Testing Serverless Workflow's

Published on Mar 27, 2020 by jhole89 on serverless, testing, aws, cloud-native

Serverless is a design pattern which aims to remove many issues development teams typically face when maintaining servers or services, enabling them to focus on delivering value and benefit quickly and efficiently. Instead of spending time and money configuring and managing servers, we can turn to a serverless paradigm and offload this burden onto Cloud Native parties who specialise in these tasks. This frees up time and resources to focus on solving problems in our core domain.

However using a large amount of serverless resources also has it’s drawbacks, in particular the difficulties in testing. In this article I aim to discuss some of these problems, and propose a solution for testing heavily serverless workflow’s.

The Different Types of Testing

When building applications it’s important that we write comprehensive test coverage to ensure our application behaves as expected, and protects us from unexpected changes during iteration. In both traditional and serverless development, when building apps and workflow’s that involve calls to other services, we need to test the boundaries. This is often done by utilising mocks to simulate responses from outside our app or workflow. A large amount of mocks often highlights a large amount of side-effects, which while something we can minimise by following functional programming paradigms, are often unavoidable. Before continuing it’s important to understand the difference between unit, integration, and regression tests, as they are often easily mixed up:

  • Unit test: the smallest type of test, where we test a function. When following Test Driven Development these are the kind of tests we write first, asserting what we expect our soon to be written function will do. We expect these tests to run automatically on a Continuous Integration server for each commit. Given a function def addOne(input: Int): Int = input + 1 we would expect a corresponding test which may look something like

    * `addOne(-1) shouldEqual 0`
    * `addOne(0) shouldEqual 1`
  • Integration test: larger tests, where we test a workflow which may call many functions. These are more behaviour focused and target how our system expects to run given different inputs. As with unit tests, we expect these tests to run automatically on a Continuous Integration server for each commit. Given an application with an entrypoint def main(args: Seq[String]): Unit we may expect an integration test to look something like

    * `main(Seq("localhost:8000", "/fake-url", "30s")) shouldRaise 404`
    * `main(Seq("localhost:8000", "/mocked-url", "30s")) shouldNotRaiseException`
  • Regression test: the largest type of test, also thought of as a systems test. While unit and integration tests look to test how our application behaves during changes to it, regression tests look to test how our systems behave due to our application changing, and prevent unexpected regressions due to development. While integration testing of an api crawler may test what happens to the app when the api goes offline by utilising a local mock, regression testing should test what happens to all services should that api go offline. We except these tests to run automatically on a Continuous Integration server for each PR rather each commit.

Testing Boundaries

Problems with Testing Serverless Workflow’s

Now that we have a firm grasp of the different types of testing, lets work through a real world example where we will see that relying only on unit and integration tests is not enough for even simple serverless workflow’s. Given this demo workflow:

  1. We have some simple code running inside an AWS Lambda to get data from an api, do some processing with it, and publish the results to an S3 bucket
  2. The S3 bucket has an event trigger that sends an alert to an SNS topic when new data is published to it
  3. The SNS topic sends an email to our users letting them know that the data is available to download from a link
  4. Users access the link, which is an AWS API Gateway endpoint to authorise access to the download

Our demo workflow

We could expect the code for the Lambda to have unit and integration tests written alongside. These may utilise mocks and utilities such as wiremock to test how this simple code would handle the various HTTP response codes, and capture the messages being sent to S3. This is testing the boundaries of the Lambda, however this leaves much of our workflow untested.

In a traditional stack, where instead of utilising serverless we would be managing servers (e.g. an FTP server for S3, an SMTP server for SNS, NGINX for API-Gateway); we could regression test these by running containers for them alongside wiremock on our CI box. By triggering the application code with a range of api paths, we can regression test the local container instances for side-effects and unexpected behaviour. However how do we do this with managed/cloud-native services which are not available in the form of local containers?

