There are two software distributions of GitLab: the open source [Community Edition](https://gitlab.com/gitlab-org/gitlab-ce/) (CE), and the open core [Enterprise Edition](https://gitlab.com/gitlab-org/gitlab-ee/) (EE). GitLab is available under [different subscriptions](https://about.gitlab.com/pricing/).
Both EE and CE require some add-on components called gitlab-shell and Gitaly. These components are available from the [gitlab-shell](https://gitlab.com/gitlab-org/gitlab-shell/tree/master) and [gitaly](https://gitlab.com/gitlab-org/gitaly/tree/master) repositories respectively. New versions are usually tags but staying on the master branch will give you the latest stable version. New releases are generally around the same time as GitLab CE releases with exception for informal security updates deemed critical.
**GitLab-shell** is a third kind of worker that takes orders from a fax machine (SSH) instead of the front desk (HTTP).
GitLab-shell communicates with Sidekiq via the “communication board” (Redis), and asks quick questions of the Unicorn workers either directly or via the front desk.
**Gitaly** is a back desk that is specialized on reaching the disks to perform git operations efficiently and keep a copy of the result of costly operations. All git operations go through Gitaly.
GitLab is primarily installed within the `/home/git` user home directory as `git` user. Within the home directory is where the gitlabhq server software resides as well as the repositories (though the repository location is configurable).
The bare repositories are located in `/home/git/repositories`. GitLab is a ruby on rails application so the particulars of the inner workings can be learned by studying how a ruby on rails application works.
To serve repositories over SSH there's an add-on application called gitlab-shell which is installed in `/home/git/gitlab-shell`.
A typical install of GitLab will be on GNU/Linux. It uses Nginx or Apache as a web front end to proxypass the Unicorn web server. By default, communication between Unicorn and the front end is via a Unix domain socket but forwarding requests via TCP is also supported. The web front end accesses `/home/git/gitlab/public` bypassing the Unicorn server to serve static pages, uploads (e.g. avatar images or attachments), and precompiled assets. GitLab serves web pages and a [GitLab API](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/api) using the Unicorn web server. It uses Sidekiq as a job queue which, in turn, uses redis as a non-persistent database backend for job information, meta data, and incoming jobs.
The GitLab web app uses MySQL or PostgreSQL for persistent database information (e.g. users, permissions, issues, other meta data). GitLab stores the bare git repositories it serves in `/home/git/repositories` by default. It also keeps default branch and hook information with the bare repository.
The add-on component gitlab-shell serves repositories over SSH. It manages the SSH keys within `/home/git/.ssh/authorized_keys` which should not be manually edited. gitlab-shell accesses the bare repositories through Gitaly to serve git objects and communicates with redis to submit jobs to Sidekiq for GitLab to process. gitlab-shell queries the GitLab API to determine authorization and access.
Gitaly executes git operations from gitlab-shell and the GitLab web app, and provides an API to the GitLab web app to get attributes from git (e.g. title, branches, tags, other meta data), and to get blobs (e.g. diffs, commits, files).
You may also be interested in the [production architecture of GitLab.com](https://about.gitlab.com/handbook/engineering/infrastructure/production-architecture/).
GitLab has several components to operate. As a system user (i.e. any user that is not the `git` user) it requires a persistent database (MySQL/PostreSQL) and redis database. It also uses Apache httpd or Nginx to proxypass Unicorn. As the `git` user it starts Sidekiq and Unicorn (a simple ruby HTTP server running on port `8080` by default). Under the GitLab user there are normally 4 processes: `unicorn_rails master` (1 process), `unicorn_rails worker` (2 processes), `sidekiq` (1 process).
Repositories get accessed via HTTP or SSH. HTTP cloning/push/pull utilizes the GitLab API and SSH cloning is handled by gitlab-shell (previously explained).
## Troubleshooting
See the README for more information.
### Init scripts of the services
The GitLab init script starts and stops Unicorn and Sidekiq.
```
/etc/init.d/gitlab
Usage: service gitlab {start|stop|restart|reload|status}
GitLab has configuration files located in `/home/git/gitlab/config/*`. Commonly referenced config files include:
-`gitlab.yml` - GitLab configuration.
-`unicorn.rb` - Unicorn web server settings.
-`database.yml` - Database connection settings.
gitlab-shell has a configuration file at `/home/git/gitlab-shell/config.yml`.
### Maintenance Tasks
[GitLab](https://gitlab.com/gitlab-org/gitlab-ce/tree/master) provides rake tasks with which you see version information and run a quick check on your configuration to ensure it is configured properly within the application. See [maintenance rake tasks](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/raketasks/maintenance.md).
Note: It is recommended to log into the `git` user using `sudo -i -u git` or `sudo su - git`. While the sudo commands provided by gitlabhq work in Ubuntu they do not always work in RHEL.
We've also detailed [our architecture of GitLab.com](https://about.gitlab.com/handbook/engineering/infrastructure/production-architecture/) but this is probably over the top unless you have millions of users.