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As mentioned previously, Google has specific notions of test size. Small tests are restricted to one thread, one process, one machine. Larger tests do not have the same restrictions. But Google also has notions of test scope. A unit test necessarily is of smaller scope than an integration test. And the largest-scoped tests (sometimes called end-to-end or system tests) typically involve several real dependencies and fewer test doubles.

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So why have larger tests? Reflect back on your coding process. How do you confirm that the programs you write actually work? You might be writing and running unit tests as you go, but do you find yourself running the actual binary and trying it out yourself? And when you share this code with others, how do they test it? By running your unit tests, or by trying it out themselves?

Unit tests can give you confidence about individual functions, objects, and modules, but large tests provide more confidence that the overall system works as intended. And having actual automated tests scales in ways that manual testing does not.

One way of envisioning fidelity is in terms of the environment. As Figure 14-1 illustrates, unit tests bundle a test and a small portion of code together as a runnable unit, which ensures the code is tested but is very different from how production code runs. Production itself is, naturally, the environment of highest fidelity in testing. There is also a spectrum of interim options. A key for larger tests is to find the proper fit, because increasing fidelity also comes with increasing costs and (in the case of production) increasing risk of failure.

Tests can also be measured in terms of how faithful the test content is to reality. Many handcrafted, large tests are dismissed by engineers if the test data itself looks unrealistic. Test data copied from production is much more faithful to reality (having been captured that way), but a big challenge is how to create realistic test traffic before launching the new code. This is particularly a problem in artificial intelligence (AI), for which the "seed" data often suffers from intrinsic bias. And, because most data for unit tests is handcrafted, it covers a narrow range of cases and tends to conform to the biases of the author. The uncovered scenarios missed by the data represent a fidelity gap in the tests.

A single unit test typically covers one class or module. Test doubles (as discussed in Test Doubles) are frequently used to eliminate heavyweight or hard-to-test dependencies. But when those dependencies are replaced, it becomes possible that the replacement and the doubled thing do not agree.

Almost all unit tests at Google are written by the same engineer who is writing the unit under test. When those unit tests need doubles and when the doubles used are mocks, it is the engineer writing the unit test defining the mock and its intended behavior. But that engineer usually did not write the thing being mocked and can be misinformed about its actual behavior. The relationship between the unit under test and a given peer is a behavioral contract, and if the engineer is mistaken about the actual behavior, the understanding of the contract is invalid.

Moreover, mocks become stale. If this mock-based unit test is not visible to the author of the real implementation and the real implementation changes, there is no signal that the test (and the code being tested) should be updated to keep up with the changes.

Unit tests cover code within a given binary. But that binary is typically not completely self-sufficient in terms of how it is executed. Usually a binary has some kind of deployment configuration or starter script. Additionally, real end-user-serving production instances have their own configuration files or configuration databases.

If there are issues with these files or the compatibility between the state defined by these stores and the binary in question, these can lead to major user issues. Unit tests alone cannot verify this compatibility.1 Incidentally, this is a good reason to ensure that your configuration is in version control as well as your code, because then, changes to configuration can be identified as the source of bugs as opposed to introducing random external flakiness and can be built in to large tests.

At Google, configuration changes are the number one reason for our major outages. This is an area in which we have underperformed and has led to some of our most embarrassing bugs. For example, there was a global Google outage back in 2013 due to a bad network configuration push that was never tested. Configurations tend to be written in configuration languages, not production code languages. They also often have faster production rollout cycles than binaries, and they can be more difficult to test. All of these lead to a higher likelihood of failure. But at least in this case (and others), configuration was version controlled, and we could quickly identify the culprit and mitigate the issue.

At Google, unit tests are intended to be small and fast because they need to fit into our standard test execution infrastructure and also be run many times as part of a frictionless developer workflow. But performance, load, and stress testing often require sending large volumes of traffic to a given binary. These volumes become difficult to test in the model of a typical unit test. And our large volumes are big, often thousands or millions of queries per second (in the case of ads, real-time bidding)!

Unit tests are limited by the imagination of the engineer writing them. That is, they can only test for anticipated behaviors and inputs. However, issues that users find with a product are mostly unanticipated (otherwise it would be unlikely that they would make it to end users as issues). This fact suggests that different test techniques are needed to test for unanticipated behaviors.

Unit tests are limited to the scope that they cover (especially with the widespread use of test doubles), so if behavior changes in areas outside of this scope, it cannot be detected. And because unit tests are designed to be fast and reliable, they deliberately eliminate the chaos of real dependencies, network, and data. A unit test is like a problem in theoretical physics: ensconced in a vacuum, neatly hidden from the mess of the real world, which is great for speed and reliability but misses certain defect categories.

Additionally, larger tests present two other challenges. First, there is a challenge of ownership. A unit test is clearly owned by the engineer (and team) who owns the unit. A larger test spans multiple units and thus can span multiple owners. This presents a long-term ownership challenge: who is responsible for maintaining the test and who is responsible for diagnosing issues when the test breaks? Without clear ownership, a test rots.

When we discussed the history of testing at Google earlier (see Testing Overview), we mentioned how Google Web Server (GWS) mandated automated tests in 2003 and how this was a watershed moment. However, we actually had automated tests in use before this point, but a common practice was using automated large and enormous tests. For example, AdWords created an end-to-end test back in 2001 to validate product scenarios. Similarly, in 2002, Search wrote a similar "regression test" for its indexing code, and AdSense (which had not even publicly launched yet) created its variation on the AdWords test.

Throughout this book, we have looked at the influence of time on software engineering, because Google has built software running for more than 20 years. How are larger tests influenced by the time dimension? We know that certain activities make more sense the longer the expected lifespan of code, and testing of various forms is an activity that makes sense at all levels, but the test types that are appropriate change over the expected lifetime of code.

As we pointed out before, unit tests begin to make sense for software with an expected lifespan from hours on up. At the minutes level (for small scripts), manual testing is most common, and the SUT usually runs locally, but the local demo likely is production, especially for one-off scripts, demos, or experiments. At longer lifespans, manual testing continues to exist, but the SUTs usually diverge because the production instance is often cloud hosted instead of locally hosted.

Even worse, if the code is difficult to unit test (because of the way it was implemented in the first place), the only automated tests that can be written are end-to-end ones, and we have inadvertently created "legacy code" within days.

It is critical for longer-term health to move toward the test pyramid within the first few days of development by building out unit tests, and then to top it off after that point by introducing automated integration tests and moving away from manual end-to-end tests. We succeeded by making unit tests a requirement for submission, but covering the gap between unit tests and manual tests is necessary for long-term health.

It would seem that larger tests should be more necessary and more appropriate at larger scales of software, but even though this is so, the complexity of authoring, running, maintaining, and debugging these tests increases with the growth in scale, even more so than with unit tests.

The rate of distinct scenarios to test in an end-to-end way can grow exponentially or combinatorially depending on the structure of the system under test, and that growth does not scale. Therefore, as the system grows, we must find alternative larger testing strategies to keep things manageable.

One way to achieve this test ratio when presented with a user journey that can require contributions from many internal systems is to "chain" tests, as illustrated in Figure 14-4, not specifically in their execution, but to create multiple smaller pairwise integration tests that represent the overall scenario. This is done by ensuring that the output of one test is used as the input to another test by persisting this output to a data repository. 17dc91bb1f