Update the package manager implementation docs

Add additional details and guidelines for implementing a new
package manager using the existing structures and
logic along with suggested MRs

doc/development/packages.md

@@ -4,12 +4,13 @@ This document will guide you through adding another [package management system](
 
 See already supported package types in [Packages documentation](../administration/packages/index.md)
 
-Since GitLab packages' UI is pretty generic, it is possible to add new
+Since GitLab packages' UI is pretty generic, it is possible to add basic new
 package system support by solely backend changes. This guide is superficial and does
 not cover the way the code should be written. However, you can find a good example
 by looking at existing merge requests with Maven and NPM support:
 
 - [NPM registry support](https://gitlab.com/gitlab-org/gitlab/merge_requests/8673).
+- [Conan repository](https://gitlab.com/gitlab-org/gitlab/issues/8248).
 - [Maven repository](https://gitlab.com/gitlab-org/gitlab/merge_requests/6607).
 - [Instance level endpoint for Maven repository](https://gitlab.com/gitlab-org/gitlab/merge_requests/8757)
 
@@ -34,7 +35,7 @@ endpoints like:
 - GET package file content.
 - PUT upload package.
 
-Since the packages belong to a project, it's expected to have project-level endpoint
+Since the packages belong to a project, it's expected to have project-level endpoint (remote)
 for uploading and downloading them. For example:
 
 ```
@@ -44,9 +45,48 @@ PUT https://gitlab.com/api/v4/projects/<your_project_id>/packages/npm/
 
 Group-level and instance-level endpoints are good to have but are optional.
 
-NOTE: **Note:**
-To avoid name conflict for instance-level endpoints we use
-[the package naming convention](../user/packages/npm_registry/index.md#package-naming-convention)
+## Naming conventions
+
+To avoid name conflict for instance-level endpoints you will need to define a package naming convention
+that gives a way to identify the project that the package belongs to. This generally involves using the project
+id or full project path in the package name. See
+[Conan's naming convention](../user/packages/conan_repository/index.md#package-recipe-naming-convention) as an example.
+
+For group and project-level endpoints, naming can be less constrained, and it will be up to the group and project
+members to be certain that there is no conflict between two package names, however the system should prevent
+a user from reusing an existing name within a given scope.
+
+Otherwise, naming should follow the package manager's naming conventions and include a validation in the `package.md`
+model for that package type.
+
+## File uploads
+
+File uploads should be handled by GitLab workhorse using object accelerated uploads. What this means is that
+the workhorse proxy that checks all incoming requests to GitLab will intercept the upload request,
+upload the file, and forward a request to the main GitLab codebase only containing the metadata
+and file location rather than the file itself. An overview of this process can be found in the
+[development documentation](uploads.md#workhorse-object-storage-acceleration).
+
+In terms of code, this means a route will need to be added to the
+[gitlab-workhorse project](https://gitlab.com/gitlab-org/gitlab-workhorse) for each level of remote being added
+(instance, group, project). [This merge request](https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/412/diffs)
+demonstrates adding an instance-level endpoint for Conan to workhorse. You can also see the Maven project level endpoint
+implemented in the same file.
+
+Once the route has been added, you will need to add an additional `/authorize` version of the upload endpoint to your API file.
+[Here is an example](https://gitlab.com/gitlab-org/gitlab/blob/398fef1ca26ae2b2c3dc89750f6b20455a1e5507/ee/lib/api/maven_packages.rb#L164)
+of the additional endpoint added for Maven. The `/authorize` endpoint verifies and authorizes the request from workhorse,
+then the normal upload endpoint is implemented below, consuming the metadata that workhorse provides in order to
+create the package record. Workhorse provides a variety of file metadata such as type, size, and different checksum formats.
+
+For testing purposes, you may want to [enable object storage](https://gitlab.com/gitlab-org/gitlab-development-kit/blob/master/doc/howto/object_storage.md)
+in your local development environment.
+
+## Services and finders
+
+Logic for performing tasks such as creating package or package file records or finding packages should not live
+within the API file, but should live in services and finders. Existing services and finders should be used or
+extended when possible to keep the common package logic grouped as much as possible.
 
 ## Configuration
 
@@ -56,7 +96,7 @@ to add anything there.
 
 Packages can be configured to use object storage, therefore your code must support it.
 
-## Database
+## Database and handling metadata
 
 The current database model allows you to store a name and a version for each package.
 Every time you upload a new package, you can either create a new record of `Package`
@@ -65,4 +105,58 @@ information like the file `name`, `side`, `sha1`, etc.
 
 If there is specific data necessary to be stored for only one package system support,
 consider creating a separate metadata model. See `packages_maven_metadata` table
-and `Packages::MavenMetadatum` model as example for package specific data.
+and `Packages::MavenMetadatum` model as an example for package specific data, and `packages_conan_file_metadata` table
+and `Packages::ConanFileMetadatum` model as an example for package file specific data.
+
+If there is package specific behavior for a given package manager, add those methods to the metadata models and
+delegate from the package model.
+
+Note that the existing package UI only displays information within the `packages_packages` and `packages_package_files`
+tables. If the data stored in the metadata tables need to be displayed, a ~frontend change will be required.
+
+## Authorization
+
+There are project and group level permissions for `read_package`, `create_package`, and `destroy_package`. Each
+endpoint should
+[authorize the requesting user](https://gitlab.com/gitlab-org/gitlab/blob/398fef1ca26ae2b2c3dc89750f6b20455a1e5507/ee/lib/api/conan_packages.rb#L84)
+against the project or group before continuing.
+
+## Keep iterations small
+
+When implementing a new package manager, it is easy to end up creating one large merge request containing all of the
+necessary endpoints and services necessary to support basic usage. If this is the case, consider putting the
+API endpoints behind a [feature flag](feature_flags/development.md) and
+submitting each endpoint or behavior (download, upload, etc) in different merge requests to shorten the review
+process.
+
+### Potential MRs for any given package system
+
+#### MVC MRs
+
+These changes represent all that is needed to deliver a minimally usable package management system.
+
+1. Empty file structure (api file, base service for this package)
+1. Authentication system for 'logging in' to the package manager
+1. Identify metadata and create applicable tables
+1. Workhorse route for [object storage accelerated uploads](uploads.md#workhorse-object-storage-acceleration)
+1. Endpoints required for upload/publish
+1. Endpoints required for install/download
+1. Endpoints required for remove/delete
+
+#### Possible post-MVC MRs
+
+These updates are not essential to be able to publish and consume packages, but may be desired as the system is
+released for general use.
+
+1. Endpoints required for search
+1. Front end updates to display additional package information and metadata
+1. Limits on file sizes
+1. Tracking for metrics
+
+## Exceptions
+
+This documentation is just guidelines on how to implement a package manager to match the existing structure and logic
+already present within GitLab. While the structure is intended to be extendable and flexible enough to allow for
+any given package manager, if there is good reason to stray due to the constraints or needs of a given package
+manager, then it should be raised and discussed within the implementation issue or merge request to work towards
+the most efficient outcome.