# Single-node functional tests for Scylla's CQL features

These tests use the Python CQL library and the pytest frameworks.
By using an actual CQL library for the tests, they can be run against *any*
implementation of CQL - both Scylla and Cassandra. Most tests - except in
rare cases - should pass on both, to ensure that Scylla is compatible with
Cassandra in most features.

To run all tests against an already-running local installation of Scylla
or Cassandra on localhost, just run `pytest`. The "--host" and "--port"
can be used to give a different location for the running Scylla or Cassanra.
The "--ssl" option can be used to use an encrypted (TLSv1.2) connection.

More conveniently, we have two scripts - "run" and "run-cassandra" - which
do all the work necessary to start Scylla or Cassandra (respectively),
and run the tests on them. The Scylla or Cassandra process is run in a
temporary directory which is automatically deleted when the test ends.

"run" automatically picks the most recently compiled version of Scylla in
`build/*/scylla` - but this choice of Scylla executable can be overridden with
the `SCYLLA` environment variable. "run-cassandra" defaults to running the
command `cassandra` from the user's path, but this can be overridden by setting
the `CASSANDRA` environment variable to the path of the `cassandra` script,
e.g., `export CASSANDRA=$HOME/apache-cassandra-3.11.10/bin/cassandra`.
A few of the tests also require the `nodetool` when running on Cassandra -
this tool is again expected to be in the user's path, or be overridden with
the `NODETOOL` environment variable. Nodetool is **not** needed to test
Scylla.

Additional options can be passed to "pytest" or to "run" / "run-cassandra"
to control which tests to run:

* To run all tests in a single file, do `pytest test_table.py`.
* To run a single specific test, do `pytest test_table.py::test_create_table_unsupported_names`.
* To run the same test or tests 100 times, add the `--count=100` option.
  This is faster than running `run` 100 times, because Scylla is only run
  once, and also counts for you how many of the runs failed.
  For `pytest` to support the `--count` option, you need to install a
  pytest extension: `pip install pytest-repeat`

Additional useful pytest options, especially useful for debugging tests:

* -v: show the names of each individual test running instead of just dots.
* -s: show the full output of running tests (by default, pytest captures the test's output and only displays it if a test fails)

# Developing new cql-pytest tests

The cql-pytest test framework is designed to encourage Scylla developers
to quickly write _extensive_ functional tests for the CQL features which
they develop. This is why cql-pytest is included in the main Scylla
repository (and not some external repository), and why the test framework
focuses on the ease of writing new tests, the ease of understanding test
failures, and the speed to run and re-run tests especially during development
of the test and/or the tested feature. Moreover, the ability to run the
same tests against Cassandra is meant to make it easier to write good tests
even before developing a feature (so-called "test-driven development").

To maintain these benefits, we recommend that the following principles
and practices be followed when writing new tests:

1. **Keep each test fast**: Ideally each test function should
   take a fraction of a second. At the time of this writing, the entire
   cql-pytest test suite of over 800 test functions takes around 80
   seconds to run, on average 0.1 second per test. Always think if your
   test really requires inserting a million items or sleeping 5 seconds -
   usually it does NOT.
   Short tests make it easy and fun to run and rerun a single test during
   development, and also allow developers to run the entire cql-pytest
   test suite during development instead of trying to guess which test might
   break.

2. **Keep each test small**: Don't write one big test function for many
   aspects of some feature. Instead, write in the same test file many small
   test functions, each for a different aspect of the feature. This makes it
   easier to understand what each small test checks. And when a test fails,
   it makes it easier to understand exactly which part of the feature broke.
   It also makes the test code easier to read and understand.

3. **Use fixtures to reduce test time**: When testing a feature with many
   small test functions (as recommended above), often all of these small
   test functions need some common setup, such as a table with a certain
   schema or with certain data in it. Instead of each small test function
   re-creating the same table (which takes time), use pytest _fixtures_.
   Fixtures allow several test functions to use the same temporary test table.
   Different tests can safely share a test table by using unique keys
   instead of hard-coded keys that can break if another test accidentally
   uses the same key.
   One of the reasons the tests in test/alternator are currently faster
   than test/cql-pytest (on average 0.03 second per test function, vs. 0.1)
   is that they make better use of fixtures, and very few tests create
   their own tables. This is an ideal we should strive for (but harder
   to achieve in CQL because different schemas require different tables).

4. **Write comments**: No, a self-explanatory test name is NOT enough.
   It may be self-explanatory to you, but when the test fails a year
   from now, nobody will remember what you were testing, or *why* you
   decided to check these specific conditions, or if it's a forgotten
   backport of a fix to an issue, which issue was that.
   Before each test function, please explain why it exists - what feature
   it intends to verify, and why it is tested in this specific way.
   If the test is meant to reproduce a specific issue, please give the
   issue number. All of this will be very helpful for later developers
   which need to understand why your test suddenly failed after their
   refactoring, or on a different Scylla branch.

