Retrying e2e test suites with Jest

April 21, 2022 , ,

At Wealthfront, we pride ourselves on reliable test coverage across our codebases. However, like many engineering teams, we haven’t been able to completely eliminate flakiness from our end to end (e2e) test suites. While migrating to puppeteer certainly helped, it didn’t eliminate the problem entirely. We’ll get into what causes these flakes below, but suffice it to say they are often complex race conditions that can be difficult to debug. Since we don’t want to block builds or deployments while we investigate, we decided to retry suites during the test runs, flagging the flakes for later investigation.

We use Jest to run our puppeteer e2e suites, and Jest has some good built in retry support. However, this retry logic only supports retrying individual tests and not full test suites. Since our test suites often look something like this:

Retrying individual tests is often not effective. In the above example, we might not even be on the same page anymore when we attempt to retry the test! Instead, we need to retry entire test suites, which Jest doesn’t support out of the box.

Fortunately, we were able to write a short script that allowed us to retry entire suites easily and effectively, dramatically reducing the amount of failed test runs in the process. The below graph shows the percentage of successful and failed runs each day both prior to and post implementing the retries. A failed test run is defined as at least one test failing when all our e2e test suites are run.

Success rate of e2e test runs before and after retry implementation

Why test suites

Before we get into how we retry our test suites, we should quickly address why our tests are structured this way to begin with. A logical question might be, “why not have each flow be tested by an individual test, instead of a suite where the individual tests represent various steps in the flow?” as this would allow us to utilize Jest’s built in retry logic.

The answer, in part, stems from how Capybara (a ruby based selenium wrapper library we used before migrating to puppeteer) prescribed tests to be structured and how we maintained much of that structure to ease the conversion process. However there are some distinct advantages to using this method as well. Namely, the benefit of having error reports quickly and clearly denote via the test name exactly where the test is failing, as you can see from this example error log:

Example e2e test failure output

Many of these individual tests are also defined in shared helpers and reused across multiple test suites, which helps us to easily identify if one helper function is causing multiple suites to flake or fail.

Retrying test suites

The initial script to get the tests retrying was a bit more straightforward than we expected. All we needed to do was to run tests normally (via the command line tools), inspect the results for failures, and call Jest again, passing it only the failing tests. Keep in mind that since it’s just JavaScript, you can also import your existing configuration file as well.

Then any task runner (we use gulp) can call our new function. Alternatively, you can call the script directly from the command line or an npm script as well.

Managing JUnit results

For many, the above script may be enough. However if you’re like us at Wealthfront, you may rely on the JUnit output from Jest for parsing, storing and analyzing test results. Unfortunately, every time Jest runs, it will clobber the previous output. So retrying the tests will result in only the tests from the final run being reported. That means we’ll miss out on the data from all the tests that passed in any other initial runs!

To solve this, we can add some logic to our script that merges JUnit results as we go. Note that while we use JUnit at Wealthfront, you can modify this to support whatever test reporter you choose.

This new function will merge the first run with the retried run by inserting each new test result adjacent to that test’s first failed run in the JUnit output.

You can also set the top level JUnit metrics in your merge function, although it’s worth noting that jest-junit doesn’t actually use the top level metrics, and instead sums the results of each individual test. Still, for completeness you can optionally add:

So now we have our test suites retrying and correctly outputting all the results, which is great. But what about the issues causing the flakes to begin with?

Flagging for investigation

Even though our tests are now retrying and hopefully passing successfully, our work isn’t quite done. It’s still important to investigate why the tests are flaking to begin with. Sometimes this can be caused by outages of other servers, especially if you’re relying on less stable development instances for your tests. There can even sometimes be network flakiness. However, e2e test flakiness is most often caused by bugs in the code! This is something I find to be commonly overlooked.

For example, in the Wealthfront account creation flow we navigate to new information gathering forms throughout the application process. If we move too quickly, something automated tests are certainly capable of, we can sometimes navigate to a new page before the data is fully updated on the server. A user navigating at a normal speed will rarely encounter this, however when automated tests speed through, they can sometimes get redirected back to a previous step that is assumed to be incomplete, thus breaking our tests. We’ve also seen some instances where a component will render, then enter a loading state, and then re-render once loaded. This flicker, typically imperceptible to humans, will often cause e2e tests to find elements in the initial render, but attempt to click only after it’s entered a loading state. While neither of these instances are major issues or user blocking bugs, they still indicate potential improvements we can make to our frontend code.

The main point here is that we shouldn’t ignore tests that flake, just because they pass on a retry. These tests are often highlighting real race conditions in our code. Fortunately, because we have good JUnit output from our merged results, we are able to parse and analyze the failure or flake rate of our tests. This allows us to make charts and flag flaking tests for review by the teams that own them, something we’d highly recommend you do as well.

Example e2e test flake rate dashboard


Good end to end tests may not always be the most alluring part of web development, but they are incredibly important tools in validating the reliability of complex software. Our e2e tests give us a lot of confidence in our production code. While retrying the tests is an incredibly valuable  tool in helping improve build and development tooling, it is not a replacement for debugging the underlying cause.

If developing solutions to problems like these sound interesting, you want to help make it even better, or you want to generally help tackle the complex engineering challenges associated with building a financial system that favors people, not institutions, check out our careers page for current opportunities!


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