref> Within the kernel, the communications sub-system is of foremost importance for a distributed OS.<ref name="Gościński1991"/>
In a distributed OS, the kernel often supports a minimal set of functions, including low-level [[address space]] management, [[thread (computing)|thread]] management, and [[inter-process communication]] (IPC). A kernel of this design is referred to as a [[microkernel]].<ref>Using LOTOS for specifying the CHORUS distributed operating system kernel Pecheur, C. 1992. Using LOTOS for specifying the CHORUS distributed operating system kernel. Comput. Commun. 15, 2 (Mar. 1992), 93-102.</ref><ref>COOL: kernel support for object-oriented environments Habert, S. and Mosseri, L. 1990. COOL: kernel support for object-oriented environments. In Proceedings of the European Conference on Object-Oriented Programming on Object-Oriented Programming Systems, Languages, and Applications (Ottawa, Canada). OOPSLA/ECOOP '90. ACM, New York, NY, 269-275.</ref> Its modular nature enhances reliability and security, essential features for a distributed OS.<ref name="Sinha1997">{{cite book|last=Sinha|first=Pradeep Kumar |title=Distributed Operating Systems: Concepts and Design|url=https://archive.org/details/distributedopera0000sinh|url-access=registration|year=1997|publisher=IEEE Press|isbn=978-0-7803-1119-0}}</ref>
[[Image:System Management Components.PNG|thumbnail|right|175px|alt=General overview of system management components that reside above the microkernel.|System management components overview]]
===
The distributed nature of the OS requires additional services to support a node's responsibilities to the global system. In addition, the system management components accept the "defensive" responsibilities of reliability, availability, and persistence. These responsibilities can conflict with each other. A consistent approach, balanced perspective, and a deep understanding of the overall system can assist in identifying [[diminishing returns]].<!--this sentence is rhetoric. say what is meant. give an example.--> Separation of policy and mechanism mitigates such conflicts.<ref name="Chow1997">{{cite book|last1=Chow|first1=Randy|author2=Theodore Johnson|title=Distributed Operating Systems and Algorithms|url=https://books.google.com/books?id=J4MZAQAAIAAJ|year=1997|publisher=Addison Wesley|isbn=978-0-201-49838-7}}</ref>
===Working together as an operating system===
The architecture and design of a distributed operating system must realize both individual node and global system goals. Architecture and design must be approached in a manner consistent with separating policy and mechanism. In doing so, a distributed operating system attempts to provide an efficient and reliable distributed computing framework allowing for an absolute minimal user awareness of the underlying command and control efforts.<ref name="Sinha1997" />
The multi-level collaboration between a kernel and the system management components, and in turn between the distinct nodes in a distributed operating system is the functional challenge of the distributed operating system. This is the point in the system that must maintain a perfect harmony of purpose, and simultaneously maintain a complete disconnect of intent from implementation. This challenge is the distributed operating system's opportunity to produce the foundation and framework for a reliable, efficient, available, robust, extensible, and scalable system. However, this opportunity comes at a very high cost in complexity.
===The price of complexity===
In a distributed operating system, the exceptional degree of inherent complexity could easily render the entire system an anathema to any user. As such, the logical price of realizing a distributed operation system must be calculated in terms of overcoming vast amounts of complexity in many areas, and on many levels. This calculation includes the depth, breadth, and range of design investment and architectural planning required in achieving even the most modest implementation.<ref>Surajbali, B., Coulson, G., Greenwood, P., and Grace, P. 2007. Augmenting reflective middleware with an aspect orientation support layer. In Proceedings of the 6th international Workshop on Adaptive and Reflective Middleware: Held At the ACM/IFIP/USENIX international Middleware Conference (Newport Beach, CA, November 26–30, 2007). ARM '07. ACM, New York, NY, 1-6.</ref>
These design and development considerations are critical and unforgiving. For instance, a deep understanding of a distributed operating system's overall architectural and design detail is required at an exceptionally early point.<ref name="Tanenbaum1993"/> An exhausting array of design considerations are inherent in the development of a distributed operating system. Each of these design considerations can potentially affect many of the others to a significant degree. This leads to a massive effort in balanced approach, in terms of the individual design considerations, and many of their permutations. As an aid in this effort, most rely on documented experience and research in distributed computing power.
==History==
Research and experimentation efforts began in earnest in the 1970s and continued through the 1990s, with focused interest peaking in the late 1980s. A number of distributed operating systems were introduced during this period; however, very few of these implementations achieved even modest commercial success.
