There are multiple different ways that tests can be run. Each has different trade-offs between cost, reliability and test coverage.
The primary goal for OpenStack test infrastructure is to deliver highly reliable testing: with 500 patches successfully getting through the OpenStack gate on a peak day, even short service interruptions have a significant impact on project velocity.
The same velocity makes it extremely risky to disable tests: once disabled a test is likely to bitrot quickly, making re-enabling such tests hard.
This gives the following principle:
Experimental jobs have low reliability requirements: they are run by hand on developer request, typically as part of bringing up a new job definition. Failures in experimental jobs are not the responsibility of openstack-infra, though they will offer best-effort assistance to developers.
Silent jobs are jobs that are not yet ready to vote on code changes. They might not be ready because of known failures, a lack of redundancy in the infrastructure or some other reason. In all other regards they are the same as Check jobs, which means we find out about the test reliability and can accurately assess whether the job is ready to promote to Check status.
Third party test jobs are able to vote on code changes (+/- 1 only). These jobs are run by third parties on code pushes, but are not able to prevent code landing. (Developers of projects usually take negative votes from third party systems seriously however). Third party test jobs cannot be gates, and cannot set the ‘+2 verified’ flag on review.
Check jobs are used to verify each patch pushed to Gerrit. Like a third party test job they run against a single pushed patch, rather than the proposed merged state of the repository. A failure reported by a check job will prevent the patch being approved. As such check jobs have to run in a highly available environment with only infra controlled components permitted to have shared failure domains.
Gate jobs are used to detect failures in patches after they are approved. They run against the state the OpenStack projects will have if the code is merged (rather than the state of the pushed code). This allows detection of semantic conflicts cross-patch (and the usual state for OpenStack is that multiple patches are going through the gate at once, so this is crucial). Failures in the gate both prevent the patch landing and cause all the pending patches after it to be retested. Gate jobs, like check jobs, have to run in a highly available environment with only infra controlled components permitted to have shared failure domains.