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Encyclopedia > Impedance mismatch

Impedance mismatch has two meanings.

It is a problem in electrical engineering that occurs when two transmission lines or circuits with different impedances are connected. This can cause signal reflection resulting in attenuation and noise. See also impedance matching. Electrical engineering is an engineering discipline that deals with the study and application of electricity and electromagnetism. ... A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electric currents, magnetic fields, acoustic waves, or electromagnetic waves. ... In electrical engineering, impedance is a measure for the manner and degree a component resists the flow of electrical current if a given voltage is applied. ... When a signal is transmitted along a transmission medium, such as a copper cable or an optical fibre, there is the possibility that some of the signal power is reflected back to its origin, rather than being carried all the way along the cable to the far end. ... Attenuation is the decrease of the amount, force, magnitude, or value of something. ... In science, and especially in physics and telecommunication, noise is fluctuations in and the addition of external factors to the stream of target information (signal) being received at a detector. ... This article or section should be merged with Maximum power theorem. ...

In programming terminology it refers to the attempt to connect two systems that have very different conceptual bases, most commonly when trying to use an SQL database from an object oriented program. Christopher J. Date would argue that a truly relational DBMS would pose no such problem, as domains and classes are essentially one and the same thing. Another significant impedance mismatch in programming occurs between the domain objects and the user interface. Computer programming (often simply programming) is the craft of implementing one or more interrelated abstract algorithms using a particular programming language to produce a concrete computer program. ... Structured Query Language (SQL) is the most popular computer language used to create, modify and query databases. ... Object-oriented programming (OOP) is a computer programming paradigm in which a software system is modeled as a set of objects that interact with each other. ... Christopher J. Date became involved with Edgar F. Codds relational model for database management while working at IBM. He is the current principal maintainer and developer of the relational model, together with Hugh Darwen. ... A database management system (DBMS) is a computer program (or more typically, a suite of them) designed to manage a database, a large set of structured data, and run operations on the data requested by numerous users. ... Domain has several meanings: some kind of territory, such as (for example) a demesne or a realm synonymous with field, e. ... In object-oriented programming, a class consists of encapsulated instance variables and subprograms, the methods mentioned below. ... A domain object is an object from a domain model. ... The user interface is the part of a system exposed to users. ...

Impedance mismatch between object-orientation and relational databases

The impedance mismatch between object-oriented programming and relational databases occurs when a relational database is being used within an object-oriented program in such way that objects are mapped in straightforward way to database tables. This practice has been recommended and documented by some object-oriented literature as a way to use databases in object-oriented programs. However, object-oriented programs are designed with methods that result in encapsulated objects whose representation is hidden. Mapping such private object representation to database tables makes such databases fragile, since there are significantly fewer constraints for design of encapsulated private representation of objects compared to database's use of public data, which must be amenable to upgrade, inspection and queries. A relational database is a database based on the relational model. ... An object is fundamental concept in object-oriented programming. ...

In particular, object-oriented modelling emphasises the essential concept of an invariant, which requires the use of encapsulation before any access to data of OOP objects is performed. These invariants cannot be represented in relational databases. In relational thinking, "private" versus "public" is relative to need rather than an absolute characteristic of a piece of state (data). Relational and OO often have conflicts over relativity versus absolutism of classifications and characteristics. It is sometimes said that OO is a Plato-like view of the world while relational is an Einstein-like view. Invariant may have meanings invariant (computer science), such as a combination of variables not altered in a loop invariant (mathematics), something unaltered by a transformation invariant (music) invariant (physics) conserved by system symmetry This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the...

Also, access to objects in object-oriented programs is allegedly best performed via interfaces that together provide the only access to the internals of an object. Similarly, essential OOP concepts for classes of objects, inheritance and polymorphism, are not supported by database systems. A proper mapping between relational database concepts and object-oriented concepts can be made if relational database tables are linked to associations found in object-oriented analysis. Data stored in OOP objects should not as such be stored in relational databases. In general usage, an interface is the point, area, or surface along which two substances or other qualitatively different things meet; it is also used metaphorically for the juncture between items. ... In object-oriented programming, a class consists of encapsulated instance variables and subprograms, the methods mentioned below. ... For other uses, see inheritance (disambiguation). ... In computer science, polymorphism is the idea of allowing the same code to be used with different types, resulting in more general and abstract implementations. ... Object-oriented analysis builds a model of a system that is composed of objects. ...

