C* DTests currently run using the python test framework nosetest. This framework has been largely abandoned with no releases since 2015 and a general strong consensus in the python community that pytest is the future.
Currently (and historically) dtests have always been difficult to run, flaky and unpredictable in CI environments, and almost impossible to debug.
On November 28th, 2017, I proposed on the dev@ list that we move the dtests from nosetests to pytests. I got replies from Jon Haddad, Philip Thompson, and kurt greaves with really only "+1" like replies to the proposal.
Since then I've been working pretty much non stop to complete the large refactor of dtests to pytests. As part of this effort (and due to the migration tools that exist require it) I also ported the code to python3 (from the current python 2.7 based code-base).
- Migrate dtests from executing using the nosetest framework to pytest
- Port the entire code base from Python 2.7 to Python 3.6
- Update run_dtests.py to work with pytest
- Add --dtest-print-tests-only option to run_dtests.py to get easily parsable list of all available collected tests
- Update README.md for executing the dtests with pytest
- Add new debugging tips section to README.md to help with some basics of debugging python3 and pytest
- Migrate all existing Enviornment Variable usage as a means to control dtest operation modes to argparse command line options with documented help on each toggles intended usage
- Migration of old unitTest and nose based test structure to modern pytest fixture approach
- Automatic detection of physical system resources to automatically determine if @pytest.mark.resource_intensive annotated tests should be collected and run on the system where they are being executed
- new pytest fixture replacements for @since and @pytest.mark.upgrade_test annotations
- Migration to python logging framework
- Upgrade thrift bindings to latest version with full python3 compatibility
- Remove deprecated cql and pycassa dependencies and migrate any remaining tests to fully remove those dependencies
- Fixed dozens of tests that would hang the pytest framework forever when run in CI enviornments
- Ran code nearly 300 times in CircleCI during the migration and to find, identify, and fix any tests capable of hanging CI
- Upgrade Tests do not yet run in CI and still need additional migration work (although all upgrade test classes compile successfully)
I started with the nose2pytest https://github.com/pytest-dev/nose2pytest migration tool. As this required python 3 language support I found myself down the 2to3 python migration path. While painful to do this, the benefits of python3 over python2.7 are numerous and moving to python3 for the additional debugging tools now available to use when fixing dtests makes the effort worth it for that reason alone!
After the automated tools did their thing I began what was a much longer and tedious manual process than I ever could have expected due to the custom many ways we did things in dtests (frequently to work around nosetest limitations of missing features that thankfully are now all included with the pytest framework). I've done nearly 300 test runs of my migration branch with circleci.
With vnodes, there are currently only 6 remaining dtest test failures.
Without vnodes, there are 12 remaining dtest failures.
It turns out that after the dtests were moved to ASF Jenkins from cassci, the jobs were misconfigured and we actually haven't been running the dtests in the non-vnodes configuration. The current most recent trunk dtest job to complete on ASF Jenkins (with vnodes) was https://builds.apache.org/view/A-D/view/Cassandra/job/Cassandra-trunk-dtest/387/. That test run had 36 test failures.
There are great performance improvements so far with pytests. With a parallism factor of 90x in CircleCI the dtests-no-vnodes job completed all tests in 16 minutes and 24 seconds and the dtests-with-vnodes job completed all tests in 14 minutes and 22 seconds.. (Compare that to the 8+ hours ASF Jenkins currently takes!).
A dtest on pytest compatible CircleCI configuration is available at https://github.com/mkjellman/cassandra/blob/trunk_circle/.circleci/config.yml
A very small patch is required to ccm for 2 places that were not yet python3 compatible. (I didn't do a full audit of ccm, but these appear to be the only two issues I've found even with a ton of testing when using ccmlib with dtests and running with python 3).
For the actual dtest work itself: I've squashed all my work down into a single commit available at https://github.com/mkjellman/cassandra-dtest/tree/dtests_on_pytest_v2
And the actual commit available at https://github.com/mkjellman/cassandra-dtest/commit/11201c149d1198c0d6d1a479e08bfb8695f14c9e
I tried to put a lot of effort into making the end user experience better for both bootstrapping and running dtests for the first time, but also day to day. I hope people like the approach! In addition to moving to argparse with commented command line arguments to control execution, I also updated the README.md to reflect the new pytest world and also added a new section with a few tips I learned while I was debugging hundreds of tests during this migration process that hopefully are helpful to the rest of the community going forward!
I'm sure the first commit above isn't 100% perfect (although I really have put a lot of effort into it) but my one request is to hopefully get consensus with this first revision of changes committed to reduce any chance of further drift with the codebase as changes are committed into master.