When pushing VCL files to Fastly, a missing semicolon or comma can lead to a failure. The configuration will fail to activate, you have to check why and what errored. Typos happen. It’s linter’s job to catch them and let us fail fast.

Fastly VCL

Fastly is one of the world’s leading CDNs and Edge Computing platforms that helps to accelerate a fairly big chunk of today’s internet. From eCommerce to streaming solutions, if they’re performing well, they’re likely to have been built around Fastly1.

What makes Fastly great is its flexibility powered by the possibility to use custom Varnish configuration files (VCLs) to route traffic, alter and handle requests, or even compute at the edge. All close to the end user.

Falco VCL parser

Falco is an Open Source VCL parser that is built against Fastly’s implementation of Varnish.

It has a comprehensive set of features one of which is dedicated to linting Fastly VCL.

Linting with Falco GitHub Action

In order to automate the linting of Fastly VCL, I’ve built the Falco GitHub Action.

Everyone who has worked with CI/CD pipelines on GitHub before, the configuration of the Falco GitHub Action is trivial and self-explanatory. You define the includes used by the VCL file in options and the VCL file to lint in target:

- name: Lint VCL
  uses: ain/falco-github-action@v1
  with:
    subcommand: lint
    options: "-v -I test/vcl/includes"
    target: test/vcl/valid_with_include.vcl

ATTN: you can also lint ACL files! Linting has a lot of value against a missing comma or invalid CIDR notation in those big dictionaries with thousands of IP ranges.

  1. Fastly Customer Stories ↩