Fundamental and pioneering implementations of primitive distributed operating system component concepts date to the early 1950s.<ref name=dyseac>{{cite journal |last1=Leiner |first1=Alan L. |title=System Specifications for the DYSEAC |journal=Journal of the ACM |date=April 1954 |volume=1 |issue=2 |pages=57–81 |doi=10.1145/320772.320773 |s2cid=15381094 |doi-access= }}</ref><ref name=lincoln_tx2>{{cite conference |title=The Lincoln TX-2 Input-Output System |first=James W. |last=Forgie |date=February 26–28, 1957 |conference=Western Joint Computer Conference: Techniques for Reliability |publisher=Association for Computing Machinery |___location=Los Angeles, California |pages=156–160 |isbn=9781450378611 |doi=10.1145/1455567.1455594 |doi-access=free }}</ref><ref name=intercomm_cells>{{cite conference |author=C. Y. Lee |title=Intercommunicating cells, basis for a distributed logic computer |date=December 4–6, 1962 |conference=Fall Joint Computer Conference |publisher=Association for Computing Machinery |___location=Philadelphia, Pennsylvania |pages=130–136 |doi=10.1145/1461518.1461531 |doi-access=free}}</ref> Some of these individual steps were not focused directly on distributed computing, and at the time, many may not have realized their important impact. These pioneering efforts laid important groundwork, and inspired continued research in areas related to distributed computing.<ref name="Dreyfus_1958_Gamma60">{{citation |title=System design of the Gamma 60 |author-first=Phillippe |author-last=Dreyfus |author-link=Philippe Dreyfus |work=Proceedings of the May 6–8, 1958, [[Western Joint Computer Conference]]: Contrasts in Computers |___location=Los Angeles |date=1958-05-08 |orig-year=1958-05-06 |id=IRE-ACM-AIEE '58 (Western) |publication-place=ACM, New York, NY, USA |pages=130–133 |url=https://www.computer.org/csdl/proceedings/afips/1958/5052/00/50520130.pdf |access-date=2017-04-03 |url-status=live |archive-url=https://web.archive.org/web/20170403224547/https://www.computer.org/csdl/proceedings/afips/1958/5052/00/50520130.pdf |archive-date=2017-04-03}}</ref><ref>Leiner, A. L., Notz, W. A., Smith, J. L., and Weinberger, A. 1958. Organizing a network of computers to meet deadlines. In Papers and Discussions Presented At the December 9–13, 1957, Eastern Joint Computer Conference: Computers with Deadlines To Meet (Washington, D.C., December 09–13, 1957). IRE-ACM-AIEE '57</ref><ref>Leiner, A. L., Smith, J. L., Notz, W. A., and Weinberger, A. 1958. PILOT, the NBS multicomputer system. In Papers and Discussions Presented At the December 3–5, 1958, Eastern Joint Computer Conference: Modern Computers: Objectives, Designs, Applications (Philadelphia, Pennsylvania, December 03–05, 1958). AIEE-ACM-IRE '58 (Eastern). ACM, New York, NY, 71-75.</ref><ref>Bauer, W. F. 1958. Computer design from the programmer's viewpoint. In Papers and Discussions Presented At the December 3–5, 1958, Eastern Joint Computer Conference: Modern Computers: Objectives, Designs, Applications (Philadelphia, Pennsylvania, December 03–05, 1958). AIEE-ACM-IRE '58 (Eastern). ACM, New York, NY, 46-51.</ref><ref>Leiner, A. L., Notz, W. A., Smith, J. L., and Weinberger, A. 1959. PILOT—A New Multiple Computer System. J. ACM 6, 3 (Jul. 1959), 313-335.</ref><ref>Estrin, G. 1960. [https://dl.acm.org/doi/abs/10.1145/1460361.1460365 Organization of computer systems: the fixed plus variable structure computer]. In Papers Presented At the May 3–5, 1960, Western Joint IRE-AIEE-ACM Computer Conference (San Francisco, California, May 03–05, 1960). IRE-AIEE-ACM '60 (Western). ACM, New York, NY, 33-40.</ref>
In the mid-1970s, research produced important advances in distributed computing. These breakthroughs provided a solid, stable foundation for efforts that continued through the 1990s.
The accelerating proliferation of [[Multiprocessing|multi-processor]] and [[multi-core processor]] systems research led to a resurgence of the distributed OS concept.
