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240 lines
12 KiB
Markdown
240 lines
12 KiB
Markdown
# Bower rewrite
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## Why?
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Bower codebase is becoming unmanageable, especially at its core.
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Main issues are:
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- __No separation of concerns. The overall codebase has grown in a patch fashion, which has lead to a bloated and tight coupled solution.__
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- __Monolithic Package.js that handles all package types (both local and remote `Git`, URL, local files, etc).__
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- __Package.js has a big nesting level of callbacks, causing confusion and making the code hard to read.__
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- Some commands, such as install and update, have incorrect behaviour ([#200](https://github.com/twitter/bower/issues/200), [#256](https://github.com/twitter/bower/issues/256))
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- This is directly related with the current implementation of bower core: Package.js and Manager.js
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- Programmatic usage needs improvement
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- Unable to spawn multiple commands in parallel in different folders
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- Some commands simply do not fire the `end` event
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- Others fire the `error` event many times
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- Some commands should fire more meaningful events (e.g.: install should fire each installed package)
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## Main goals
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- Ease the process of gathering more contributors.
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- Clear architecture and separation of concerns.
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- Installation/update speedup.
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- Named endpoints on the CLI install.
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- Offline installation of packages, thanks to the cache.
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- Ability to easily add package types (`SVN`, etc).
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- Support for commit hashes and branches in targets for `Git` endpoints.
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- Improved output after installation/update.
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- Integrate with update-notifier and yeomen insight.
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## Implementation details
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### Term dictionary
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- **Canonical package:** A folder containing all the files that belong to a package. May include a `bower.json` file inside. (typically what gets installed)
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- **Source:** URL, git endpoint, etc.
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- **Target:** `semver` range, commit hash, branch (indicates a version).
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- **Endpoint:** source#target
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- **Named endpoint:** name@endpoint#target
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- **UoW:** Unit of Work
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- **Components folder:** The folder in which components are installed (`bower_components` by default).
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### Overall strategy
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Bower is composed of the following components:
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- `CLI`: Command line interface for Bower.
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- `.bowerrc`: Allows for customisations of Bower behaviour at the project/user level.
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- `bower.json`: Main purpose is to declare the component dependencies and other component related information.
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- `Manager`: Main coordinator, responsible for:
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- Checking which packages are already installed in the current `bower folder`.
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- Deciding which version of the dependencies should be fetched from the `PackageRepository`, while keeping every dependant compatible (note that the `Manager` is `server` aware).
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- Tracking which dependencies have been fetched, which ones failed to fetch, and which ones are being fetched.
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- Requesting the `PackageRepository` to fail-fast, in case it realises there is no resolution for the current dependency tree.
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- `PackageRepository`: Abstraction to the underlying complexity of heterogeneous source types. Responsible for:
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- Storing new entries in `ResolveCache`.
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- Queueing resolvers into the `UoW`, if no suitable entry is found in the `ResolveCache`.
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- `ResolveCache`: Keeps a cache of previously resolved endpoints. Lookup can be done using an endpoint.
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- `UnitOfWork`: Work coordinator, responsible for:
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- Keeping track of which resolvers are being resolved.
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- Limiting amount of parallel resolutions.
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- `ResolverFactory`: Parses an endpoint and returns a `Resolver` capable of resolving the source type.
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- `Resolver`: Base resolver, which can be extended by concrete resolvers, like `UrlResolver`, `GitRemoteResolver`, etc.
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You can find additional details about each of these components below, in [Architecture components details](#architecture-components-details).
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#### Resolve process
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Here's an overview of the dependency resolve process:
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1. **INSTALL/UPDATE** - A set of named endpoints and/or endpoints is requested to be installed/updated, and these are passed to the `Manager`.
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2. **ANALIZE COMPONENTS FOLDER** - `Manager` starts by reading the *components folder* and understanding which packages are already installed.
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3. **ENQUEUE ENDPOINTS** - For each endpoint that should be fetched, the `Manager` enqueues the *named endpoints*/endpoints in the `PackageRepository`. Some considerations:
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- If a package should be fetched or not depends on the following conditions:
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- What operation is being done (install/update).
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- If package is already installed.
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- If `Manager` has already enqueued that *named endpoint*/endpoint in the current runtime (regardless of the fetch being currently in progress, already complete, or failed).
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- Additional flags (force, etc).
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4. **FABRICATE RESOLVERS** - For each of the endpoints, the `PackageRepository` requests the `ResolverFactory` for suitable resolvers, capable of handling the source type. Some considerations:
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- The factory method takes the source string as its main argument, and is also provided with target, and package name if possible (all this information is extracted from the *named endpoint*/endpoint).
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- This method is asynchronous, in order to allow for I/O operations to happen, without blocking the whole process (e.g., querying registry, etc).
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- There is a runtime internal cache of sources that have already been analysed, and what type of `Resolver` resulted from that analysis. This speeds up the decision process, particularly for aliases (registered packages), and published packages, which would required HTTP requests.
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5. **LOOKUP CACHE** - `PackageRepository` looks up the `ResolveCache` using the endpoint, for a cached *canonical package* that complies to the endpoint target. Some considerations:
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- The lookup is performed using an endpoint that is fetched from the `Resolver`. This allows the resolver to guarantee that the endpoint has been normalised (twitter/bootstrap -> git://github.com/twitter/bootstrap.git, etc).
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- The `ResolveCache` is `semver` aware. What this means, is that if you try to lookup `~1.2.1`, and the cache has a entries for versions `1.2.3` and `1.2.4`, it will give a hit with `1.2.4`.
