Firs pass at improving the testing documentation

[ci skip]
Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/development/fe_guide/testing.md

@@ -80,18 +80,17 @@ If an integration test depends on JavaScript to run correctly, you need to make
 sure the spec is configured to enable JavaScript when the tests are run. If you
 don't do this you'll see vague error messages from the spec runner.
 
-To enable a JavaScript driver in an `rspec` test, add `js: true` to the
+To enable a JavaScript driver in an `rspec` test, add `:js` to the
 individual spec or the context block containing multiple specs that need
 JavaScript enabled:
 
 ```ruby
-
 # For one spec
-it 'presents information about abuse report', js: true do
+it 'presents information about abuse report', :js do
     # assertions...
 end
 
-describe "Admin::AbuseReports", js: true do
+describe "Admin::AbuseReports", :js do
     it 'presents information about abuse report' do
         # assertions...
     end

doc/development/testing.md

@@ -9,6 +9,144 @@ this guide defines a rule that contradicts the thoughtbot guide, this guide
 takes precedence. Some guidelines may be repeated verbatim to stress their
 importance.
 
+## Definitions
+
+### Unit tests
+
+Formal definition: https://en.wikipedia.org/wiki/Unit_testing
+
+These kind of tests ensure that a single unit of code (a method) works as expected (given an input, it has a predictable output). These tests should be isolated as much as possible (for instance model methods that don't do anything with the database shouldn't need a DB record).
+
+| Code path | Tests path | Testing engine | Notes |
+| --------- | ---------- | -------------- | ----- |
+| `app/finders/` | `spec/finders/` | RSpec | |
+| `app/helpers/` | `spec/helpers/` | RSpec | |
+| `app/migrations/` | `spec/migrations/` | RSpec | |
+| `app/policies/` | `spec/policies/` | RSpec | |
+| `app/presenters/` | `spec/presenters/` | RSpec | |
+| `app/routing/` | `spec/routing/` | RSpec | |
+| `app/serializers/` | `spec/serializers/` | RSpec | |
+| `app/services/` | `spec/services/` | RSpec | |
+| `app/tasks/` | `spec/tasks/` | RSpec | |
+| `app/uploaders/` | `spec/uploaders/` | RSpec | |
+| `app/views/` | `spec/views/` | RSpec | |
+| `app/workers/` | `spec/workers/` | RSpec | |
+| `app/assets/javascripts/` | `spec/javascripts/` | Karma | More details in the [JavaScript](#javascript) section. |
+
+### Integration tests
+
+Formal definition: https://en.wikipedia.org/wiki/Integration_testing
+
+These kind of tests ensure that individual parts of the application work well together, without the overhead of the actual app environment (i.e. the browser). These tests should assert at the request/response level: status code, headers, body. They're useful to test permissions, redirections, what view is rendered etc.
+
+| Code path | Tests path | Testing engine | Notes |
+| --------- | ---------- | -------------- | ----- |
+| `app/controllers/` | `spec/controllers/` | RSpec | |
+| `lib/api/` | `spec/requests/api/` | RSpec | |
+| `lib/ci/api/` | `spec/requests/ci/api/` | RSpec | |
+| `app/assets/javascripts/` | `spec/javascripts/` | Karma | More details in the [JavaScript](#javascript) section. |
+
+#### About controller tests
+
+In an ideal world, controllers should be thin. However, when this is not the
+case, it's acceptable to write a system test without JavaScript instead of a
+controller test. The reason is that testing a fat controller usually involves a
+lot of stubbing, things like:
+
+```ruby
+controller.instance_variable_set(:@user, user)
+```
+
+and use methods which are deprecated in Rails 5 ([#23768]).
+
+[#23768]: https://gitlab.com/gitlab-org/gitlab-ce/issues/23768
+
+#### About Karma
+
+As you may have noticed, Karma is both in the Unit tests and the Integration
+tests category. That's because Karma is a tool that provides an environment to
+run JavaScript tests, so you can either run unit tests (e.g. test a single
+JavaScript method), or integration tests (e.g. test a component that is composed
+of multiple components).
+
+### System tests or Feature tests
+
+Formal definition: https://en.wikipedia.org/wiki/System_testing.
+