===
One of the first efforts was the [[DYSEAC]], a general-purpose [[Synchronization (computer science)|synchronous]] computer. In one of the earliest publications of the [[Association for Computing Machinery]], in April 1954, a researcher at the [[National Bureau of Standards]]{{snd}} now the National [[nist|Institute of Standards and Technology]] ([[nist|NIST]]){{snd}} presented a detailed specification of the DYSEAC. The introduction focused upon the requirements of the intended applications, including flexible communications, but also mentioned other computers:
{{blockquote|Finally, the external devices could even include other full-scale computers employing the same digital language as the DYSEAC. For example, the SEAC or other computers similar to it could be harnessed to the DYSEAC and by use of coordinated programs could be made to work together in mutual cooperation on a common task… Consequently[,] the computer can be used to coordinate the diverse activities of all the external devices into an effective ensemble operation.|ALAN L. LEINER|''System Specifications for the DYSEAC''}}
The specification discussed the architecture of multi-computer systems, preferring peer-to-peer rather than master-slave.
{{blockquote|Each member of such an interconnected group of separate computers is free at any time to initiate and dispatch special control orders to any of its partners in the system. As a consequence, the supervisory control over the common task may initially be loosely distributed throughout the system and then temporarily concentrated in one computer, or even passed rapidly from one machine to the other as the need arises. …the various interruption facilities which have been described are based on mutual cooperation between the computer and the external devices subsidiary to it, and do not reflect merely a simple master-slave relationship.|ALAN L. LEINER|''System Specifications for the DYSEAC''}}
This is one of the earliest examples of a computer with distributed control. The [[United States Department of the Army|Dept. of the Army]] reports<ref>Martin H. Weik, "A Third Survey of Domestic Electronic Digital Computing Systems," Ballistic Research Laboratories Report No. 1115, pg. 234-5, Aberdeen Proving Ground, Maryland, March 1961</ref> certified it reliable and that it passed all acceptance tests in April 1954. It was completed and delivered on time, in May 1954. This was a "[[portable computer]]", housed in a [[Tractor-trailer#Types of trailers|tractor-trailer]], with 2 attendant vehicles and [[Refrigerator truck|6 tons of refrigeration]] capacity.
===
Described as an experimental input-output system, the [[Lincoln TX-2]] emphasized flexible, simultaneously operational input-output devices, i.e., [[multiprogramming]]. The design of the TX-2 was modular, supporting a high degree of modification and expansion.<ref name=lincoln_tx2/>
The system employed The Multiple-Sequence Program Technique. This technique allowed multiple [[program counter]]s to each associate with one of 32 possible sequences of program code. These explicitly prioritized sequences could be interleaved and executed concurrently, affecting not only the computation in process, but also the control flow of sequences and switching of devices as well. Much discussion related to device sequencing.
Similar to DYSEAC the TX-2 separately programmed devices can operate simultaneously, increasing [[throughput]]. The full power of the central unit was available to any device. The TX-2 was another example of a system exhibiting distributed control, its central unit not having dedicated control.<!-- seems questionable unless the devices were explicitly other computers-->
===Intercommunicating Cells===
One early effort at abstracting memory access was Intercommunicating Cells, where a cell was composed of a collection of [[Computer data storage|memory]] elements. A memory element was basically a binary electronic [[flip-flop (electronics)|flip-flop]] or [[relay]]. Within a cell there were two types of elements, ''symbol'' and ''cell''. Each cell structure stores [[data]] in a [[String (computer science)|string]] of symbols, consisting of a [[Identifier|name]] and a set of [[parameter]]s. Information is linked through cell associations.<ref name=intercomm_cells/>
The theory contended that addressing is a wasteful and non-valuable [[indirection|level of indirection]]. Information was accessed in two ways, direct and cross-retrieval. Direct retrieval accepts a name and returns a parameter set. Cross-retrieval [[Projection (mathematics)|projects]] through parameter sets and returns a set of names containing the given [[subset]] of parameters. This was similar to a modified [[hash table]] [[data structure]] that allowed multiple [[Value (mathematics)|values]] (parameters) for each [[Unique key|key]] (name).
