Managing flaky tests in MergeQueue

Flaky tests can be extremely frustrating for the health of your CI. These can especially be deteriorating when you want to incorporate a merge queue in your development workflow. Aviator MergeQueue offers capabilities to reduce the impact of flaky tests on your queue.

Optimistic validation

To understand the queue workflow around flaky tests, it’s worth first understanding how optimistic validation works. Note that these capabilities specifically work in parallel mode, including all variations of parallel mode (affected targets, fast forwarding, and batching).
Consider a simple example of parallel mode where Aviator creates Draft PRs that combine changes from one or more queued PRs. These Draft PRs run the CIs in parallel to reduce the wait time. Let’s say there are two PRs PR #1 and PR #2 that are queued, and corresponding draft PRs are created such that draft PR #3 has changes from PR #1 and draft PR #4 have changes from PR #1 and PR #2.
Parallel mode with two PRs
In a standard MergeQueue workflow, if the CI of PR #3 pass we will merge PR #1, and then when the CI of PR #4 pass we merge PR #2. Now when optimistic validation is enabled, Aviator validates the status checks of both PR #3 and PR #4 together. In this case, if CI validation of PR #4 passes before PR #3, Aviator will merge both PR #1 and PR #2 as the combined changes are valid even though the CI of PR #3 is not completed.
To keep the API limits in order, Aviator analyzes only the top 3 PRs in the queue.

Flaky tests

This optimistic validation approach builds the foundation of managing flaky tests in MergeQueue. Alongside enabling optimistic validation, you can also specify the optimistic_validation_failure_depth rule that informs Aviator to not dequeue a failed CI immediately. So, let’s again look at the example above.
Let’s say you set the optimistic_validation_failure_depth to 2. And in the example above, if some tests are flaky, causing the CI on PR #3 to fail, Aviator will not immediately dequeue PR #1 and reset the queue. Instead, we will wait for 2 consecutive CI to fail before dequeuing. So if later the CI of PR #4 passes, we use the same optimistic logic above to merge both PR #1 and PR #2 creating eventual consistency.
Resets in a merge queue can impact the performance (time to merge) for the PRs. By introducing some resistance for failed tests, you can significantly improve the overall time to merge in your queue.
Note that there is a side effect of this configuration. By introducing a failure depth, real errors will take longer to be kicked out of the queue. Therefore, the failure depth should generally be kept small. Although, We have noticed with most of the users of Aviator that this side effect is typically acceptable as the build failures in the queue are relatively small. Eventually it also depends on how flaky your tests are.

Configuring optimistic validation

There are two settings to configure optimistic validation. Both are specified within the parallel mode section of the configuration.
optimistic_validation - boolean value that configures only the look-ahead of CI. If the CI of the top of the PR fails, it will still immediately dequeue the PR. Defaults to true.
optimistic_validation_failure_depth - integer value that defines how many subsequent failures are required to dequeue a PR. Requires optimistic_validation to also be enabled.
trigger: "mergequeue"
type: "parallel"
max_parallel_builds: 10
optimistic_validation: true
optimistic_validation_failure_depth: 2


If you are looking for an end-to-end capability to manage flaky tests in your workflow, also checkout TestDeck.