This chapter will tell you how to make your library installable through composer.
## Every project is a package
As soon as you have a `composer.json` in a directory, that directory is a package. When you add a `require` to a project, you are making a package that depends on other packages. The only difference between your project and libraries is that your project is a package without a name.
In order to make that package installable you need to give it a name. You do this by adding a `name` to `composer.json`:
```json
{
"name": "acme/hello-world",
"require": {
"monolog/monolog": "1.0.*"
}
}
```
In this case the project name is `acme/hello-world`, where `acme` is the vendor name. Supplying a vendor name is mandatory.
**Note:** If you don't know what to use as a vendor name, your GitHub username is usually a good bet. While package names are case insensitive, the convention is all lowercase and dashes for word separation.
You need to specify the version some way. Depending on the type of repository you are using, it might be possible to omit it from `composer.json`, because the repository is able to infer the version from elsewhere.
If you do want to specify it explicitly, you can just add a `version` field:
```json
{
"version": "1.0.0"
}
```
However if you are using git, svn or hg, you don't have to specify it. Composer will detect versions as follows:
### Tags
For every tag that looks like a version, a package version of that tag will be created. It should match 'X.Y.Z' or 'vX.Y.Z', with an optional suffix for RC, beta, alpha or patch.
Here are a few examples of valid tag names:
1.0.0
v1.0.0
1.10.5-RC1
v4.4.4beta2
v2.0.0-alpha
v2.0.4-p1
**Note:** If you specify an explicit version in `composer.json`, the tag name must match the specified version.
### Branches
For every branch, a package development version will be created. If the branch name looks like a version, the version will be `{branchname}-dev`. For example a branch `2.0` will get a version `2.0-dev`. If the branch does not look like a version, it will be `dev-{branchname}`. `master` results in a `dev-master` version.
**Note:** When you install a dev version, it will install it from source. See [Repositories] for more information.
## Publishing to a VCS
Once you have a vcs repository (version control system, e.g. git) containing a `composer.json` file, your library is already composer-installable. In this example we will publish the `acme/hello-world` library on GitHub under `github.com/composer/hello-world`.
Now, To test installing the `acme/hello-world` package, we create a new project locally. We will call it `acme/blog`. This blog will depend on `acme/hello-world`, which in turn depends on `monolog/monolog`. We can accomplish this by creating a new `blog` directory somewhere, containing a `composer.json`:
```json
{
"name": "acme/blog",
"require": {
"acme/hello-world": "dev-master"
}
}
```
The name is not needed in this case, since we don't want to publish the blog as a library. It is added here to clarify which `composer.json` is being described.
Now we need to tell the blog app where to find the `hello-world` dependency. We do this by adding a package repository specification to the blog's `composer.json`:
For more details on how package repositories work and what other types are available, see [Repositories].
That's all. You can now install the dependencies by running composer's `install` command!
**Recap:** Any git/svn/hg repository containing a `composer.json` can be added to your project by specifying the package repository and declaring the dependency in the `require` field.
## Publishing to packagist
Alright, so now you can publish packages. But specifying the vcs repository every time is cumbersome. You don't want to force all your users to do that.
The other thing that you may have noticed is that we did not specify a package repository for `monolog/monolog`. How did that work? The answer is packagist.
Packagist is the main package repository for composer, and it is enabled by default. Anything that is published on packagist is available automatically through composer. Since monolog [is on packagist](http://packagist.org/packages/monolog/monolog), we can depend on it without having to specify any additional repositories.
Assuming we want to share `hello-world` with the world, we would want to publish it on packagist as well. And this is really easy.
You simply hit the big "Submit Package" button and sign up. Then you submit the URL to your VCS, at which point packagist will start crawling it. Once it is done, your package will be available to anyone.