{| style="width=100%;"
|- valign="top"
| colspan="3" |Cellular memory would have many advantages:
|- valign="top"
| width="20px" | || width="10px" | [[File:Writing bullet.svg|top]] || A major portion of a system's [[Boolean logic|logic]] is distributed within the associations of information stored in the cells,
|- valign="top"
| width="20px" | || width="10px" | [[File:Writing bullet.svg|top]] || This flow of information association is somewhat guided by the act of storing and retrieving,
|- valign="top"
| width="20px" | || width="10px" | [[File:Writing bullet.svg|top]] || The time required for storage and [[Information retrieval|retrieval]] is mostly [[constant time|constant]] and completely unrelated to the size and fill-factor of the memory
|- valign="top"
| width="20px" | || width="10px" | [[File:Writing bullet.svg|top]] || Cells are logically indistinguishable, making them both flexible to use and relatively simple to extend in size
|}
This [[Computer configuration|configuration]] was ideal for distributed systems. The constant-time projection through memory for storing and retrieval was inherently [[Atomic operation|atomic]] and [[Mutual exclusion|exclusive]]. The cellular memory's intrinsic distributed characteristics<!-- are these intrinsically distributed or merely abstract?--> would be invaluable. The impact on the [[User interface|user]], [[Computer hardware|hardware]]/[[Peripheral|device]], or [[Application programming interface]]s was indirect. The authors were considering distributed systems, stating:
{{blockquote|We wanted to present here the basic ideas of a distributed logic system with... the macroscopic concept of logical design, away from scanning, from searching, from addressing, and from counting, is equally important. We must, at all cost, free ourselves from the burdens of detailed local problems which only befit a machine low on the evolutionary scale of machines.|Chung-Yeol (C. Y.) Lee|''Intercommunicating Cells, Basis for a Distributed Logic Computer''}}
===Foundational work===
====Coherent memory abstraction====
{{pad|2em}} Algorithms for scalable synchronization on shared-memory multiprocessors <ref>Mellor-Crummey, J. M. and Scott, M. L. 1991. [https://dl.acm.org/doi/abs/10.1145/103727.103729 Algorithms for scalable synchronization on shared-memory multiprocessors]. ACM Trans. Comput. Syst. 9, 1 (Feb. 1991), 21-65.</ref>
===
{{pad|2em}}Measurements of a distributed file system<ref>Baker, M. G., Hartman, J. H., Kupfer, M. D., Shirriff, K. W., and Ousterhout, J. K. 1991. [http://people.csail.mit.edu/ledlie/resources/papers/1991/baker.ps Measurements of a distributed file system]. In Proceedings of the Thirteenth ACM Symposium on Operating Systems Principles (Pacific Grove, California, United States, October 13–16, 1991). SOSP '91. ACM, New York, NY, 198-212.</ref>
<br />{{pad|2em}}Memory coherence in shared virtual memory systems <ref>Li, K. and Hudak, P. 1989. Memory coherence in shared virtual memory systems. ACM Trans. Comput. Syst. 7, 4 (Nov. 1989), 321-359.</ref>
===
{{pad|2em}}''Transactions''
<br />{{pad|4em}} Sagas <ref>Garcia-Molina, H. and Salem, K. 1987. Sagas. In Proceedings of the 1987 ACM SIGMOD international Conference on Management of Data (San Francisco, California, United States, May 27–29, 1987). U. Dayal, Ed. SIGMOD '87. ACM, New York, NY, 249-259.</ref>
{{pad|2em}}''Transactional Memory''
<br />{{pad|4em}}Composable memory transactions<ref>Harris, T., Marlow, S., [[Simon Peyton Jones|Peyton-Jones, S.]], and Herlihy, M. 2005. [http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.77.3476&rep=rep1&type=pdf Composable memory transactions]. In Proceedings of the Tenth ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming (Chicago, IL, USA, June 15–17, 2005). PPoPP '05. ACM, New York, NY, 48-60.</ref>
<br />{{pad|4em}}Transactional memory: architectural support for lock-free data structures <ref>Herlihy, M. and Moss, J. E. 1993. [http://hpl.americas.hp.net/techreports/Compaq-DEC/CRL-92-7.pdf Transactional memory: architectural support for lock-free data structures]. In Proceedings of the 20th Annual international Symposium on Computer Architecture (San Diego, California, United States, May 16–19, 1993). ISCA '93. ACM, New York, NY, 289-300.</ref>
<br />{{pad|4em}}Software transactional memory for dynamic-sized data structures<ref>Herlihy, M., Luchangco, V., Moir, M., and Scherer, W. N. 2003. [http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.59.8787&rep=rep1&type=pdf Software transactional memory for dynamic-sized data structures]. In Proceedings of the Twenty-Second Annual Symposium on Principles of Distributed Computing (Boston, Massachusetts, July 13–16, 2003). PODC '03. ACM, New York, NY, 92-101.</ref>
