Scalability Requirements

Definition

Scalability Requirement: any quality requirement that specifies a minimum required amount of the quality factor scalability

Objectives

The typical objectives of an scalability requirement are to:

Minimize the effort and expense required to ensure that one of the following continues to function properly and adequatly as its load increases over time:
- Application.
- Application functional capability (e.g., use case, use case path).
- Component:
  - Data Component.
  - Hardware Component (e.g. client computer, server computer, network).
  - Software Component.
- Data Center.
Ensure that the thing being specified can be upgraded to handle increases in the:
- Number of users (both total and simultaneous).
- Number of transactions (both total and simultaneous).
- Size of databases (or records within the databases).
- Amount of network traffic (e.g., number of data packets per time).
- Amount of hardware (e.g., increases in the number of Web servers may require corresponding increases in the number of application servers).

Measurements

Scalability requirements are typically specified in terms of the following measurements:

Amount/increase per release/year

Examples

The following are typical examples of scalability requirements for an eMarket application:

“The application shall be able to scale as specified in the following table:”

Release	Open Sales	Sales Per Year	Total Employees	Total Customers	Simultaneous Interactions
0	10	N/A	10	10	10
1	10,000	50,000	50	500,000	10,000
2	25,000	250,000	250	5,000,000	50,000
3	100,000	1,000,000	500	10,000,000	250,000

Guidelines

The following guidelines have been found to be useful when producing scalability requirements:

The scope of a scalability requirement can be:
- A business enterprise
- An application
- A component
- A center
- A document
Scalability requirements are to developers as capacity requirements are to users.
Scalability requirements may specify the future ability to handle more simultaneous users or interactions, or to store more information in its databases.
Scalability requirements should be tagged with future planned releases of the application or component.
Scalability requirements can be specified as:
- Individual textual requirements.
- Tables of related requirements (see above example table).
Scalability requirements are different from and not to be confused with the scalability mechanisms that may be used to implement them:
- Increasing the amount of hardware components and subcomponents:
  - Additional servers.
  - Additional CPUs per server.
  - Additional RAM per server.
  - Additional storage libraries.
  - Additional ports.
  - Load balancing software components or hardware components (a.k.a, layer 3–7 network switches) allocating work to clusters of servers (e.g., web server clusters, application server clusters, directory server clusters, database server clusters).
- Scaling up (a.k.a., upgrading) current components and subcomponents by increasing:
  - CPU power.
  - RAM chip size.
  - Harddisk capacity.
  - Storage library capacity.
  - The capacity of removable storage such as disks and tape cartradges.
  - Network performance by upgrading the media (e.g., replacing copper wire with fiber optics) and network connectivity devices (e.g., upgrading routers).
- Performance tuning:
  - Reconfigurating/optimizing databases.
  - Identifying bottlenecks, and then redesigning and reimplementing the associated software components.