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6. **CACHE HIT VALIDATION** - At this stage, and only for the cache hits, the `PackageRepository` will question the `Resolver` if there is any version higher than the one fetched from cache that also complies with the endpoint target. Some considerations:
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- This step is ignored in case a flag like `offline` is passed.
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- How the `Resolver` checks this, depends on the `Resolver` type. (e.g. `GitRemoteResolver` would fetch the git refs with `git ls-remote --tags --heads`, and check if there is a higher version that complies with the target).
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- This check should be as quick as possible. If the process of checking a new version is too slow, it's preferable to just assume there is a new version.
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- If there is no way to check if there is a higher version, assume that there is.
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- If the `Resolver` indicates that the cached version is outdated, then it is treated as a cache miss.
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7. **RESOLVE CACHE MISSES** - Any cache miss needs to be resolved, so the `PackageRepository` requests each of the remaining resolvers to resolve, and waits.
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8. **CACHE RESOLVED PACKAGES** - As the resolvers complete the resolution, the `PackageRepository` stores the canonic packages in the `ResolveCache`, along with the source, version, and any additional information that the `Resolver` provides. This allows resolvers to store additional details about the fetched package to be used for future *cache hit validations* (e.g. store HTTP expiration headers in the case of the `UrlPackage`).
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9. **RETURN PACKAGE TO MANAGER** - The `PackageRepository` returns the canonical package to the `Manager`.
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10. **EVALUATE RESOLVED PACKAGE DEPENDENCIES** - The `Manager` checks if the returned canonical packages have a `bower.json` file describing additional dependencies and, if so, continue in point #3. If there are no more unresolved dependencies, finish up the installation procedure.
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### Architecture components details
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#### Manager
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TODO
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#### PackageRepository
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TODO
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#### ResolverFactory
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Simple function that takes a *named endpoint*/endpoint with options and creates an instance of a `Resolver` that obeys the base `Resolver` interface.
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```js
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function createResolver(endpoint, options) -> Promise
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```
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This function could perform transformations/normalisations to the tuple endpoint.
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For instance, if `endpoint` is a shorthand it would expand it.
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The function is actually async to allow query the bower registry to know the real endpoint.
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#### ResolveCache
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TODO
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#### Resolver
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The `Resolver` class extends `EventEmitter`.
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Think of it as an abstract class that implements the resolver interface as well as serving as a base for other resolver types.
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##### Events
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- `name_change`: fired when the name of the package has changed
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- `action`: fired to inform the current action being performed by the resolver
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- `warn`: fired to inform a warning, e.g.: deprecation
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##### Constructor
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Resolver(source, options)
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Options:
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- `name` - the name (if none is passed, one will be guessed from the endpoint)
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- `target` - the target (defaults to *)
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- `config` - the config to use (defaults to the global config)
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------------
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Public functions
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##### Resolver#getName() -> String
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Returns the name.
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##### Resolver#getSource() -> String
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Returns the source.
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##### Resolver#getTarget() -> String
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Returns the target.
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##### Resolver#getTempDir() -> String
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Returns the temporary directory that the resolver can use to resolve itself.
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##### Resolver#hasNew(oldVersion, oldResolution) -> Promise
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Checks if there is a new version. Takes the old version and resolution to be used when comparing.
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Resolves to a boolean when done.
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##### Resolver#resolve() -> Promise
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Resolves the resolver.
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The resolve process obeys a very explicit flow:
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- calls #_createTempDir and waits
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- When done, calls #_resolveSelf and waits
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- When done, calls #_readJson and waits
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- When done, calls #_parseJson and waits
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- When done, resolves the promise with the resolution.
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##### Resolver#getJson() -> Object
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Get the `bower.json` of the resolved package.
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Throws an error if the resolver is not yet resolved.
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-----------
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Protected functions
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##### Resolver#_createTempDir() -> Promise
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Creates a temporary dir.
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##### Resolver#_readJson() -> Promise
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Reads `bower.json`, possibly by using a dedicated `read-json` package that will be available in the Bower organization. It will ensure everything is valid.
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##### Resolver#_parseJson(json) -> Promise
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Parses the json:
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- Checks if the resolver name is different from the json one. If so and if the name was "guessed", the name of the package will be updated and a `name_change` event will be emitted.
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- Deletes files that are specified in the `ignore` property of the json from the temporary directory.
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--------
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Abstract functions that must be implemented by concrete resolvers.
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##### Resolver#_resolveSelf() -> Promise
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Resolves self. This method should be implemented by the concrete resolvers. For instance, the UrlResolver would download the contents of a URL into the temporary directory.
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#### Types of Resolvers
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The following resolvers will extend from `Resolver.js` and will obey its interface.
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- `LocalResolver` extends `Resolver` (dependencies pointing to files of folders in the own system)
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- `UrlResolver` extends `Resolver` (dependencies pointing to downloadable resources)
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- `GitFsResolver` extends `Resolver` (git dependencies available in the local file system)
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- `GitRemoteResolver` extends `Resolver` or `GitFsResolver` (remote git dependencies)
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- `PublishedResolver` extends `Resolver` (? makes sense if bower supports a publish model, just like `npm`).
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These type of resolvers will be known and created (instantiated) by the `ResolverFactory`.
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This architecture will make it very easy for the community to create others package types, for instance, a `MercurialLocalPackage`, `MercurialRemotePackage`, `SvnResolver`, etc.
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#### Unit of work
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TODO
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