Monorepo Composer Magic

Symfony is by far the best source to learn about the monorepo practice. I learned most just by looking into it's composer.json - monorepo and package one.

Today, I'd like to share secrets behind biggest WTF that monorepo composer setup has.

In each Symplify, Symfony, Shopsys or Sylius package composer.json, you'll find:

{
    "minimum-stability": "dev",
    "prefer-stable": true
}

"So I get unstable dev dependencies to my project by installing symplify/*?"

No! These section are root-only. The most known is require-dev and also:

When you install a package, e.g. composer require symplify/easy-coding-standard --dev, you won't download PHPUnit even if that package has it in require-dev.

"So why is this needed? Do these monorepo packages use unstable versions?"

Of course not. When you install any package standalone, you'll get stable dependencies. It's not even possible to install dev dependency without explicitly saying that (e.g. composer require symplify/statie:@dev).

What is Branch Alias for?

Here comes the magical combo. Each package composer.json also includes:

{
    "extra": {
        "branch-alias": {
            "dev-master": "5.5-dev"
        }
    }
}

Do you think it will install composer require symplify/statie:5.5-dev? No.

The rule is: when last minor version is 5.4, the alias will be +0.1 version (5.5). E.g. the latest released Symfony version is 4.2.x, next one will be 4.3, so the alias is:

{
    "extra": {
        "branch-alias": {
            "dev-master": "4.3-dev"
        }
    }
}

Is that clear? Good.

"So what the branch-alias and the minimum-stability have in common?"

After-Split Interdependency

The main goal of all this magic is simple. To use the most recent mutual dependencies. E.g., I added a new feature in the monorepo that changed the code of symplify/statie and symplify/package-builder.

When the monorepo is split, the dev symplify/statie uses the dev version of symplify/package-builder.

Monorepo Release Maintenance

Let's say we'll release next Symplify version v5.4.2. What we have to handle before it's tagged and released?

1. Bump each Interdependency to the Release Version

 {
     "name": "symplify/statie",
     "require": {
-        "symplify/package-builder": "^5.5"
+        "symplify/package-builder": "^5.4.2"
     }
 }

2. Tag Current Version

git tag v5.4.2

3. Push the Tag

git push --tags

4. Bump the branch-alias if Needed

5.4.x still uses 5.5-dev as the alias, so we don't have to change anything for 5.4.2 version.

In case we'll release 5.5 in the future, it would look like this:

 {
     "extra": {
         "branch-alias": {
-            "dev-master": "5.5-dev"
+            "dev-master": "5.6-dev"
         }
     }
 }

Now we can continue committing and packages will always use last committed code.


"Monorepo is Hell of Work"

← You must be thinking right now. Well, these steps are basically find and replace - perfect weak spot to automate. All these 4 steps can replace single command from Symplify/MonorepoBuilder:

vendor/bin/monorepo-builder release v5.4.2

The lazy job is done :)


Happy coding!


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