gitlab-org--gitlab-foss/doc/development/gitaly.md

GitLab Developers Guide to Working with Gitaly

Gitaly is a high-level Git RPC service used by GitLab CE/EE, Workhorse and GitLab-Shell. All Rugged operations in GitLab CE/EE are currently being phased out to be replaced by Gitaly API calls.

Visit the Gitaly Migration Board for current status of the migration.

Developing new Git features

Starting with GitLab 10.8, all new Git features should be developed in Gitaly.

This is a new process that is not clearly defined yet. If you want to contribute a Git feature and you're getting stuck, reach out to the Gitaly team or @jacobvosmaer-gitlab.

By 'new feature' we mean any method or class in lib/gitlab/git that is called from outside lib/gitlab/git. For new methods that are called from inside lib/gitlab/git, see 'Modifying existing Git features' below.

There should be no new code that touches Git repositories via disk access (e.g. Rugged, git, rm -rf) anywhere outside lib/gitlab/git.

The process for adding new Gitaly features is:

• exploration / prototyping
• design and create a new Gitaly RPC in gitaly-proto
• release a new version of gitaly-proto
• write implementation and tests for the RPC in Gitaly, in Go or Ruby
• release a new version of Gitaly
• write client code in gitlab-ce/ee, gitlab-workhorse or gitlab-shell that calls the new Gitaly RPC

These steps often overlap. It is possible to use an unreleased version of Gitaly and gitaly-proto during testing and development.

• See the Gitaly repo for instructions on writing server side code with an unreleased protocol.
• See below for instructions on running gitlab-ce tests with a modified version of Gitaly.
• In GDK run gdk install and restart gdk run (or gdk run app) to use a locally modified Gitaly version for development

Gitaly-ruby

It is possible to implement and test RPC's in Gitaly using Ruby code, in gitaly-ruby. This should make it easier to contribute for developers who are less comfortable writing Go code.

There is documentation for this approach in the Gitaly repo.

Modifying existing Git features

If you modify existing Git features in lib/gitlab/git you need to make sure the changes also work in Gitaly. Because we are still in the migration process there are a number of subtle pitfalls. Features that have been migrated have dual implementations (Gitaly and local). The Gitaly implementation may or may not use a vendored (and therefore possibly outdated) copy of the local implementation in lib/gitlab/git.

To avoid unexpected problems and conflicts, all changes to lib/gitlab/git need to be approved by a member of the Gitaly team.

For the time being, while the Gitaly migration is still in progress, there should be no Enterprise Edition-only Git code in lib/gitlab/git. Also no mixins.

Feature Flags

Gitaly makes heavy use of feature flags.

Each Rugged-to-Gitaly migration goes through a series of phases:

• Opt-In: by default the Rugged implementation is used.
  • Production instances can choose to enable the Gitaly endpoint by enabling the feature flag.
  • For testing purposes, you may wish to enable all feature flags by default. This can be done by exporting the following environment variable: GITALY_FEATURE_DEFAULT_ON=1.
  • On developer instances (ie, when Rails.env.development? is true), the Gitaly endpoint is enabled by default, but can be disabled using feature flags.
• Opt-Out: by default, the Gitaly endpoint is used, but the feature can be explicitly disabled using the feature flag.
• Mandatory: The migration is complete and cannot be disabled. The old codepath is removed.

Enabling and Disabling Feature

In the Rails console, type:

Feature.enable(:gitaly_feature_name)
Feature.disable(:gitaly_feature_name)

Where gitaly_feature_name is the name of the Gitaly feature. This can be determined by finding the appropriate gitaly_migrate code block, for example:

gitaly_migrate(:tag_names) do
...
end

Since Gitaly features are always prefixed with gitaly_, the name of the feature flag in this case would be gitaly_tag_names.

Gitaly-Related Test Failures

If your test-suite is failing with Gitaly issues, as a first step, try running:

rm -rf tmp/tests/gitaly

TooManyInvocationsError errors

During development and testing, you may experience Gitlab::GitalyClient::TooManyInvocationsError failures. The GitalyClient will attempt to block against potential n+1 issues by raising this error when Gitaly is called more than 30 times in a single Rails request or Sidekiq execution.

As a temporary measure, export GITALY_DISABLE_REQUEST_LIMITS=1 to suppress the error. This will disable the n+1 detection in your development environment.

Please raise an issue in the GitLab CE or EE repositories to report the issue. Include the labels ~Gitaly ~performance ~"technical debt". Please ensure that the issue contains the full stack trace and error message of the TooManyInvocationsError. Also include any known failing tests if possible.

Isolate the source of the n+1 problem. This will normally be a loop that results in Gitaly being called for each element in an array. If you are unable to isolate the problem, please contact a member of the Gitaly Team for assistance.

Once the source has been found, wrap it in an allow_n_plus_1_calls block, as follows:

# n+1: link to n+1 issue
Gitlab::GitalyClient.allow_n_plus_1_calls do
  # original code
  commits.each { |commit| ... }
end

Once the code is wrapped in this block, this code-path will be excluded from n+1 detection.

Request counts

Commits and other git data, is now fetched through Gitaly. These fetches can, much like with a database, be batched. This improves performance for the client and for Gitaly itself and therefore for the users too. To keep performance stable and guard performance regressions, Gitaly calls can be counted and the call count can be tested against. This requires the :request_store flag to be set.

describe 'Gitaly Request count tests' do
  context 'when the request store is activated', :request_store do
    it 'correctly counts the gitaly requests made' do
      expect { subject }.to change { Gitlab::GitalyClient.get_request_count }.by(10)
    end
  end
end

Running tests with a locally modified version of Gitaly

Normally, gitlab-ce/ee tests use a local clone of Gitaly in tmp/tests/gitaly pinned at the version specified in GITALY_SERVER_VERSION. The GITALY_SERVER_VERSION file supports =my-branch syntax to use a custom branch in gitlab-org/gitaly. If you want to run tests locally against a modified version of Gitaly you can replace tmp/tests/gitaly with a symlink. This is much faster because the =my-branch syntax forces a Gitaly re-install each time you run rspec.

rm -rf tmp/tests/gitaly
ln -s /path/to/gitaly tmp/tests/gitaly

Make sure you run make in your local Gitaly directory before running tests. Otherwise, Gitaly will fail to boot.

If you make changes to your local Gitaly in between test runs you need to manually run make again.

Note that CI tests will not use your locally modified version of Gitaly. To use a custom Gitaly version in CI you need to update GITALY_SERVER_VERSION. You can use the format =revision to use a non-tagged commit from https://gitlab.com/gitlab-org/gitaly in CI.

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