How RSpec helped me with resolving random spec failures

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How RSpec helped me with resolving random spec failures

Photo available thanks to the courtesy of Robert Kash. CC BY 2.0

Recently we started experiencing random spec failures in one of our customer’s project. When the test was run in an isolation, everything was fine. The problem appeared only when some of the specs were run before the failing spec.


We use CI with four workers in the affected environment. The all of our specs are divided into the four groups which are run with the same seed. In the past, we searched for the cause of such problem doing manual bisection. It was time-consuming and a bit frustrating for us.

RSpec can do a bisection for you

You probably already know RSpec’s --seed and --order flags. They are really helpful when trying surface flickering examples like the one mentioned in the previous paragraphs. RSpec 3.4 comes with a nifty flag which is able to do that on behalf of a programmer. It’s called --bisect. According to the docs, RSpec will repeatedly run subsets of your suite in order to isolate the minimal set of examples that reproduce the same failures.

How I solved the problem using RSpec’s --bisect flag

I simply copied the rspec command from the CI output with all the specs run on given worker with the --seed option and just added --bisect at the end. What happened next? See the snippet below:

Running suite to find failures... (7 minutes 48 seconds)
Starting bisect with 4 failing examples and 1323 non-failing examples.
Checking that failure(s) are order-dependent... failure appears to be order-dependent

Round 1: bisecting over non-failing examples 1-1323 .. ignoring examples 663-1323 (6 minutes 41 seconds)
Round 2: bisecting over non-failing examples 1-662 .. ignoring examples 332-662 (4 minutes 44.5 seconds)
Round 3: bisecting over non-failing examples 1-331 .. ignoring examples 166-331 (3 minutes 25 seconds)
Round 4: bisecting over non-failing examples 1-166 .. ignoring examples 84-166 (2 minutes 14 seconds)
Round 5: bisecting over non-failing examples 1-83 .. ignoring examples 1-42 (44.45 seconds)
Round 6: bisecting over non-failing examples 43-83 .. ignoring examples 64-83 (56.97 seconds)
Round 7: bisecting over non-failing examples 43-63 .. ignoring examples 43-53 (20.71 seconds)
Round 8: bisecting over non-failing examples 54-63 .. ignoring examples 54-58 (20.02 seconds)
Round 9: bisecting over non-failing examples 59-63 .. ignoring examples 59-61 (20.23 seconds)
Round 10: bisecting over non-failing examples 62-63 .. ignoring example 62 (20.49 seconds)
Bisect complete! Reduced necessary non-failing examples from 1323 to 1 in 19 minutes 53 seconds.

The minimal reproduction command is:
  rspec './payment_gateway/spec/stripe/payment_gateway_spec.rb[1:8,1:9,1:10,1:11]' \
        './spec/services/backstage/fill_in_shipping_details_spec.rb[1:1:1]' \
         --color --format Fivemat --require spec_helper --seed 42035


It took almost 20 minutes to find the spec which interfered with other ones. Usually, I had to spend 1-2 hours to find the issue. During this 20 minutes run of an automated task, I was simply working on a feature. The --bisect flag is a pure gold.

But what was the reason for the failure?

It was simply before(:all) {} used to set up the test. You shouldn’t use that unless you really know what you’re doing. You can read more about the differences between before(:each) and before(:all) in this 3.years.old, but still valid blog post.

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Szymon Fiedler

I solve problems. This is the place where I share my thoughts on Software Engineering.