Most software is simple: you have a codebase, and existing patterns for testing at a unit and integration level.

Sometimes though, you’ll face problems that aren’t just adding code to an existing project. Maybe your problem involves many codebases, uses tricky infrastructure, or perhaps you’re not trying to test ‘code’, per-say.

I’ve faced my share of problems outside the usual mold, and found you can combat the complexity if you’ll willing to get creative about investing in test or dev tooling. Most of these approaches generalise, and once you get how they work, you start seeing opportunities to use them everywhere.

So here’s some examples of problems that required out-of-box thinking to test, and how you can tackle them.

Snapshot tests

The frontend community have popularised ‘snapshot testing’, where HTML output is compared to a known-good snapshot as part of the CI pipeline, often by textually diff’ing the HTML and sometimes by visually comparing screenshots.

Snapshots are useful because small changes to the frontend code – say, changing a CSS selector – can have outsized impact on the final result, and it can be difficult to catch that during code review. It’s much easier to realise your sidebar has become a footer when CI flags a screenshot, or catch a bad condition in your React component when large chunks of HTML disappear from the snapshot file.

The concept is applicable outside of frontend though, and can be cheap to implement depending on what you’d like to test.

One success story around snapshot tests comes from my time at GoCardless, when we first started using Helm charts to deploy apps into Kubernetes. If you’re unfamiliar, Helm charts allow you to write Kubernetes config files (big YAML documents) using Go templates, helping DRY-up your infrastructure code.

An example Helm template would be:

# Extract of helm/charts/stable/buildkite:
{{- if .Values.agent.token }}
apiVersion: extensions/v1beta1
kind: Deployment
  name: {{ template "buildkite.fullname" . }}
  replicas: {{ .Values.replicaCount }}
        app: {{ template "" . }}
        release: {{ .Release.Name }}
        # ...
{{- end }}

Which when evaluated by Helm - using a given set of values that provide things like the replicaCount field - will become:

apiVersion: extensions/v1beta1
kind: Deployment
  name: buildkite-production
  replicas: 3
        app: buildkite
        release: production
        # ...

Seems simple enough, but if you’ve ever written Helm templates you’ll know that’s a lie.

While the Go template language might be ok alone, using it to produce large, whitespace sensitive, heavily nested YAML files is horrible. It’s extremely easy to screw up the whitespace such that the output totally changes, and the subtle {{- trailers in the Go conditional blocks can drive you mad.

But when you can say the following statements about your code:

  1. Small changes can cause large differences in output
  2. You care a lot about catching regressions (god forbid we accidentally delete a database statefulset)
  3. It’s cheap to calculate a ‘snapshot’ of your output

Your problem might benefit from snapshot tests, to regain some sanity/control.

Applying this to our Helm charts, we started creating a values directory in each chart that contains fixtures of chart values (such as the replicaCount in the buildkite example) each analagous to a test case.

For a generic app chart that supports deploying server backends and async workers, we might have something like:

├── Chart.yaml
├── templates                                                                                                           
│   ├── _helpers.tpl
│   ├── ...
│   └── deployment.yaml
├── values
│   ├── backend.yaml
│   ├── backend.snapshot
│   │   └── app
│   │       └── templates
│   │           ├── ...
│   │           └── deployment.yaml
│   ├── async-workers.yaml
│   └── async-workers.snapshot
│       └── app
│           └── templates
│               ├── ...
│               └── deployment.yaml
└── values.yaml

Each use case of the chart – in this case deploying a backend, or provisioning async workers – is represented as a .yaml values fixture which compiles into a .snapshot directory, using the helm template command. Snapshots are built using a Makefile target which regenerates snapshots from their fixtures, taking only a couple of seconds to refresh the entire lot.

The resulting snapshots are checked into the repo alongside the chart, which means:

  1. Whenever changes to the chart alters YAML output, the resultant Kubernetes manifests are included in the pull request diff, helping catch unexpected errors or clarify complex templating.
  2. If you want to see what a chart might produce, you can look at the snapshot and see plain YAML instead of trying to mentally evaluate the Go template.
  3. Values fixtures are great documentation for how to use the chart, as you know you can trust they work because they’re checked via the snapshots on every change to the repo.

Adding a CI task that regenerates snapshots and checks the git state means you can’t forget to generate them, and is an easy way to fail the build if the snapshots have got stale.

# CircleCI
    - checkout
    - run: make snapshots
    - run:
        name: Ensure snapshots were updated
        command: |
          if [ ! -z "$(git status --porcelain)" ]; then
            git status
            echo -e '\033[1;93m!!! "make snapshots" resulted in changes. Please run locally and commit the changes.\e[0m'
            git diff
            exit 1

When we deprecated Helm in favour of Jsonnet (which can also be used to build YAML manifests) we continued to generate YAML snapshots. This allowed us to use more complex Jsonnet templating – because the snapshots confirm the output – and even power CODEOWNER rules if a dependent Jsonnet file changes the output of a snapshot in a different location.

See the Jsonnet snapshots in action at Utopia: Getting Started: Jsonnet snapshots.

Snapshot tests like this are language independent, and can be applied to all sorts of problems. They take almost no time to setup, and can improve code reviews by bringing the generated artifact front-and-centre of the review.

I encourage you to give them a go!

Real infrastructure

On occassion, you’ll want to test something that isn’t just code. It might be a real system like a database cluster, and you may want to use tests not on a continual basis in CI but as part of an operational event, like a migration.

Either way, the key is realising you can use the same tools and testing frameworks as in normal software engineering, but applied to the real world system.

When migrating GoCardless’ infrastructure from Softlayer to GCP, we needed to lift-and-shift everything from compute to DNS. One of the thorniest parts of this move was the ‘routing tier’, a collection of proxies held together by scrappy Chef templating that provided network ingress and layer 7 rule for all GoCardless traffic.

