• Yorick Peterse's avatar
    Rewrite the GitHub importer from scratch · 86c05c535691
    Yorick Peterse authored
    Prior to this MR there were two GitHub related importers:
    * Github::Import: the main importer used for GitHub projects
    * Gitlab::GithubImport: importer that's somewhat confusingly used for
      importing Gitea projects (apparently they have a compatible API)
    This MR renames the Gitea importer to Gitlab::LegacyGithubImport and
    introduces a new GitHub importer in the Gitlab::GithubImport namespace.
    This new GitHub importer uses Sidekiq for importing multiple resources
    in parallel, though it also has the ability to import data sequentially
    should this be necessary.
    The new code is spread across the following directories:
    * lib/gitlab/github_import: this directory contains most of the importer
      code such as the classes used for importing resources.
    * app/workers/gitlab/github_import: this directory contains the Sidekiq
      workers, most of which simply use the code from the directory above.
    * app/workers/concerns/gitlab/github_import: this directory provides a
      few modules that are included in every GitHub importer worker.
    == Stages
    The import work is divided into separate stages, with each stage
    importing a specific set of data. Stages will schedule the work that
    needs to be performed, followed by scheduling a job for the
    "AdvanceStageWorker" worker. This worker will periodically check if all
    work is completed and schedule the next stage if this is the case. If
    work is not yet completed this worker will reschedule itself.
    Using this approach we don't have to block threads by calling `sleep()`,
    as doing so for large projects could block the thread from doing any
    work for many hours.
    == Retrying Work
    Workers will reschedule themselves whenever necessary. For example,
    hitting the GitHub API's rate limit will result in jobs rescheduling
    themselves. These jobs are not processed until the rate limit has been
    == User Lookups
    Part of the importing process involves looking up user details in the
    GitHub API so we can map them to GitLab users. The old importer used
    an in-memory cache, but this obviously doesn't work when the work is
    spread across different threads.
    The new importer uses a Redis cache and makes sure we only perform
    API/database calls if absolutely necessary.  Frequently used keys are
    refreshed, and lookup misses are also cached; removing the need for
    performing API/database calls if we know we don't have the data we're
    looking for.
    == Performance & Models
    The new importer in various places uses raw INSERT statements (as
    generated by `Gitlab::Database.bulk_insert`) instead of using Rails
    models. This allows us to bypass any validations and callbacks,
    drastically reducing the number of SQL queries and Gitaly RPC calls
    necessary to import projects.
    To ensure the code produces valid data the corresponding tests check if
    the produced rows are valid according to the model validation rules.