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Shift-left testing is an approach to software testing and system testing in which testing is performed earlier in the lifecycle (i.e. moved left on the project timeline). It is the first half of the maxim "Test early and often." It was coined by Larry Smith in 2001.
Harm due to late testing
Shift-left testing is important because it helps to prevent the following types of harm due to late testing:
- Testers may be less involved in initial planning, often resulting in insufficient resources being allocated to testing.
- Defects in requirements, architecture, and design remain undiscovered while significant effort is wasted implementing them.
- Debugging (including identifying, localizing, fixing, and regression testing defects) becomes harder as more software is produced and integrated.
- Encapsulation impedes white-box testing, reducing code coverage during testing.
- There is less time to fix defects found by testing, thereby increasing the likelihood that they will be postponed until later increments or versions of the system. This creates a “bow wave” of technical debt that can sink projects if it grows too large.
Types of shift-left testing
There are four basic ways to shift testing earlier in the life-cycle (that is, leftward on the classic V-model). These can be referred to as traditional shift-left testing, incremental shift-left testing, Agile/DevOps shift-left testing, and model-based shift-left testing.
Traditional shift-left testing
As illustrated in the following figure, traditional shift-left moves the emphasis of testing lower down (and therefore slightly to the left) on the right hand side of the classic V model. Instead of emphasizing acceptance and system level testing (e.g., GUI testing with record and playback tools), traditional shift-left concentrates on unit testing and integration testing (e.g., using API testing and modern test tools). The transition to traditional shift-left testing has largely been completed.[by whom?]
Incremental shift-left testing
As illustrated in the following figure, many projects developing large and complex software-reliant systems decompose development into a small number of increments (Vs) having correspondingly shorter durations. The shift-left illustrated by the dashed red arrows occurs because parts of the single, large waterfall V model’s types of testing (shown in gray) are shifted left to become increments of the corresponding types of testing in the smaller incremental V models. When each increment is also a delivery to the customer and operations, then incremental shift-left testing shifts both developmental testing and operational testing to the left. Incremental shift-left testing is popular when developing large, complex systems, especially those incorporating significant amounts of hardware. Like traditional shift-left, the transition to incremental shift-left has also been largely completed.
Agile/DevOps shift-left testing
As illustrated in the following figure, Agile and DevOps projects have numerous short duration Vs (sprints) in lieu of a single or small number of V as in the previous two examples of shift-left testing. These small Vs would also be modified if one or more early sprints are used to block out the basic requirements and architecture or if test-first and test-driven development (TDD) are being performed. The shift-left occurs because the types of testing on the right sides of the earliest of these tiny Vs are to the left of the corresponding types of testing on right side of the larger V(s) they replace. While the following figure appears remarkably the same for Agile and DevOps, Agile testing is typically restricted to developmental testing and does not include operational testing, which occurs once the system is placed into operation. The transition to Agile/DevOps shift-left testing is currently popular and ongoing.
Model-based shift-left testing
The previous forms all concentrated on testing earlier in the development cycle. However, they all test after software exists and seek to uncover only implementation defects.
Model-based testing moves testing to the left side of the Vs, by testing requirements, architecture, and design models. This shift begins testing almost immediately, instead of waiting a long time (traditional testing), medium time (incremental testing), or short time (Agile/DevOps) for software to become available to the right side of the Vs. This trend is just beginning.
- Donald Firesmith (23 March 2015). "Four Types of Shift Left Testing". Archived from the original on 2015-09-05. Retrieved 27 March 2015.
- Microsoft (2012). "Test Early and Often". Retrieved 27 March 2015.
- Smith, Larry (September 2001). "Shift-Left Testing". Dr. Dobb's Journal. 26 (9): 56, 62.
- "Sep01: Shift-Left Testing". 2014-08-10. Archived from the original on 2014-08-10. Retrieved 2019-09-06.
- Velocity Partners (28 January 2014). "Agile Testing - The Agile Test Automation Pyramid". Retrieved 27 March 2015.
- Paul Bahrs (6 November 2014). "Shift Left: Approaches and Practices". Retrieved 27 March 2015.
- Dibbe Edwards (18 September 2014). "Enabling DevOps Success with Shift Left Continuous Testing". Retrieved 27 March 2015.
- Donald Firesmith (11 November 2013). "Using V Models for Testing". Retrieved 27 March 2015.
- Microsoft (2013). "Record and Playback Manual Tests". Retrieved 27 March 2015.