The Agile Modeling (AM) Method

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Quality of Service (QoS) Requirements: An Agile Introduction

A quality of service (QoS) requirement concerns the technical constraints your solution must meet, such as performance, reliability, and availability. QoS requirements are often called technical requirements, service-level requirements or non-functional requirements (I don’t like that term as it makes them sound like requirements that won’t work). Examples of QoS requirements are presented in Figure 1. As you can see, QoS requirements are summarized in a manner similar to business rules: they have a name and a unique identifier (my convention is to use the format QoS#). You document QoS requirements in the same manner as business rules, including a description, an example, a source, references to related technical requirements, and a revision history.Figure 1. QoS requirements (summary form) for a seminar enrollment system at a university.

  • QoS34 The system shall be available 99.99% of the time for any 24-hour period.
  • QoS78 A seminar search will occur within less than three seconds 95 percent of the time.
  • QoS79 A seminar search will occur within no more than ten seconds 99 percent of the time.

When capturing a QoS requirement, it’s important to describe it in terms of ranges and sometimes even probabilities. For example, when building a data warehouse (DW) for a bank, knowing the current average number of customers and transactions is good, but it’s better to know the ranges. For example, say we built our DW to handle up to 20,000 transactions a day. That would certainly be more than sufficient for a typical day, but it wouldn’t be for tax season.

Figure 2. Detailed QoS requirement for a bank’s data warehouse.

Volume Requirement
Domain: Wealth Management
Main entities:
Customer

  • Current: 360,000
  • Expected annual growth rate: 2-3% (max 7%)
  • Annual update rate: 33%
  • Record size: 4-8K

Transaction

  • Daily rate: 12,000-17,000
  • Daily rate maximum: 50,000
  • Expected growth rate: 5-8%
  • Record size: 1K

 

Types of QoS Requirements

Potential types of QoS requirements include:

  • Access control
  • Accessibility
  • Accuracy
  • Concurrency
  • Consumability/usability
  • Customization points
  • Environment/green
  • Internationalization
  • Latency
  • Maintenance
  • Operations
  • Performance
  • Privacy
  • Reliability
  • Reuse
  • Resiliency
  • Scalability
  • Security
  • Serviceability
  • Supportability
  • Throughput
  • Timeliness

 

QoS Requirements in Practice

I’m a firm believer in minimizing the number of purely technology-focused QoS requirements. Technology changes quickly, and often requirements based on technology change just as quickly. Whenever you have a requirement driven purely by technology, try to determine the real underlying business needs it expresses. To do this, keep asking why your application must meet a requirement. For example, when asked why your application must be written in Java, the reply was that it has to run on the Internet. When asked why it must run on the Internet, the reply was that your organization wants to take orders for its products and services on the Internet. The real requirement is to sell to consumers at their convenience; one technical solution to this need (and a good one) is to write the component in Java and make it accessible via the Internet. A big difference exists between writing the entire application in Java and supporting the sale of products and services to consumers over the Internet.

QoS requirements are often thought of as constraints, and in fact, constraints can apply to either technical or business issues.

 

Source

This artifact description is excerpted from Chapter 7 of The Object Primer 3rd Edition: Agile Model Driven Development with UML 2. The banking example was taken from the book Not Just Data: How to Deliver Continuous Enterprise Data.