AWS SNS and a traditional SMTP server may be similar, but they’re not the same, and any tests using it as a replacement would provide little benefit. However if we only test the Lambda code then we are leaving much of our workflow untested. What happens if someone logs into the console and changes the SNS topic name? The Lambda will still pass it’s unit and integration tests, and it will still publish data to the S3 bucket. However the SNS topic will no longer receive the event, and won’t be able to pass on alert to our users - our workflow is broken, and even worse we’re not aware of it.

Worfkflow boundaries

This is the catch-22 of testing managed/cloud-native serverless - as our workflow’s become more complicated, we need rigorous testing, but the more services we include, the less tested our workflow becomes. This is why regression/systems testing becomes more important with serverless workflow’s, and why it should become more of the norm.

Regression Testing Serverless Workflow’s

So now that we understand what we want to test, and why its important, we need to find a way of testing it; and to achieve this we need to be using regression tests.

The traditional approach still used by many would be to deploy the stack onto an environment, where someone can manually trigger and evaluate the workflow. This is testing the happy path, as it doesnt evaluate all the permutations of different components changing. Additionally, due to the manual process involved we are unlikely to be able to evaluate this on each PR, and instead may only do this once per release which could contain many changes. Should we find any regressions, it becomes harder to identify the root cause due to the multiple changes that have been implemented between releases. This also doesn’t scale well when we have more complex workflow’s that utilise parallel and diverging streams (for an example of such read my blog on building serverless data pipelines).

So how do we do better? How do we thoroughly test the workflow and ensure that our workflow remains stable when individual components are able to change? Well, what we can do is take the same approach used for unit and integration tests, and look at how we can test our remit (in this case our entire workflow) as a black box. We can achieve this by spinning up infrastructure around our workflow, then run a suite of tests to start the workflow, assert on the results at the end of the workflow, and finally destroy our test infrastructure afterwards - to do which we need to leverage IaC (Infrastructure as Code) tools such as terraform.

For our demo workflow, we would achieve this by deploying managed/cloud-native services, which the Lambda at the start of our workflow will connect to, in lieu of the real external API. We can then run a suite of tests to trigger the Lambda, and assert the expected results exist at the end of our workflow via the via the workflow API Gateway.

Regression workflow

Conceptually, this is very similar to running a local wiremock server as part of our test suites - well, why can’t we just do that instead of worrying about building infrastructure? The problem here is that running wiremock within our test suite would only be local to that process, and we wouldn’t be able to expose the wiremock endpoint to the Lambda - we would need a DNS for that. By launching API Gateway, we generate a public (or private) URL which we can pass to our applications and test them from the outside in; compared to unit or integration tests, where we run tests alongside our code.

With this approach, we can now automate the traditional manual QA testing, and ensure we cover a much wider spectrum of BDD test cases, including scenarios such as “What alert do/should our users receive if the API is unavailable?”. In traditional unit/integration testing we wouldn’t be able to answer or test for this, as this process is handled outside of the Lambda. We could test what happens to the Lambda in the event of the external API becoming available, but not how downstream processes would react - we’d be reliant on someone manually trying to mimic this scenario, which doesn’t scale. Furthermore, utilising IaC we can run a huge barrage of these larger workflow tests in parallel, and easily scale these up to incorporate elements of load and chaos testing. Instead of being reactive to our workflow breaking, we can push the limits to establish our redundancy prior to experiencing event outages.


Hopefully I’ve sold you on the idea of regression/systems testing, and why as we move to a more serverless world, we need to establish a more holistic view on testing our systems as a whole, rather than only the components in isolation. That’s not to say that we should abandon the faithful unit test in favour of systems testing, but why we should not fall into the fallacy that just because our “code” is tested, our systems and workflow’s are also tested. This also highlights why Development, QA, and DevOps are not activities do be done in isolation by separate teams. Having a key understanding of each is required to implement and test such a workflow, and that ideally both the workflow and test framework should be implemented by a single cross functional team, rather than throwing tasks over the fence.

If any of that sounds interesting and you’d like to know more, you can reach me at There is a corresponding live demo that implements this workflow and the ideals behind it, which can be shown on request. Feel free to reach out for assistance or training with Cloud, Data, or DevOps solutions, or any of our other workstreams here at Manta Innovations Ltd.

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