“Shadow table”的意思、由来-开放百科全书

Shadow tables, as an abstract concept, have been used since the beginning of modern computing. However, widespread usage of the specific phrase "shadow table" began when relational database management systems (RDBMS) became widely used in the 1970s.^[4]

The initial usage of relational DBMs for commercial purposes lead to the term "shadow tables" becoming widespread. A relational DBM uses related data fields (columns) to correlate information between tables.^[5] For example, two tables, transaction_user and transaction_amount, would both contain the column "key", and keys between tables would match, making it easy to find both the user and the amount of a specific transaction if the key is known. This relational technology allowed people to correlate information stored in a primary table and its shadow.

Applications

Since shadow tables are such an abstract concept, their applications remain in the realm of computer science. Although their usage may not be specifically declared as "shadow table(s)", the concept remains the same. Shadow tables are usually used in order to improve the performance, capacity, and ability of an existing computer/network system. In most applications, shadow tables are usually a carbon copy of their primary tables' structure, but with unique data.

Theoretical Application

Since shadow tables are a specific type of object in computer science, the applications vary greatly, because their application depends on what data is stored in the shadow table and how that data is used. The following is a list of general, abstract applications for shadow tables that span all real-world applications.

Engineering Applications

When shadow tables are used to solve current problems in today's computer/network systems, usually a combination of more than one of the aforementioned theoretical/abstract applications of shadow tables are used. The following list is a very tiny subset of all real-world applications of shadow tables and is only shown to give an example of common applications of shadow tables.

Database Management Systems

For example, shadow tables could be used in an efficient backup system that supports large data tables that rarely change.

Interfacing

Interfacing is the process of using "layers" to simplify the communications between technologies and between people and technologies. One example of layered interfacing is the buttons and menus used by home computer operating systems. These graphical objects exist as a link to the underlying "buttons" of the operating system. The command console is one level below these graphical objects as it gives you an even closer link to the underlying functions of the operating system.

Shadow tables are often used as layers between the end-user and the database. For example, if a user logs into his/her bank account and requests a history of all his/her past transactions, the database usually stores all transactions for all users in one huge table and distinguishes the parties involved in each transaction in one specific column of that table. At this point the server has two options:

The second option is usually more favorable because it saves bandwidth and processing power on the user's end. It also keeps others' transaction data secure.

Operating System Virtualization

Operating system virtualization is the process of simulating the operation of a computer within another computer. This technique is useful for someone who wants to run more than one type of operating system on his/her PC concurrently. Shadow page tables are often used in simulating more than one operating system on a single set of memory and processor. A page table is used by an operating system to map the virtual memory, the actual memory used by programs and the operating system to store information, to its location on the physical memory, the hardware-specific memory stored in bytes on the RAM (Random Access Memory).^[8] A shadow page table is a pseudo-page table within a computer's main page table which allows a system to run more than one kind of operating system concurrently.^[9]

References

1. ^{{cite web|title=Shadow Databases|url=http://supportingadvancement.com/records/shadow_databases/01_shadow_databases.htm|work=Supporting Advancements|accessdate=26 October 2011}}
2. ^{{cite web|last=Ambler|first=Scott|title=Shadow Information and Scaffolding|url=http://www.agiledata.org/essays/mappingObjects.html#ShadowData|work=Mapping Objects to Relational Databases: O/R Mapping In Detail|publisher=Agile Data|accessdate=28 October 2011}}
3. ^{{cite web|last=Wehlou|first=Martin|title=Shadows and Trails|url=http://www.wehlou.com/Code/shadows_and_trails.htm|work=Wehlou|accessdate=26 October 2011}}
4. ^{{cite web|last=Brown|first=Pete|title=Mountain Man|url=http://www.mountainman.com.au/software/history/it2.html|work=A Brief History of Modern RDBMS IT Management|accessdate=30 October 2011}}
5. ^¹{{cite journal|last=Codd|first=Edgar|title=A Relational Model of Data for Large Shared Data Banks|journal=Communications of the ACM|date=June 1970|volume=13|issue=6|pages=377–387|url=http://www.seas.upenn.edu/~zives/03f/cis550/codd.pdf|accessdate=28 October 2011|doi= 10.1145/362384.362685 }}
6. ^{{cite book|last=Speaker|first=Devin Spackman, Mark|title=Enterprise integration solutions|year=2005|publisher=Microsoft Press|location=Redmond, Wash.|isbn=978-0-7356-2060-5|page=45}}
7. ^Kent, Jack, and Hector Garcia-Molina. 1988. p. 13.
8. ^{{cite web|last=Denning|first=P.J.|title=Page Tables|url=http://cs.gmu.edu/cne/modules/vm/purple/ptable.html|accessdate=30 October 2011}}
9. ^{{cite web|title=Virtualization: Architectural Considerations And Other Evaluation Criteria|url=http://www.vmware.com/pdf/virtualization_considerations.pdf|accessdate=30 October 2011}}

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