+These kind of tests ensure the application works as expected from a user point
+of view (aka black-box testing). These tests should test a happy path for a
+given page or set of pages, and a test case should be added for any regression
+that couldn't have been caught at lower levels with better tests (i.e. if a
+regression is found, regression tests should be added at the lowest-level
+possible).
+
+| Tests path | Testing engine | Notes |
+| ---------- | -------------- | ----- |
+| `spec/features/` | [Capybara] + [RSpec] | If your spec has the `:js` metadata, the browser driver will be [Poltergeist], otherwise it's using [RackTest]. |
+| `features/` | Spinach | Spinach tests are deprecated, [you shouldn't add new Spinach tests](#spinach-feature-tests). |
+
+[Capybara]: https://github.com/teamcapybara/capybara
+[RSpec]: https://github.com/rspec/rspec-rails#feature-specs
+[Poltergeist]: https://github.com/teamcapybara/capybara#poltergeist
+[RackTest]: https://github.com/teamcapybara/capybara#racktest
+
+#### Good practices
+
+- Create only the necessary records in the database
+- Test a happy path and a less happy path but that's it
+- Every other possible paths should be tested with Unit or Integration tests
+- Test what's displayed on the page, not the internal of ActiveRecord models
+- It's ok to look for DOM elements but don't abuse it since it makes the tests
+  more brittle
+
+If we're confident that the low-level components work well (and we should be if
+we have enough Unit & Integration tests), we shouldn't need to duplicate their
+thorough testing at the System test level.
+
+It's very easy to add tests, but a lot harder to remove or improve tests, so one
+should take care of not introducing too many (slow and duplicated) specs.
+
+The reason why we should follow these good practices are as follows:
+
+- System tests are slow to run since they spin up the entire application stack
+  in a headless browser, and even slower when they integrate a JS driver
+- With System tests run with a driver that supports JavaScript, the tests are
+  run in different thread than the application. This means it does not share a
+  database connection and your test will have to commit the transactions in
+  order for the running application to see the data (and vice-versa). In that
+  case we need to truncate the database after each spec instead of simply
+  rolling back a transaction (the faster strategy that's in use for other kind
+  of tests). This is slower than transactions, however, so we want to use
+  truncation only when necessary.
+
+## How to test at the correct level?
+
+As many things in life, deciding what to test at each level of testing is a
+trade-off:
+
+- Unit tests are usually cheap, and you should consider them like the basement
+  of your house: you need them to be confident that your code is behaving
+  correctly. However if you run only unit tests without integration / system tests, you might miss the [big] [picture]!
+- Integration tests are bit more expensive but don't abuse them. A feature test
+  is often better than an integration test that is stubbing a lot of internals.
+- System tests are expensive (compared to unit tests), even more if they require
+  a JavaScript driver. Make sure to follow the guidelines in the [Speed](#test-speed)
+  section.
+
+Another way to see it is to think about the "cost of tests", this is well
+explained [in this article][tests-cost] and the basic idea is that the cost of a
+test includes:
+
+- The time it takes to write the test
+- The time it takes to run the test every time the suite runs
+- The time it takes to understand the test
+- The time it takes to fix the test if it breaks and the underlying code is OK
+- Maybe, the time it takes to change the code to make the code testable.
+
+[big]: https://twitter.com/timbray/status/822470746773409794
+[picture]: https://twitter.com/withzombies/status/829716565834752000
+[tests-cost]: https://medium.com/table-xi/high-cost-tests-and-high-value-tests-a86e27a54df#.2ulyh3a4e
+
 ## Factories
 
 GitLab uses [factory_girl] as a test fixture replacement.
@@ -117,11 +255,20 @@ it 'is overdue' do
 end
 ```
 