<br />{{pad|4em}}Software transactional memory<ref>Shavit, N. and Touitou, D. 1995. [http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.474.5928&rep=rep1&type=pdf Software transactional memory]. In Proceedings of the Fourteenth Annual ACM Symposium on Principles of Distributed Computing (Ottawa, Ontario, Canada, August 20–23, 1995). PODC '95. ACM, New York, NY, 204-213.</ref>
===
{{pad|2em}}OceanStore: an architecture for global-scale persistent storage <ref>Kubiatowicz, J., Bindel, D., Chen, Y., Czerwinski, S., Eaton, P., Geels, D., Gummadi, R., Rhea, S., Weatherspoon, H., Wells, C., and Zhao, B. 2000. [http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.439.4822&rep=rep1&type=pdf OceanStore: an architecture for global-scale persistent storage]. In Proceedings of the Ninth international Conference on Architectural Support For Programming Languages and Operating Systems (Cambridge, Massachusetts, United States). ASPLOS-IX. ACM, New York, NY, 190-201.</ref>
===
{{pad|2em}} Weighted voting for replicated data <ref>Gifford, D. K. 1979. [http://pages.cs.wisc.edu/~remzi/Classes/739/Spring2004/Papers/p150-gifford.pdf Weighted voting for replicated data]. In Proceedings of the Seventh ACM Symposium on Operating Systems Principles (Pacific Grove, California, United States, December 10–12, 1979). SOSP '79. ACM, New York, NY, 150-162</ref>
<br />{{pad|2em}} Consensus in the presence of partial synchrony <ref>Dwork, C., Lynch, N., and Stockmeyer, L. 1988. [https://groups.csail.mit.edu/tds/papers/Lynch/MIT-LCS-TM-270.pdf Consensus in the presence of partial synchrony]. J. ACM 35, 2 (Apr. 1988), 288-323.</ref>
===
{{pad|2em}}''Sanity checks''
<br />{{pad|4em}}The Byzantine Generals Problem <ref>Lamport, L., Shostak, R., and Pease, M. 1982. [http://people.cs.uchicago.edu/~shanlu/teaching/33100_wi15/papers/byz.pdf The Byzantine Generals Problem]. ACM Trans. Program. Lang. Syst. 4, 3 (Jul. 1982), 382-401.</ref>
<br />{{pad|4em}}Fail-stop processors: an approach to designing fault-tolerant computing systems <ref>Schlichting, R. D. and Schneider, F. B. 1983. Fail-stop processors: an approach to designing fault-tolerant computing systems. ACM Trans. Comput. Syst. 1, 3 (Aug. 1983), 222-238.</ref>
{{pad|2em}}''Recoverability''
<br />{{pad|4em}}''Distributed'' snapshots: determining global states of distributed systems<ref>Chandy, K. M. and Lamport, L. 1985. Distributed snapshots: determining global states of distributed systems. ACM Trans. Comput. Syst. 3, 1 (Feb. 1985), 63-75.</ref>
<br />{{pad|4em}}Optimistic recovery in distributed systems <ref>Strom, R. and Yemini, S. 1985. Optimistic recovery in distributed systems. ACM Trans. Comput. Syst. 3, 3</ref>
==Distributed computing models==
{{More citations needed section|date=January 2012}}
===Three basic distributions===
To better illustrate this point, examine three system [[Software architecture|architectures]]; centralized, decentralized, and distributed. In this examination, consider three structural aspects: organization, connection, and control. Organization describes a system's physical arrangement characteristics. Connection covers the communication pathways among nodes. Control manages the operation of the earlier two considerations.
====Organization====
A [[Centralized computing|centralized system]] has one level of structure, where all constituent elements directly depend upon a single control element. A [[decentralized system]] is hierarchical. The bottom level unites subsets of a system's entities. These entity subsets in turn combine at higher levels, ultimately culminating at a central master element. A distributed system is a collection of autonomous elements with no concept of levels.
====Connection====
Centralized systems connect constituents directly to a central master entity in a hub and spoke fashion. A decentralized system (aka [[Network operating system|network system]]) incorporates direct and indirect paths between constituent elements and the central entity. Typically this is configured as a hierarchy with only one shortest path between any two elements. Finally, the distributed operating system requires no pattern; direct and indirect connections are possible between any two elements. Consider the 1970s phenomena of “[[string art]]” or a [[spirograph]] drawing as a [[Fully connected network|fully connected system]], and the [[spider web|spider's web]] or the [[Interstate Highway System]] between U.S. cities as examples of a ''partially connected system''.
