Stress Tests

Stress Tests determine the load under which a system fails, and how itfails. This is in contrast to Load Testing, which attempts to simulateanticipated load. It is important to know in advance if a ‘stress’situation will result in a catastrophic system failure, or if everything just“goes really slow”. There are various varieties of Stress Tests, includingspike, stepped and gradual ramp-up tests. Catastrophic failures requirerestarting various infrastructure and contribute to downtime, a stress-fullenvironment for support staff and managers, as well as possible financiallosses. If a major performance bottleneck is reached, then the systemperformance will usually degrade to a point that is unsatisfactory, butperformance should return to normal when the excessive load is removed.

Before conducting a Stress Test, it is usually advisable to conduct targeted infrastructure tests on each of thekey components in the system. A variation on targetedinfrastructure tests would be to execute each one as a mini stress test.

The diagram below shows an unexpectedly high amount of demand on atypical web system. Stress situations are not expected under normalcircumstances.

The following table lists possible situations for a variety ofapplications where stress situations may occur.

Type of Application	Circumstances that could give rise to Stress levels of activity.
Online Banking	After an outage - when many clients have been waiting for access to the application to do their banking transactions.
Marketing / Sales Application	Very successful advertising campaign - or substantial error in advertising campaign that understates pricing details.
Various applications	Unexpected publicity - for example, in a news article in a national online newspaper.

Focus of stress test.

In a stress event, it is most likely that many more connections will berequested per minute than under normal levels of expected peak activity. In many stress situations, the actions of each connected user will not betypical of actions observed under normal operating conditions. Thisis partly due to the slow response and partly due to the root cause of thestress event.

Let’s take an example of a large holiday resort web site. Normalactivity will be characterized by browsing, room searches and bookings. If a national online news service posted a sensational article about the resortand included a URL in the article, then the site may be subjected to a hugenumber of hits, but most of the visits would probably be a quick browse. It is unlikely that many of the additional visitors would search for rooms andit would be even less likely that they would make bookings. However, ifinstead of a news article, a national newspaper advertisement erroneouslyunderstated the price of accommodation, then there may well be an influx ofvisitors who clamour to book a room, only to find that the price did not matchtheir expectations.

In both of the above situations, the normal traffic would be increasedwith traffic of a different usage profile. So a stress test design wouldincorporate a Load Test as well as additional virtual users running a specialseries of 'stress' navigations and transactions.

For the sake of simplicity, one can just increase the number of usersusing the business processes and functions coded in the Load Test. However, onemust then keep in mind that a system failure with that type of activity may bedifferent to the type of failure that may occur if a special series of 'stress'navigations were utilized for stress testing.

Stress test execution.

Typically, a stress test starts with a Load Test, and then additionalactivity is gradually increased until something breaks. An alternativetype of stress test is a Load Test with sudden bursts of additionalactivity. The sudden bursts of activity generate substantial activity assessions and connections are established, where as a gradual ramp-up inactivity pushes various values past fixed system limitations.

Ideally, stress tests should incorporate two runs, one with burst typeactivity and the other with gradual ramp-up as per the diagram above, to ensurethat the system under test will not fail catastrophically under excessiveload. System reliability under severe load should not be negotiable andstress testing will identify reliability issues that arise under severe levelsof load.

An alternative, or supplemental stress test is commonly referred to as aspike test, where a single short burst of concurrent activity is applied to asystem. Such tests are typical of simulating extreme activity where a'count-down' situation exists. For example, a system that will not takeorders for a new product until a particular date and time. If demand isvery strong, then many users will be poised to use the system the moment thecount down ends, creating a spike of concurrent requests and load.

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