- If a packages exists in a higher priority repository, it will now be entirely ignored in lower priority repositories. See [repository priorities](https://getcomposer.org/repoprio) for details.
-`PluginInterface` added a deactivate (so plugin can stop whatever it is doing) and an uninstall (so the plugin can remove any files it created or do general cleanup) method.
- Plugins implementing `EventSubscriberInterface` will be deregistered from the EventDispatcher automatically when being deactivated, nothing to do there.
-`Pool` objects are now created via the `RepositorySet` class, you should use that in case you were using the `Pool` class directly.
- The `Composer\Installer` class changed quite a bit internally, but the inputs are almost the same:
-`setAdditionalInstalledRepository` is now `setAdditionalFixedRepository`
-`setUpdateWhitelist` is now `setUpdateAllowList`
-`setWhitelistDependencies`, `setWhitelistTransitiveDependencies` and `setWhitelistAllDependencies` are now all rolled into `setUpdateAllowTransitiveDependencies` which takes one of the `Request::UPDATE_*` constants
-`setSkipSuggest` is gone
-`vendor/composer/installed.json` format changed:
- packages are now wrapped into a `"packages"` top level key instead of the whole file being the package array
- packages now contain an `"installed-path"` key which lists where they were installed
- there is a top level `"dev"` key which stores whether dev requirements were installed or not
-`PreFileDownloadEvent` now receives an `HttpDownloader` instance instead of `RemoteFilesystem`, and that instance can not be overridden by listeners anymore
-`IOInterface` now extends PSR-3's `LoggerInterface`, and has new `writeRaw` + `writeErrorRaw` methods
-`RepositoryInterface` changes:
- A new `loadPackages(array $packageNameMap, array $acceptableStabilities, array $stabilityFlags)` function was added for use during pool building
-`search` now has a third `$type` argument
- A new `getRepoName()` function was added to describe the repository
- A new `getProviders()` function was added to list packages providing a given package's name
- Removed `BaseRepository` abstract class
-`DownloaderInterface` changes:
-`download` now receives a third `$prevPackage` argument for updates
-`download` should now only do network operations to prepare the package for installation but not actually install anything
-`prepare` (do user prompts or any checks which need to happen to make sure that install/update/remove will most likely succeed), `install` (should do the non-network part that `download` used to do) and `cleanup` (cleaning up anything that may be left over) were added as new steps in the package install flow
- All packages get first downloaded, then all together prepared, then all together installed/updated/uninstalled, then finally cleanup is called for all. Therefore for error recovery it is important to avoid failing during install/update/uninstall as much as possible, and risky things or user prompts should happen in the prepare step rather. In case of failure, cleanup() will be called so that changes can be undone as much as possible.
- If you used `RemoteFilesystem` you probably should use `HttpDownloader` instead now
-`PRE_DEPENDENCIES_SOLVING` and `POST_DEPENDENCIES_SOLVING` events have been removed, use the new `PRE_OPERATIONS_EXEC` or other existing events instead or talk to us if you think you really need this
## For Composer repository implementors
Composer 2.0 adds support for a new Composer repository format.
It is possible to build a repository which is compatible with both Composer v1 and v2, you keep everything you had and simply add the new fields in `packages.json`.
Here are examples of the new values from packagist.org:
### metadata-url
`"metadata-url": "/p2/%package%.json",`
This new metadata-url should serve all packages which are in the repository.
- Whenever Composer looks for a package, it will replace `%package%` by the package name, and fetch that URL.
- If dev stability is allowed for the package, it will also load the URL again with `$packageName~dev` (e.g. `/p2/foo/bar~dev.json` to look for `foo/bar`'s dev versions).
- Caching is done via the use of If-Modified-Since header, so make sure you return Last-Modified headers and that they are accurate.
- Any requested package which does not exist MUST return a 404 status code, which will indicate to Composer that this package does not exist in your repository.
- The `foo/bar.json` and `foo/bar~dev.json` files containing package versions MUST contain only versions for the foo/bar package, as `{"packages":{"foo/bar":[ ... versions here ... ]}}`.
- The array of versions can also optionally be minified using `Composer\Util\MetadataMinifier::minify()`. If you do that, you should add a `"minified": "composer/2.0"` key at the top level to indicate to Composer it must expand the version list back into the original data. See https://repo.packagist.org/p2/monolog/monolog.json for an example.
If your repository only has a small number of packages, and you want to avoid the 404-requests, you can also specify an `"available-packages"` key in `packages.json` which should be an array with all the package names that your repository contain.
The providers-api is optional, but if you implement it it should return packages which provide a given package name, but not the package which has that name. For example https://packagist.org/providers/monolog/monolog.json lists some package which have a "provide" rule for monolog/monolog, but it does not list monolog/monolog itself.