====Control====
Centralized and decentralized systems have directed [[Software flow control|flows of connection]] to and from the central entity, while distributed systems communicate along arbitrary paths. This is the pivotal notion of the third consideration. Control involves allocating tasks and data to system elements balancing efficiency, responsiveness, and complexity.
Centralized and decentralized systems offer more control, potentially easing administration by limiting options. Distributed systems are more difficult to explicitly control, but scale better horizontally and offer fewer points of system-wide failure. The associations conform to the needs imposed by its design but not by organizational chaos
==Design considerations==
===Transparency===
''Transparency'' or ''single-system image'' refers to the ability of an application to treat the system on which it operates without regard to whether it is distributed and without regard to hardware or other implementation details. Many areas of a system can benefit from transparency, including access, ___location, performance, naming, and migration. The consideration of transparency directly affects decision making in every aspect of design of a distributed operating system. Transparency can impose certain requirements and/or restrictions on other design considerations.
Systems can optionally violate transparency to varying degrees to meet specific application requirements. For example, a distributed operating system may present a hard drive on one computer as "C:" and a drive on another computer as "G:". The user does not require any knowledge of device drivers or the drive's ___location; both devices work the same way, from the application's perspective. A less transparent interface might require the application to know which computer hosts the drive. Transparency domains:
* ''Location transparency'' – Location transparency comprises two distinct aspects of transparency, naming transparency and user mobility. Naming transparency requires that nothing in the physical or logical references to any system entity should expose any indication of the entity's ___location, or its local or remote relationship to the user or application. User mobility requires the consistent referencing of system entities, regardless of the system ___location from which the reference originates.<ref name="Sinha1997" />{{rp|20}}
* ''Access transparency'' – Local and remote system entities must remain indistinguishable when viewed through the user interface. The distributed operating system maintains this perception through the exposure of a single access mechanism for a system entity, regardless of that entity being local or remote to the user. Transparency dictates that any differences in methods of accessing any particular system entity—either local or remote—must be both invisible to, and undetectable by the user.<ref name="Gościński1991"/>{{rp|84}}<!--what is the difference between referencing and access?-->
* ''Migration transparency'' – Resources and activities migrate from one element to another controlled solely by the system and without user/application knowledge or action.<ref name="Galli2000">{{cite book|last=Galli|first=Doreen L.|title=Distributed Operating Systems: Concepts and Practice|url=https://archive.org/details/distributedopera00gall |url-access=registration|year=2000|publisher=Prentice Hall|isbn=978-0-13-079843-5}}</ref>{{rp|16}}
* ''Replication transparency'' – The process or fact that a resource has been duplicated on another element occurs under system control and without user/application knowledge or intervention.<ref name="Galli2000" />{{rp|16}}
* ''Concurrency transparency'' – Users/applications are unaware of and unaffected by the presence/activities of other users.<ref name="Galli2000" />{{rp|16}}
* ''Failure transparency'' – The system is responsible for detection and remediation of system failures. No user knowledge/action is involved other than waiting for the system to resolve the problem.<ref name="Chow1997" />{{rp|30}}
* ''Performance Transparency'' – The system is responsible for the detection and remediation of local or global performance shortfalls. Note that system policies may prefer some users/user classes/tasks over others. No user knowledge or interaction. is involved.<ref name="Sinha1997" />{{rp|23}}
* ''Size/Scale transparency'' – The system is responsible for managing its geographic reach, number of nodes, level of node capability without any required user knowledge or interaction.<ref name="Sinha1997" />{{rp|23}}
* ''Revision transparency'' – The system is responsible for upgrades and revisions and changes to system infrastructure without user knowledge or action.<ref name="Chow1997" />{{rp|30}}
* ''Control transparency'' – The system is responsible for providing all system information, constants, properties, configuration settings, etc. in a consistent appearance, connotation, and denotation to all users and applications.<ref name="Gościński1991"/>{{rp|84}}
* ''Data transparency'' – The system is responsible for providing data to applications without user knowledge or action relating to where the system stores it.<ref name="Gościński1991"/>{{rp|85}}
* ''Parallelism transparency'' – The system is responsible for exploiting any ability to parallelize task execution without user knowledge or interaction. Arguably the most difficult aspect of transparency, and described by Tanenbaum as the "Holy grail" for distributed system designers.<ref name="Tanenbaum1995">{{cite book|last=Tanenbaum|first=Andrew S.|title=Distributed Operating Systems|url=https://archive.org/details/unset0000unse_h1q3|url-access=registration|year=1995|publisher=Prentice Hall|isbn=978-0-13-219908-7}}</ref>{{rp|23–25}}
===Inter-process communication===
[[Inter-Process Communication]] (IPC) is the implementation of general communication, process interaction, and [[dataflow]] between [[Thread (computer science)|threads]] and/or [[Process (computing)|processes]] both within a node, and between nodes in a distributed OS. The intra-node and inter-node communication requirements drive low-level IPC design, which is the typical approach to implementing communication functions that support transparency. In this sense, Interprocess communication is the greatest underlying concept in the low-level design considerations of a distributed operating system.
