Tight coupling

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Not to be confused with the related concept in software architecture, tight coupling between two components of a distributed system, means that they are so closely interconnected or interrelated that the failure of one quickly or inevitably triggers failure of the other so that it is not practicable to shut the system down, isolate the failing component or otherwise reconfigure the wider system to operate safely before the failing components precipitate a catastrophic, system-wide failure.

Originates from Charles Perrow’s Normal Accidents.

Tightly coupled systems include Dams, power grids, nuclear power stations, aeroplanes and DNA.

Loosely coupled systems include post offices, universities, research and development and most manufacturing.

Features of tight and loose coupling

Tight coupling	Loose coupling
Production delays not possible Difficult to stop the component operating Mandatory sequence of operation No scope for changing sequence on the fly Few buffers or redundancy in the system Existing buffers are hard-wired and inflexible Little scope for improvisation	Processing delays possible Production sequence can be changed Redundancy and slack on resources and personnel Fortuitous buffers and redundancies in the system Scope for improvision and substitution

See also

• Charles Perrow’s magnificent Normal Accidents, which introduced the concept of tight coupling in complex systems.
• DEPOSE components of a distributed system