From memory, the path of a request would be:

1.    Cloudflare (DNS + HTTP proxy)
2. -> nginx (edge, public internet)
3. -> HAProxy (internal, private network)
   -> Compute VMs {
4.    -> nginx
5.    -> Containers

Cloudflare hosted our DNS and was enabled in proxy mode for DDoS protection, meaning DNS requests would resolve Cloudflare’s edge load balancer (1).

Requests would be proxied to our nginx edge LBs (2) which handled a mix of static sites proxied onto S3 and internal services, which were forwarded into the private network.

HAProxy load balancers (3) received internal requests which (in addition to the edge nginx load balancers) handled some proxying of static sites to S3 buckets, though its primary purpose was round-robin’ing requests to our compute machines (4 + 5). If you’d made it this far, the compute machines ran an nginx (4) which was managed by our homegrown container orchestration system (conductor) such that incoming service HTTP requests would be round-robin’d to the right containers.

For reasons that I hope are now clear, we wanted to simplify this when moving to GCP, with an aim of:

1.    Google Cloud Load Balancer (HTTP proxy)
2. -> HAProxy (deployed to GKE)

Running a single HAProxy inside GKE that would accept traffic for all existing domains (about ~30, if memory serves) and preserve legacy routing behaviour, such as HTTP redirects and static site hosting.

That’s more context than you might need, but it helps highlight the signs that mean it might be worth investing in some real world tests to manage the craziness:

  1. Behaviour is the sum of many production systems, some of which (e.g. Cloudflare) not able to be easily simulated.
  2. Many parts and complex interactions mean there are likely to be awkward edge cases you can’t easily predict.
  3. Serving everything from marketing site to GoCardless API requests, mistakes will be visible and customer impacting, justifying additional care and time to reduce the risk.

So taking inspiration from tools like Chef InSpec which runs tests against external systems, we built a test suite that covered legacy routing behaviour that could be applied to an arbitrary server, which would allow us to verify both legacy and new systems.

This isn’t as difficult as it sounds, especially when the external system can be verified with something as standard as black-box HTTP requests. I no longer have access to the test suite we used, but writing something similar for’s site might look like:

require "excon"
require "rspec"

# Make a GET request with HOST_OVERRIDE applied.
def request(url)
  Excon.get(build_url(url), headers: {
    "Host" => URI.parse(url).hostname,

# Alter the given URL to override the hostname, port and
# scheme according to the HOST_OVERRIDE envar.
def build_url(url)
  uri = URI.parse(url)
  if override = ENV["HOST_OVERRIDE"]
    override_uri = URI.parse(override)

    uri.hostname = override_uri.hostname
    uri.port = override_uri.port
    uri.scheme = override_uri.scheme


RSpec.describe "" do
  describe "redirects" do
      "" => "",
      "" => "/careers",
    }.each do |from_url, to_url|
      specify "#{from_url} -> #{to_url}" do
        resp = request(from_url)

        expect(resp.status).to match(301..302)
        expect(resp.headers["location"]).to eql(to_url)

The request method makes an HTTP request that can be overriden to target a specific host via the HOST_OVERRIDE environment variable. This means you can invoke the test suite with no parameters to test the existing production setup (for, Cloudflare and Netlify):

$ bundle exec rspec --format=doc routing.rb
  redirects -> ✔️ -> /careers ✔️

Finished in 0.48689 seconds (files took 0.06897 seconds to load)
2 examples, 0 failures

Or, if you’re running a local server like I do to preview this blog, you can direct it at that instead:

$ HOST_OVERRIDE=http://localhost:4000/ \
  bundle exec rspec --format=doc routing.rb
  redirects -> (FAILED - 1) -> /careers (FAILED - 2)
Finished in 0.01104 seconds (files took 0.06916 seconds to load)
2 examples, 2 failures

Obviously my local blog server doesn’t provide these redirects, but the requests have been directed there: that’s why the tests failed.

Returning to the example of migrating the routing tier, the new setup with a single HAProxy was much more testable than the legacy system: we could boot the HAProxy locally and point this test suite at it, allowing us to catch regressions or bad HAProxy config before it was deployed.

There’s nothing too outlandish about this setup other than realising you can use the same testing tools as in normal software to help work with real systems. And perhaps a bit of “oh, software engineering can help with my operations!” instead of keeping the two areas – SWE & SRE – separate.

It’ll depend on the situation at hand, but I’ve written test suites for big migrations or critical operational processes many times now. Even the more complex tests become possible with a helper that executes commands against the machines that are involved: we once broke a 10+ step Postgres cluster migration into a series of Ruby script/RSpec test files that executed commands via ssh, making the migration as simple as:

$ ruby scripts/01_enable_maintenance.rb
timestamp="2022-12-28T15:59:41Z" msg="enabling maintenance mode"
$ rspec scripts/01_enable_maintenance_spec.rb

  checking maintenance is on
    server01 ✔️
    server02 ✔️
    server03 ✔️

Finished in 0.0234 seconds (files took 0.09123 seconds to load)
3 examples, 3 failures

$ ruby scripts/02_run_checkpoint.rb

Don’t limit yourself

This might all seem like common sense, but I’ve rarely seen techniques like these applied outside whichever small area of engineering they normally occupy. And that feels a shame, especially when they can be such help elsewhere.

I think it’s a similar problems as SREs being slow to adopt soft-eng best practices (see Platform software is just software, you’re not special) in that cross-polination requires someone to see the potential, figure out what needs changing to make it work, then socialise it.

There’s little stopping people from trying this, other than an assumption that if a tool or process you’ve used successfully isn’t used here, it’s because it doesn’t work. But you won’t know until you try, and I think we could benefit from more people trying!

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