falco

falco is VCL parser and linter optimized for Fastly.

Disclaimer

This is a VCL parser, but dedicated to Fastly's VCL (version 2.x), so we don't care about the latest Varnish (7.x or later) syntax. The Varnish may have additional syntax, builtin function, predefined variables, but this tool may not parse correctly.

Additionally, Fastly provides its special builtin function, predefined variables. It's not compatible with Varnish. But this tool is optimized for them, we could parse and lint their declarations.

Motivation

Fastly is a really fantastic CDN, but sometimes we have problems with deployment operations. On deploy custom VCL to the Fastly, VCLs are validated when activating a new service version. Typically our deployment flow using custom VCLs is following:

Clone active service and create new version
Delete existing custom VCLs
Upload new VCL files to the Fastly
Activate new device version <= Validate VCLs on the Fastly cloud

Above flows take a time, and then if we have some mistakes on VCL e.g. missing semicolon X(, the deployment will fail. Additionally, unnecessary service versions will be created by our trivial issue.

To solve them, we made a Fastly dedicated VCL parser and linter tool to notice syntax errors and unexpected mistakes before starting the deployment flow.

Installation

Download binary from releases page according to your platform and place it under the $PATH.

Usage

Command help displays following:

falco -h
=======================================
  falco: Fastly VCL parser / linter
=======================================
Usage:
    falco [main vcl file]

Flags:
    -I, --include_path : Add include path
    -t, --transformer  : Specify transformer
    -h, --help         : Show this help
    -r, --remote       : Communicate with Fastly API
    -V, --version      : Display build version
    -v,                : Verbose warning lint result
    -vv,               : Verbose all lint result

Example:
    falco -I . -vv /path/to/vcl/main.vcl

Note:

Your VCL will have dependent modules loaded via include [module]. falco accept include path from -I, --include_path flag and search and load destination module from include path.

User defined subroutine

On linting, falco could not recognize when the user-defined subroutine is called, so you should apply the subroutine scope by adding annotation or its subroutine name. falco understands call scope by following rules:

Subroutine name

If the subroutine name has a suffix of _[scope], falco lint within that scope.

sub custom_recv { // name has `_recv` suffix, lint with RECV scope
  ...
}

sub custom_fetch { // name has `_fetch` suffix, lint with FETCH scope
  ...
}

Following table describes subroutine name and recognizing scope:

suffix	scope	example
_recv	RECV	sub custom_recv {}
_miss	MISS	sub custom_miss {}
_hash	HASH	sub custom_hash {}
_pass	PASS	sub custom_pass {}
_fetch	FETCH	sub custom_fetch {}
_error	ERROR	sub custom_error {}
_deliver	DELIVER	sub custom_deliver {}
_log	LOG	sub custom_log {}

Annotation

For some reasons, the subroutine name could not be changed. Then, if you apply a hint of scope on annotation, falco also understands scope:

// @recv
sub custom_process { // subroutine has `recv` annotation, lint with RECV scope
  ...
}

// @fetch
sub custom_request { // subroutine has `fetch` annotation, lint with FETCH scope
  ...
}

Following table describes annotation name and recognizing scope:

annotation	scope	example
@recv	RECV	// @recv sub custom {}
@miss	MISS	// @miss sub custom {}
@hash	HASH	// @hash sub custom {}
@pass	PASS	// @pass sub custom {}
@fetch	FETCH	// @fetch sub custom {}
@error	ERROR	// @error sub custom {}
@deliver	DELIVER	// @deliver sub custom {}
@log	LOG	// @log sub custom {}

Fastly related features

Partially supports fetching Fastly managed VCL snippets. See remote.md in detail.

Lint error

falco has built in lint rules. see rules in detail. falco may report lots of errors and warnings because falco lints with strict type checks, disallows implicit type conversions even VCL is fuzzy typed language.

Overriding Severity

To avoid them, you can override severity levels by putting a configuration file named .falcorc on working directory. the configuration file contents format is following:

## /path/to/working/directory/.falcorc
regex/matched-value-override: IGNORE
...

Format is simply a yaml key-value object. The key is rule name, see rules.md and value should be one of IGNORE, INFO, WARNING and ERROR, case insensitive.

In the above case, the rule of regex/matched-value-override reports INFO as default, but overrides IGNORE which does not report it.

Error Levels

falco reports three of severity on linting:

ERROR

VCL may cause errors on Fastly, or may cause unexpected behavior for actual works.

WARNING

VCL could work, but may have potential bug and cause unexpected behavior for actual works.

falco does not output warnings as default. To see them, run with -v option.

INFORMATION

VCL is fine, but we suggest you improve your VCL considering Fastly recommendation.

falco does not output information as default. To see them, run with -vv option.

Transforming

falco is planning to transpile Fastly VCL to the other programming language e.g Go (HTTP service), node.js (Lambda@Edge) to use temporal CDN instead of Fastly.

xordspar0/falco