= Rapid elasticity
<Rapid elasticity> is the ability to provision and release computing capacity quickly in response to demand, often automatically. The consumer may experience capacity as readily available even though the provider operates finite physical resources.<The NIST Definition of Cloud Computing by Peter Mell and Timothy Grance>[§2, Rapid elasticity]{ref}
For example, a service can add workers when its queue grows and remove them after the backlog clears. A control rule needs an observation signal, a scaling policy and time for new workers to become ready. Elastic capacity does not make every program parallel: a serial bottleneck, shared <database> or slow startup can prevent additional workers from increasing <throughput>. Releasing capacity also requires preserving needed <state> outside workers that may disappear.
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