Review and merge guidelines

Much of our active infrastructure is configured and automatically updated via CI/CD pipelines. This means that changes in this repository often immediately impact the infrastructure that we run. As such, we follow team policies for review/merge that are more specific than our general development merge policies.

This document codifies our guidelines for doing code review and merging pull requests on active infrastructure (ie, anything in the infrastructure/ codebase).

A Foolish Consistency is the Hobgoblin of Little Minds

  • PEP 8

Changing active infrastructure in general

The following general policies apply to any change that will affect active infrastructure. Policies specific to Terraform changes are described below.

Here are some guidelines for merging and reviewing in this case:

  • The PR author must also merge the PR. (REQUIRED) Because a PR is a reflection of a deploy, the author of the PR should also be the one that merges it, since only they know what infrastructure has actually been deployed.

  • Explicitly list a back-up if you must step away. (REQUIRED) PR authors are responsible for the infrastructure changes that they make. You should generally only make a change to live infrastructure if you’ll have the bandwidth to ensure it is fixed if something goes wrong. If for some reason you must step away, make it clear who else is responsible for shepherding the PR.

  • Be careful when self-merging without review. Because changing active infrastructure is potentially confusing or disruptive to users, be extra careful if you self-merge without a review approval. Consider whether your commit will cause a change that might be destructive and ask if you really need to merge now or can wait for review. That said, sometimes the only way to understand the impact of a change is to merge and see how things go, so use your best judgment!

    Common things that often don’t need an approval

    • Updating admin users for a hub

    • Changing basic hub configuration such as the URL of its landing page image

    • Updating the user image of a hub.

Terraform changes

Terraform is used for directly provisioning and modifying cloud infrastructure (as opposed to just deploying applications on pre-existing cloud infrastructure). This lets us treat our infrastructure as code, so we can use regular code quality practices (code review, linters, extensive documentation, etc) to keep the quality of our infrastructure high. However, it comes with a few caveats:

  1. Locally and iteratively testing terraform is impossible - as we develop it, the infrastructure is deployed / modified / destroyed as we go.

  2. Behavior of terraform code differs based on the current state of the infrastructure (as represented in the state file).

  3. While terraform plan can tell us what terraform thinks it is going to do, it might not always succeed. Permission errors or organizational restrictions can cause it to fail. Timeouts can happen. Runtime errors pop up.

Thanks to all these, it is impossible to either:

  • Accurately gauge the impact of a terraform code change just from looking at the text

  • Automatically deploy it via continuous deployment

As a result, we have some workflows that are specific to using Terraform, described below.

Changes to running infrastructure

In this scenario, there is already-running infrastructure and you are deploying changes to it.

To deploy changes, follow these steps:

  1. Propose changes. Change the codebase and open a PR. Do not deploy changes yet. Use terraform plan rather than terraform apply do understand the change before deploying it.

  2. Post the output of terraform plan. In your PR’s top comment, post the output of terraform plan whenever you update the PR. This helps others review the likely impact on our infrastructure.

  3. Wait for review and approval. Leave the PR open for other team members to review and approve.

    Guidelines for reviewing changes to existing infrastructure

    Focus is on both making sure the change is minimally disruptive, and on the quality of the infrastructure design.

  4. Deploy the change. Once approved, leave a comment in the PR that you will start deploying the changes. Then terraform apply your change and verify it works ok.

  5. Iterate as needed. If things break or you need to iterate on the deployment, follow this cycle until the infrastructure is in the state you wish:

    1. Amend your PR with changes

    2. Deploy with terraform apply

    3. Comment that you’ve made a deployment

  6. Communicate that you’ve finished deploying. Leave a comment in the PR that the changes have been deployed. You should always communicate when a deploy has been made!

  7. Self-merge or request another review. If you didn’t need to make a change to the PR, merge the PR. If you made changes to the PR, use your judgement to decide if you should request another review or if you should self-merge it directly. If you merge, just leave a comment about changes made since the initial approval.

Changes to set up new infrastructure

For creating new infrastructure, you are less-likely to deploy something disruptive to users, and you have a bit more flexibility to experiment.

The suggested workflow for setting up new infrastructure is:

  1. Iterate deploys locally. Iteratively develop the code locally, testing it with terraform apply as you go.

  2. Start a PR early, and solicit feedback. The primary goal here is to do a review of the infrastructure design, rather than of any particular terraform plan output.

    Guidelines for reviewing changes to existing infrastructure

    Focus of code review is just on infrastructure design.

  3. After approval, re-deploy from scratch. Once someone else approves your PR, you should re-deploy the infrastructure from scratch to make sure that it exactly matches the PR’s code. Do the following:

    1. Destroy the current infra you have setup with terraform destroy

    2. terraform apply from scratch.

    3. If things break, amend your PR until it unbreaks. This might solicit adding another cycle of code review.

    4. Self-merge the PR once the infra works as you would like.

    This way, terraform code will have been ‘realized’ by the time it is merged.