rules-tutorial.md

Writing dependency-cruiser rules - a tutorial

Introduction

This tutorial takes you step by step through writing a rule for dependency-cruiser. If you're one of those 'quick-to-grasp' types, or you're just looking for a reference there's a rules reference that might better suit your needs.

Basics

A simple rule - take 1

Let's say you want to prevent the use of the node core 'http' library for one reason or other.

{
  "forbidden": [
    {
      "from": {},
      "to": { "path": "http" }
    }
  ]
}

This rule says it is forbidden to have a relation from anything to things with a path that contains the string http.

You run a depcruise with these rules and lo and behold it comes up with something:

warn unnamed: src/secure/index.ts → node_modules/@supsec/http/index.js
warn unnamed: src/secure/knappekop.ts → node_modules/@supsec/http/index.js
warn unnamed: node_modules/yudelyo/index.js → http
warn unnamed: src/secure/index.ts → http
warn unnamed: src/secure/index.ts → https

There's a few things you notice:

• dependency-cruiser generates warnings. This is the default, but maybe you want to stop the build. You'd need errors.
• The rule has no name. For this one rule - no probs. If there's more rules it might be handy for your future self (and your co-workers) to reference the rule.
• The rule matches a little too much for your taste.

Let's see how we can fix that - name and severity first.

A simple rule - take 2 - adding name and severity

{
  "forbidden": [
    {
      "name": "not-to-core-http",
      "comment": "Don't rely on node's http module because of internal guideline BOYLE-839 - use https and the internal @supsec variant in stead",
      "severity": "error",
      "from": {},
      "to": { "path": "http" }
    }
  ]
}

error not-to-core-http: src/secure/index.ts → node_modules/@supsec/http/index.js
error not-to-core-http: src/secure/knappekop.ts → node_modules/@supsec/http/index.js
error not-to-core-http: node_modules/yudelyo/index.js → http
error not-to-core-http: src/secure/index.ts → http
error not-to-core-http: src/secure/index.ts → https

That's a lot easier to understand - and it will stop the build from happening.

A simple rule - take 3 - tightening the rule down

The rule as it is matches not only the core module, but also @supsec/http which is module you should actually use according to BOYLE-839. So we should make sure we only match the core module. We can do that by specifying we want to also match the dependencyType core:

{
  "forbidden": [
    {
      "name": "not-to-core-http",
      "comment": "Don't rely on node's http module because of internal guideline BOYLE-839 - use https and the internal @supsec variant in stead",
      "severity": "error",
      "from": {},
      "to": { "dependencyTypes": ["core"], "path": "http" }
    }
  ]
}

The result:

error not-to-core-http: node_modules/yudelyo/index.js → http
error not-to-core-http: src/secure/index.ts → http

Now you can go about fixing so src/secure/index.ts relies on the internal @supsec/http module, so you're all BOYLE-839 compliant.

But that pesky node_module relies on a forbidden dependency as well? Watnu?

Yep. Don't you just love those 1500 npm packages you drag in and rely on for your website to run :grimace: .

Luckily you know kpttraag, the author of yudelyo - you submit a PR and wait. In the mean time you don't want to have the build break until kpttraag has found the time to merge your PR.

You realize there might be more npm packages using http too, so ...

• You change the not-to-core-http to only generate errors for paths outside node_modules.
• You add a new rule for node_modules, that just generate a warning. You'll still see it in the build logs, but you can go on developing for the time being.

{
  "forbidden": [
    {
      "name": "not-to-core-http",
      "comment": "Don't rely on node's http module because of internal guideline BOYLE-839 - use https and the internal @supsec variant in stead",
      "severity": "error",
      "from": { "pathNot": "^node_modules" },
      "to": { "dependencyTypes": ["core"], "path": "^http$" }
    },
    {
      "name": "node_mods-not-to-http",
      "comment": "Some node_modules use http - warn about these so we can replace them/ make PR's so we're BOYLE compliant",
      "severity": "warn",
      "from": { "path": "^node_modules" },
      "to": { "dependencyTypes": ["core"], "path": "^http$" }
    }
  ]
}

warn node_mods-not-to-http: node_modules/yudelyo/index.js → http
error not-to-core-http: src/secure/index.ts → http

{} over { "path": ".+" }

Functionally, "from": {} and "from": { "path": ".+" } are the same. The way depencency-cruiser is wired today, however, makes the former faster than the latter. So - unless you have CPU cycles to spare - use the former one ("from": {}).