| description |
|---|
Using optimistic merging and pending-failure-depth to protect your Merge Queue from flaky failures |
Some CI jobs fail for reasons unrelated to a PR's code change, such as due to flaky tests or a CI runner disconnecting. These failures are usually cleared when the CI job is rerun. If a second PR that depends on the first does pass, it is very likely that the first PR was good and simply experienced a transient failure. Trunk Merge Queue can use the combination of Optimistic Merging and Pending Failure Depth to merge pull requests that would otherwise be rejected from the queue.
{% hint style="success" %} If you have a lot of flaky tests in your projects, you should track and fix them with Trunk Flaky Tests. Anti-flake protection helps reduce the impact of flaky tests but doesn't help you detect, track, and eliminate them. {% endhint %}
In the video below, you can see an example of this anti-flake protection:
{% embed url="https://share.vidyard.com/watch/5YxXzJ5Szy7vP4F7awgTsc" %} anti-flake protection with optimistic merging + pending failure depth {% endembed %}
| what's happening? | queue |
|---|---|
| A, B, C begin predictive testing | main <- A <- B+a <- C+ba |
| B fails testing | main <- A <- B+a <- C+ba |
| predictive failure depth keeps B from being evicted while C tests | main <- A <- B+a (hold) <- C+ba |
| C passes | main <- A <- B+a <- C+ba |
| optimistic merging allows A, B, C to merge | merge A B C |
{% hint style="info" %} Optimistic Merging only works when the Pending Failure Depth is set to a value greater than zero. When zero or disabled, Merge will not hold any failed tests in the queue. {% endhint %}
Achieve anti-flake protection works by enabling Optimistic Merge and setting Pending Failure Depth greater than 0 in the Merge UI settings:
setting to enable anti-flake protection
The Fine Print
There is a small tradeoff to be made when optimistic merging is used. You can get into a situation where an actually broken test in say change 'B' is corrected by a change in 'C'. In this case if you later reverted 'C' your build would be broken.