Elasticity (physics)

In physics, elasticity (from Greek ἐλαστός "ductible") is the ability of a body to resist a distorting influence or stress and to return to its original size and shape when the stress is removed. Solid objects will deform when forces are applied on them. If the material is elastic, the object will return to its initial shape and size when these forces are removed.

The physical reasons for elastic behavior can be quite different for different materials. In metals, the atomic lattice changes size and shape when forces are applied (energy is added to the system). When forces are removed, the lattice goes back to the original lower energy state. For rubbers and other polymers, elasticity is caused by the stretching of polymer chains when forces are applied.

Perfect elasticity is an approximation of the real world, and few materials remain purely elastic even after very small deformations. In engineering, the amount of elasticity of a material is determined by two types of material parameter. The first type of material parameter is called a modulus, which measures the amount of force per unit area (stress) needed to achieve a given amount of deformation. The units of modulus are pascals (Pa) or pounds of force per square inch (psi, also lbf/in²). A higher modulus typically indicates that the material is harder to deform. The second type of parameter measures the elastic limit. The limit can be a stress beyond which the material no longer behaves elastic and deformation of the material will take place. If the stress is released, the material will elastically return to a permanent deformed shape instead of the original shape.

Elasticity

Elasticity may refer to:

Numerous uses are derived from this physical sense of the term, which is inherently mathematical, such as used in Engineering, Chemistry, Construction and variously in Economics:

Elasticity (data store)

The elasticity of a data store relates to the flexibility of its data model and clustering capabilities. The greater the number of data model changes that can be tolerated, and the more easily the clustering can be managed, the more elastic the data store is considered to be.

Types

Clustering elasticity

Clustering elasticity is the ease of adding or removing nodes from the distributed data store. Usually, this is a difficult and delicate task to be done by an expert in a relational database system. Some NoSQL data stores, like Apache Cassandra have an easy solution, and a node can be added/removed with a few changes in the properties and by adding specifying at least one seed.

Data-modelling elasticity

Relational databases are most often very inelastic, as they have a predefined data model that can only be adapted through redesign. Most NoSQL data stores, however, do not have a fixed schema. Each row can have a different number and even different type of columns. Concerning the data store, modifications in the schema are no problem. This makes this kind of data stores more elastic concerning the data model. The drawback is that the programmer has to take into account that the data model may change over time.

Elasticity (cloud computing)

In cloud computing, elasticity is defined as the degree to which a system (or a particular cloud layer) autonomously adapts its capacity to workload over time. In the cloud computing context, ″capacity″ refers to the maximum workload a system (or a particular cloud layer) can handle as bound by its service level objectives. Elasticity is a defining characteristic that differentiates cloud computing from previously proposed computing paradigms, such as grid computing. The dynamic adaptation of capacity, e.g., by altering the use of computing resources, to meet a varying workload is called "elastic computing". In general, an elastic cloud application or process has three elasticity dimensions,Cost, Quality, and Resources, enabling it to increase and decrease its cost, quality, or available resources, as to accommodate specific requirements.

Example

Let us illustrate elasticity through a simple example of a service provider who wants to run a website on an IaaS cloud. At moment , the website is unpopular and a single machine (most commonly a virtual machine) is sufficient to serve all web users. At moment , the website suddenly becomes popular, for example, as a result of a flash crowd, and a single machine is no longer sufficient to serve all users. Based on the amount of web users simultaneously accessing the website and the resource requirements of the web server, it might be that ten machines are needed. An elastic system should immediately detect this condition and provision nine additional machines from the cloud, so as to serve all web users responsively.