Solving the impedance mismatch problem for object-oriented programs requires recognition of the difference in scale between object-oriented programs and relational database systems. In particular, relational database transactions, as the smallest unit of work performed by databases, are much larger than any operations performed by objects in object-oriented design. Transactions are collections of related associations. Therefore, objects can only represent data relating to a small portion of a database transaction. Object-oriented design can be used as a component of a system with a database, for example, as an implementation methodology for database resources. However, direct mapping between objects and some entities in databases are not possible without running into the impedance mismatch problem. A database transaction is a unit of interaction with a database management system or similar system that is treated in a coherent and reliable way independent of other transactions. ...

There have been some attempts at building object-oriented database systems (OODBMS) that would by default solve the impedance mismatch problem. They have been less successful in practice than relational databases however.

Key differences between OOP and relational can be summarized as follows:

  • Declarative Vs. Imperative Interfaces - Relational thinking tends to use data as interfaces, not behavior as interfaces. It thus has a declarative tilt in design philosophy in contrast to OO's behavioral tilt. (However, some relational proponents may perhaps propose using lots of triggers, etc. to provide complex behavior, but this is not a common viewpoint.)
  • Schema Bound - Objects don't have to follow a "parent schema" as far as which attributes or accessors an object has while table rows must follow the entity's schema. However "dynamical relational" theory may also provide this, although it does not exist in practice yet. Still, a given row must belong to one and only one entity. The closest thing in OO is inheritance, but it is generally tree-shaped and optional.
  • Access Rules - In relational databases, attributes are accessed and altered through predefined relational operators, while OO allows each class to create its own state alteration interface and practices. The "self-handling noun" viewpoint of OO gives independence to each object that relational does not permit. This boils down to a "standards versus local freedom" debate. OO tends to argue that relational standards limit expressiveness, while relational proponents suggest the rule adherence allows more abstract math-like reasoning, integrity, and design consistency.
  • Uniqueness Observation - Row identities (keys) generally have a text-representable form, but objects don't require an externally-viewable unique identifier.
  • Normalization - Relational normalization practices are often ignored by OO designs. However, this may just be a bad habit instead of a native feature of OO.
  • Schema Inheritance - Most relational databases do not support schema inheritance. Although such a feature could be added in theory to reduce the conflict with OOP, relational proponents are less likely to believe in the utility of hierarchical taxonomies and sub-typing because they tend to view set-based taxonomies as more powerful and flexible than trees.
  • Structure vs. Behaviour - OO primarily focuses on ensuring that the structure of the program is reasonable (maintainable, understandable, extensible, reusable, safe), whereas relational systems focus on what kind of behaviour the resulting run-time system has (efficiency, adaptability, fault-tolerance, liveness etc.). [This needs some clarification.]

  Results from FactBites:
impedance matching: Definition and Much More from Answers.com (2976 words)
Impedance matching is the practice of attempting to make the output impedance of a source equal to the input impedance of the load to which it is ultimately connected, usually in order to maximise the power transfer and minimise reflections from the load.
In general, impedance is a complex number, which means that loads generally have a resistance to the source that is in phase with the source signal and a reactance to the source that is in quadrature to the phase of the source.
Impedance bridging is unsuitable for RF connections because it causes power to be reflected back to the source from the boundary between the high impedance and the low impedance.
Impedance mismatch - Wikipedia, the free encyclopedia (1171 words)
Impedance mismatch is a term in systems analysis which describes an inadequate or excessive ability of one system to accommodate input from another.
On one end of the spectrum, Impedance mismatch labels a common problem in electrical engineering that occurs when two transmission lines or circuits with different impedances are connected.
The Object-Relational impedance mismatch is a philosophical difference that exists between object-oriented programming and the Relational Model in database technology.
  More results at FactBites »



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