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Editing
Scalability Testing.
Seven steps to success
: Mark Trellis
Systems that work well during development, deployed on a small scale, can
fail to meet performance goals when the deployment is scaled up to support real
levels of use.
An apposite example of this comes from a major blue chip company that recently
outsourced the development of an innovative high technology platform. Though
development was behind schedule this was deemed acceptable. The system gradually
passed through functional elements of the user acceptance testing and eventually
it looked like a deployment date could be set. But then the supplier started
load testing and scalability testing. There followed a prolonged and costly
period of architectural changes and changes to the system requirements. The
supplier battled heroically to provide an acceptable system, until finally the
project was mothballed.
This is not an isolated case. IT folklore abounds with similar tales. From
ambulance dispatch systems to web-sites for the electronic submission of tax
returns, systems fail as they scale and experience peak demands. All of these
projects appear not to have identified and ordered the major risks they faced.
This is a fundamental stage of risk based testing, and applies equally to
scalability testing or load testing as it does to functionality testing or
business continuity testing. With no risk assessment they did not recognise that
scaling was amongst the biggest risks, far more so that delivering all the
functionality
Recent trends towards Service Oriented Architecture (SOA) attempt to address the
issue of scalability but also introduce new issues. Incorporating externally
provided services into your overall solution means that your ability to scale
now depends upon these external system operate under load. Assuring this is a
demanding task and sadly the load testing and stress testing here is often
overlooked.
Better practice is to start the development of a large scale software system
with its performance clearly in mind, particularly scalability testing, volume
testing and load testing. To create this performance testing focus:
1. Research and quantify the data volumes and transaction volumes the target
market implies. Some of these figures can be eye openers and help the business
users realise the full scale of the system. This alone can lead to reassessment
of the priority of many features.
2, Determine the way features could be presented to users and the system
structured in order to make scaling of the system easier. Do not try and have
the same functionality you would have for a single user desktop solution provide
an appropriate scalable alternative.
3. Recognise that an intrinsic part of the development process is load testing
at representative scale on each incremental software release. This is continual
testing, focusing on the biggest risk to the project: the ability to operate at
full scale.
4. Ensure load testing is adequate both in scope and rigour. Load testing is not
just about measuring response times with a performance test. The load testing
programme needs to include other types of load testing including stress testing,
reliability testing, and endurance testing.
5. Don’t forget that failures will occur. Large scale systems generally include
server clusters with fail-over behaviour. Failure testing, fail-over testing and
recovery testing carried out on representative scale systems operating under
load should be included.
6. Don’t forget catastrophic failure could occur. For large scale problems,
disaster testing and disaster recovery testing should be carried out at
representative scale and loads. These activities can be considered the technical
layers of business continuity testing.
7. Recognise external services if you use them. Where you are adopting an SOA
approach and are dependent on external services you need to be certain that the
throughput and turnaround time on these services will remain acceptable as your
system scales and its demands increase. A smart system architecture will include
a graceful response and fall-back operation should the external service
behaviour deteriorate or fail.
© Acutest 2005 –
http://www.acutest.co.uk
About the author
Mark is an experienced consultant working in performance testing, scalability
testing and load testing. For further information visit:
http://www.acutest.co.uk
or
http://www.acutest.co.uk/performance-testing.html
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