===Process management===
[[Process management (computing)|Process management]] provides policies and mechanisms for effective and efficient sharing of resources between distributed processes. These policies and mechanisms support operations involving the allocation and de-allocation of processes and ports to processors, as well as mechanisms to run, suspend, migrate, halt, or resume process execution. While these resources and operations can be either local or remote with respect to each other, the distributed OS maintains state and synchronization over all processes in the system.
As an example, [[Load balancing (computing)|load balancing]] is a common process management function. Load balancing monitors node performance and is responsible for shifting activity across nodes when the system is out of balance. One load balancing function is picking a process to move. The kernel may employ several selection mechanisms, including priority-based choice. This mechanism chooses a process based on a policy such as 'newest request'. The system implements the policy
===Resource management===
[[Resource (computer science)|Systems resources]] such as memory, files, devices, etc. are distributed throughout a system, and at any given moment, any of these nodes may have light to idle workloads. ''Load sharing'' and load balancing require many policy-oriented decisions, ranging from finding idle CPUs, when to move, and which to move. Many [[algorithm]]s exist to aid in these decisions; however, this calls for a second level of decision making policy in choosing the algorithm best suited for the scenario, and the conditions surrounding the scenario.<!--how is this different from process management?-->
===Reliability===
Distributed OS can provide the necessary resources and services to achieve high levels of ''reliability'', or the ability to prevent and/or recover from errors. [[Fault (technology)|Faults]] are physical or logical defects that can cause errors in the system. For a system to be reliable, it must somehow overcome the adverse effects of faults.
The primary methods for dealing with faults include ''fault avoidance'', [[Fault-tolerant design|fault tolerance]], and ''fault detection and recovery''. Fault avoidance covers proactive measures taken to minimize the occurrence of faults. These proactive measures can be in the form of ''[[transaction processing|transactions]]'', [[Replication (computer science)|replication]] and [[Replication (computer science)#Primary-backup and multi-primary replication|backups]]. Fault tolerance is the ability of a system to continue operation in the presence of a fault. In the event, the system should detect and recover full functionality. In any event, any actions taken should make every effort to preserve the ''single system image''.
===Availability===
[[Availability]] is the fraction of time during which the system can respond to requests.
===Performance===
Many [[Benchmark (computing)|benchmark metrics]] quantify [[Computer performance|performance]]; throughput, response time, job completions per unit time, system utilization, etc. With respect to a distributed OS, performance most often distills to a balance between [[Parallel computing|process parallelism]] and IPC.{{Citation needed|date=January 2012}} Managing the [[Granularity#In computing|task granularity]] of parallelism in a sensible relation to the messages required for support is extremely effective.{{Citation needed|date=January 2012}} Also, identifying when it is more beneficial to [[Process migration|migrate a process]] to its data, rather than copy the data, is effective as well.{{Citation needed|date=January 2012}}
===Synchronization===
Cooperating [[Concurrent computing|concurrent processes]] have an inherent need for [[Synchronization (computer science)|synchronization]], which ensures that changes happen in a correct and predictable fashion. Three basic situations that define the scope of this need:
:* one or more processes must synchronize at a given point for one or more other processes to continue,
:* one or more processes must wait for an asynchronous condition in order to continue,
:* or a process must establish exclusive access to a shared resource.