5. **Run your test against Cassandra**: It is not enough to run your
   test against Scylla and see that it passes. Run it against Cassandra as
   well, using test/cql-pytest/run-cassandra. If the feature being tested
   is Scylla-only, the test can be skipped on Cassandra by using the
   `scylla_only` fixture. But most of Scylla's CQL features are identical
   to Cassandra's and therefore most of our CQL tests should pass on
   Cassandra. If a test does not pass on Cassandra, the test itself is
   likely wrong, and should be fixed. In rare cases, the test fails on
   Cassandra because of a known bug in Cassandra or a deliberate difference
   between Scylla and Cassandra; In these rare cases, the test can be
   skipped by using the `cassandra_bug` fixture. However, make sure that
   you explain in a comment why you did this - often this means linking
   to an open Cassandra issue, or to a Scylla issue where we decided to
   diverge from Cassandra's implementation - or just explaining the
   difference in text.

6. **Think about risky cases, don't just randomize**: One of the benefits
   of a developer also developing the tests during development of a feature
   (instead of someone else doing it later) is that specific edge cases
   can be considered and tested. For example, consider some operation taking
   a string. If a developer knows that an empty string or a very long
   string required special code and are at risk of being mis-handled or
   are at risk to break during some future refactoring, the developer should
   write separate tests for these cases.
   If, instead, we write a test that loops 1000 times testing a random
   string of length 10 - the result will be slow **and** will also miss
   the interesting cases - neither the empty or very long strings will
   come up as random draws of strings of length 10.
   Another danger of randomized tests is that they tend to obscure what
   is actually being tested: For example, a reviewer of the test may think
   the empty string is included in the test, while actually it isn't.
   Randomized "fuzz" testing has its benefits, but it is almost always the
   wrong thing to do in the context of the cql-pytest framework. We should
   probably have a separate framework (or at least separate files) for
   these tests.

7. **Write tests, not a test library**: Developers are often told that
   long functions are evil, and are tempted to take maybe-useful sections
   of code from their test function and split them out to utility functions.
   Having a small number of these utility functions is indeed convenient,
   and we have some in `util.py`, `nodetool.py` and `rest_api.py`. However
   please resist the urge to add more and more of these utility functions.
   Utility functions are bad for several reasons. First, they make it
   harder to read tests - any reader will understand what "cql.execute(...)"
   does, but not be familiar with dozens of obscure utility functions.
   Second, when a utility function is written for the benefit of a single
   test, it is often much less general than its author thought, and when
   it comes time to reuse it in a second test, it turns out it needs to be
   duplicated or changed, and the result is many confusingly-similar
   utility functions, or utility functions with many confusing parameters.
   We've seen this happening in other frameworks such as dtest.
   If you believe something _should_ be a utility function, start by
   putting it inside the single test file that needs it - and only move
   it to util.py if several test files can benefit from it.
   At the time of this writing, cql-pytest has over 20,000 lines of test
   code, and around of 500 lines of library code. Please keep this ratio.
   We're writing a collection of tests - not a library.

8. **Do not over-design**: Continuing the "we're writing tests, not a
   library" theme, please focus on making individual tests easy and fast
   to write as well as later read. Do not over-design the test suite to use
   cool Python features like classes, strong typing, and other features that
   are useful for big projects but only make writing small tests more
   difficult. Putting tests inside Classes, in particular (as we do in dtest),
   just make it more cumbersome to run an individual test - that now needs
   to specify not just the file and test names, but also the class name.
   To share functions between different tests, a test file is good enough -
   we don't need a class inside the file.

9. **Put tests in the right file**: Try to keep related tests functions -
   tests which check the same feature, have a similar theme, or perhaps use
   some shared fixture or set of convenience functions, in the same test file.
   There is no overhead involved with having many small test files
   (unlike C++ tests where compiling each file has a large fixed overhead),
   but when there are too many small test files there is a cognitive
   burden for developers trying to find tests or trying to decide where
   to place new tests. So when writing a new test please try to consider
   whether it fits the theme of an already-existing test file, and if not try
   to create a new test file that you can explain, in a comment, which
   additional tests might belong there in the future.

10. **Test user-visible CQL features**: Usually (but not always), we should
   strive of testing CQL features that a user might access through a
   CQL driver. We do have test that check log messages, traces, and so
   on, but these should be the minority. The majority of the tests should
   not check log messages which aren't visible to a CQL application.
   Tests that do check for error conditions _should_ check the error message
   but should usually focus on the _type_ of the error and important
   substrings, not entire error messages. We don't want dozens of tests to
   break every time we change a trivial detail in an error message.

11. **Leave `cassandra_tests` alone**: The subdirectory `cassandra_tests/`
   contains test that were translated from Cassandra's unit tests, and
   they use a small compatibility layer (`cassandra_tests/porting.py`)
   to make this translation easier. If you are not translating additional
   tests, please avoid modifying this directory. In particular, avoid
   changing those tests without good reason, and don't add new tests to
   any file in it. Put new tests in the `cql-pytest` directory, in any
   place except the `cassandra_tests` subdirectory.