-### Test speed
+### System / Features tests
 
-GitLab has a massive test suite that, without parallelization, can take more
-than an hour to run. It's important that we make an effort to write tests that
-are accurate and effective _as well as_ fast.
+- Feature specs should be named `ROLE_ACTION_spec.rb`, such as
+  `user_changes_password_spec.rb`.
+- Use only one `feature` block per feature spec file.
+- Use scenario titles that describe the success and failure paths.
+- Avoid scenario titles that add no information, such as "successfully".
+- Avoid scenario titles that repeat the feature title.
+
+## Test speed
+
+GitLab has a massive test suite that, without [parallelization], can take hours
+to run. It's important that we make an effort to write tests that are accurate
+and effective _as well as_ fast.
 
 Here are some things to keep in mind regarding test performance:
 
@@ -132,38 +279,40 @@ Here are some things to keep in mind regarding test performance:
 - Use `create(:empty_project)` instead of `create(:project)` when you don't need
   the underlying Git repository. Filesystem operations are slow!
 - Don't mark a feature as requiring JavaScript (through `@javascript` in
-  Spinach or `js: true` in RSpec) unless it's _actually_ required for the test
+  Spinach or `:js` in RSpec) unless it's _actually_ required for the test
   to be valid. Headless browser testing is slow!
 
-### Features / Integration
+[parallelization]: #test-suite-parallelization-on-the-ci
 
-GitLab uses [rspec-rails feature specs] to test features in a browser
-environment. These are [capybara] specs running on the headless [poltergeist]
-driver.
+### Monitoring
 
-- Feature specs live in `spec/features/` and should be named
-  `ROLE_ACTION_spec.rb`, such as `user_changes_password_spec.rb`.
-- Use only one `feature` block per feature spec file.
-- Use scenario titles that describe the success and failure paths.
-- Avoid scenario titles that add no information, such as "successfully."
-- Avoid scenario titles that repeat the feature title.
+The GitLab test suite is [monitored] and a [public dashboard] is available for
+everyone to see. Feel free to look at the slowest test files and try to improve
+them.
 
-[rspec-rails feature specs]: https://github.com/rspec/rspec-rails#feature-specs
-[capybara]: https://github.com/teamcapybara/capybara
-[poltergeist]: https://github.com/teampoltergeist/poltergeist
+[monitored]: /development/performance.html#rspec-profiling
+[public dashboard]: https://redash.gitlab.com/public/dashboards/l1WhHXaxrCWM5Ai9D7YDqHKehq6OU3bx5gssaiWe?org_slug=default
 
-## Spinach (feature) tests
+## Test suite parallelization on the CI
 
-GitLab [moved from Cucumber to Spinach](https://github.com/gitlabhq/gitlabhq/pull/1426)
-for its feature/integration tests in September 2012.
+Our current CI parallelization setup is as follows:
 
-As of March 2016, we are [trying to avoid adding new Spinach
-tests](https://gitlab.com/gitlab-org/gitlab-ce/issues/14121) going forward,
-opting for [RSpec feature](#features-integration) specs.
+1. The `knapsack` job in the prepare stage that is supposed to ensure we have a `knapsack/rspec_report.json` file:
+  - The `knapsack/rspec_report.json` file is fetched from the cache with the
+  `knapsack` key, if it's not here we initialize the file with `{}`.
+1. Each `rspec x y` job are run with `knapsack rspec` and should have an evenly
+  distributed share of tests:
+  - It works because the jobs have access to the `knapsack/rspec_report.json`
+  since the "artifacts from all previous stages are passed by default". [^1]
+  - the jobs set their own report path to `KNAPSACK_REPORT_PATH=knapsack/spinach_node_${CI_NODE_INDEX}_${CI_NODE_TOTAL}_report.json`
+  - if knapsack is doing its job, test files that are run should be listed under
+  `Report specs`, not under `Leftover specs`
+1. The `update-knapsack` job takes all the `knapsack/spinach_node_${CI_NODE_INDEX}_${CI_NODE_TOTAL}_report.json` files from
+  the `rspec x y` jobs and merge them all together into a single
+  `knapsack/rspec_report.json` that is then cached with the `knapsack` key
 
-Adding new Spinach scenarios is acceptable _only if_ the new scenario requires
-no more than one new `step` definition. If more than that is required, the
-test should be re-implemented using RSpec instead.
+After that, the next pipeline will use the up-to-date
+`knapsack/rspec_report.json` file.
 
 ## Testing Rake Tasks
 
@@ -201,6 +350,21 @@ describe 'gitlab:shell rake tasks' do
 end
 ```
 
+## Spinach (feature) tests
+
+GitLab [moved from Cucumber to Spinach](https://github.com/gitlabhq/gitlabhq/pull/1426)
+for its feature/integration tests in September 2012.
+
+As of March 2016, we are [trying to avoid adding new Spinach
+tests](https://gitlab.com/gitlab-org/gitlab-ce/issues/14121) going forward,
+opting for [RSpec feature](#features-integration) specs.
+
+Adding new Spinach scenarios is acceptable _only if_ the new scenario requires
+no more than one new `step` definition. If more than that is required, the
+test should be re-implemented using RSpec instead.
+
 ---
 
 [Return to Development documentation](README.md)
+
+[^1]: /ci/yaml/README.html#dependencies