Improper synchronization can lead to multiple failure modes including loss of [[ACID|atomicity, consistency, isolation and durability]], [[Deadlock (computer science)|deadlock]], [[livelock]] and loss of [[serializability]].{{Citation needed|date=January 2012}}
===Flexibility===
[[Flexibility (engineering)|Flexibility]] in a distributed operating system is enhanced through the modular characteristics of the distributed OS, and by providing a richer set of higher-level services. The completeness and quality of the kernel/microkernel simplifies implementation of such services, and potentially enables service providers greater choice of providers for such services.{{Citation needed|date=April 2012}}
==Research==
===Replicated model extended to a component object model===
<br />{{pad|2em}}The Cronus distributed operating system<ref>Vinter, S. T. and Schantz, R. E. 1986. The Cronus distributed operating system. In Proceedings of the 2nd Workshop on Making Distributed Systems Work (Amsterdam, Netherlands, September 08–10, 1986). EW 2. ACM, New York, NY, 1-3.</ref>
<br />{{pad|2em}}Design and development of MINIX distributed operating system<ref>Ramesh, K. S. 1988. Design and development of MINIX distributed operating system. In Proceedings of the 1988 ACM Sixteenth Annual Conference on Computer Science (Atlanta, Georgia, United States). CSC '88. ACM, New York, NY, 685.</ref>
===Complexity/Trust exposure through accepted responsibility===
:Scale and performance in the Denali isolation kernel.<ref>Whitaker, A., Shaw, M., and Gribble, S. D. 2002. In Proceedings of the 5th Symposium on Operating Systems Design and Implementation</ref>
===Multi/Many-core focused systems===
:Corey: an Operating System for Many Cores.<ref>S. Boyd-Wickizer, H. Chen, R. Chen, Y. Mao, F. Kashoek, R. Morris, A. Pesterev, L. Stein, M. Wu, Y. Dai, Y. Zhang, and Z. Zhang. Proceedings of the 2008 Symposium on Operating Systems Design and Implementation (OSDI), December 2008.</ref>
:Almos: Advanced Locality Management Operating System for cc-NUMA Many-Cores.<ref>Almaless, G. and Wajsbürt, F. 2011. In Proceedings of the 5th national seminar of GDR SoC-SIP, Lyon, France, 2011.</ref>
===Distributed processing over extremes in heterogeneity===
:Helios: heterogeneous multiprocessing with satellite kernels.<ref>Nightingale, E. B., Hodson, O., McIlroy, R., Hawblitzel, C., and Hunt, G. 2009. In Proceedings of the ACM SIGOPS 22nd Symposium on Operating Systems Principles (Big Sky, Montana, USA, October 11–14, 2009). SOSP '09.</ref>
===Effective and stable in multiple levels of complexity===
:Tessellation: Space-Time Partitioning in a Manycore Client OS.<ref>Rose Liu, Kevin Klues, and Sarah Bird, University of California at Berkeley; Steven Hofmeyr, Lawrence Berkeley National Laboratory; [[Krste Asanović]] and John Kubiatowicz, University of California at Berkeley. HotPar09.</ref>
==See also==
* {{annotated link|Distributed computing}}
* {{annotated link|HarmonyOS}}
* {{annotated link|OpenHarmony}}
* {{annotated link|BlueOS}}
* {{annotated link|Plan 9 from Bell Labs}}
* {{annotated link|Inferno (operating system)|Inferno}}
* {{annotated link|MINIX}}
* {{annotated link|Single system image}} (SSI)
* {{annotated link|Computer systems architecture}}
* {{annotated link|Multikernel}}
* {{annotated link|Operating System Projects}}
* {{annotated link|Edsger W. Dijkstra Prize in Distributed Computing}}
* {{annotated link|List of distributed computing conferences}}
* {{annotated link|List of volunteer computing projects}}
==References==
{{Reflist}}
==Further reading==
* {{cite book|last1=Chow|first1=Randy|author2=Theodore Johnson|title=Distributed Operating Systems and Algorithms|url=https://books.google.com/books?id=J4MZAQAAIAAJ|year=1997|publisher=Addison Wesley|isbn=978-0-201-49838-7}}
* {{cite book|last=Sinha|first=Pradeep Kumar |title=Distributed Operating Systems: Concepts and Design|url=https://archive.org/details/distributedopera0000sinh|url-access=registration|year=1997|publisher=IEEE Press|isbn=978-0-7803-1119-0}}
* {{cite book|last=Galli|first=Doreen L.|title=Distributed Operating Systems: Concepts and Practice|url=https://archive.org/details/distributedopera00gall |url-access=registration|year=2000|publisher=Prentice Hall|isbn=978-0-13-079843-5}}
==External links==
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