gitlab-org--gitlab-foss/doc/development/geo/framework.md

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Geo self-service framework

NOTE: This document is subject to change as we continue to implement and iterate on the framework. Follow the progress in the epic. If you need to replicate a new data type, reach out to the Geo team to discuss the options. You can contact them in #g_geo on Slack or mention @geo-team in the issue or merge request.

Geo provides an API to make it possible to easily replicate data types across Geo sites. This API is presented as a Ruby Domain-Specific Language (DSL) and aims to make it possible to replicate data with minimal effort of the engineer who created a data type.

Nomenclature

Before digging into the API, developers need to know some Geo-specific naming conventions:

• Model: A model is an Active Model, which is how it is known in the entire Rails codebase. It usually is tied to a database table. From Geo perspective, a model can have one or more resources.

• Resource: A resource is a piece of data that belongs to a model and is produced by a GitLab feature. It is persisted using a storage mechanism. By default, a resource is not a Geo replicable.

• Data type: Data type is how a resource is stored. Each resource should fit in one of the data types Geo supports:

  • Git repository
  • Blob
  • Database

  For more detail, see Data types.

• Geo Replicable: A Replicable is a resource Geo wants to sync across Geo sites. There is a limited set of supported data types of replicables. The effort required to implement replication of a resource that belongs to one of the known data types is minimal.

• Geo Replicator: A Geo Replicator is the object that knows how to replicate a replicable. It's responsible for:

  • Firing events (producer)
  • Consuming events (consumer)

  It's tied to the Geo Replicable data type. All replicators have a common interface that can be used to process (that is, produce and consume) events. It takes care of the communication between the primary site (where events are produced) and the secondary site (where events are consumed). The engineer who wants to incorporate Geo in their feature will use the API of replicators to make this happen.

• Geo Domain-Specific Language: The syntactic sugar that allows engineers to easily specify which resources should be replicated and how.

Geo Domain-Specific Language

The replicator

First of all, you need to write a replicator. The replicators live in ee/app/replicators/geo. For each resource that needs to be replicated, there should be a separate replicator specified, even if multiple resources are tied to the same model.

For example, the following replicator replicates a package file:

module Geo
  class PackageFileReplicator < Gitlab::Geo::Replicator
    # Include one of the strategies your resource needs
    include ::Geo::BlobReplicatorStrategy

    # Specify the CarrierWave uploader needed by the used strategy
    def carrierwave_uploader
      model_record.file
    end

    # Specify the model this replicator belongs to
    def self.model
      ::Packages::PackageFile
    end
  end
end

The class name should be unique. It also is tightly coupled to the table name for the registry, so for this example the registry table will be package_file_registry.

For the different data types Geo supports there are different strategies to include. Pick one that fits your needs.

Linking to a model

To tie this replicator to the model, you need to add the following to the model code:

class Packages::PackageFile < ApplicationRecord
  include ::Geo::ReplicableModel

  with_replicator Geo::PackageFileReplicator
end

API

When this is set in place, it's easy to access the replicator through the model:

package_file = Packages::PackageFile.find(4) # just a random ID as example
replicator = package_file.replicator

Or get the model back from the replicator:

replicator.model_record
=> <Packages::PackageFile id:4>

The replicator can be used to generate events, for example in ActiveRecord hooks:

  after_create_commit -> { replicator.publish_created_event }

Library

The framework behind all this is located in ee/lib/gitlab/geo/.

Existing Replicator Strategies

Before writing a new kind of Replicator Strategy, check below to see if your resource can already be handled by one of the existing strategies. Consult with the Geo team if you are unsure.

Blob Replicator Strategy

Models that use CarrierWave's Uploader::Base are supported by Geo with the Geo::BlobReplicatorStrategy module. For example, see how Geo replication was implemented for Pipeline Artifacts.

Each file is expected to have its own primary ID and model. Geo strongly recommends treating every single file as a first-class citizen, because in our experience this greatly simplifies tracking replication and verification state.

To implement Geo replication of a new blob-type Model, open an issue with the provided issue template.

To view the implementation steps without opening an issue, view the issue template file.

Repository Replicator Strategy

Models that refer to any Git repository on disk are supported by Geo with the Geo::RepositoryReplicatorStrategy module. For example, see how Geo replication was implemented for Group-level Wikis. Note that this issue does not implement verification, since verification of Git repositories was not yet added to the Geo self-service framework. An example implementing verification can be found in the merge request to Add Snippet repository verification.

Each Git repository is expected to have its own primary ID and model.

To implement Geo replication of a new Git repository-type Model, open an issue with the provided issue template.

To view the implementation steps without opening an issue, view the issue template file.