Add Gitaly to the architecture of the application

Added both to the graph, and to the high level overview. Explaining what
is the goal of the system and how does it connect to the other systems.

doc/development/architecture.md

@@ -6,7 +6,7 @@ There are two editions of GitLab: [Enterprise Edition](https://about.gitlab.com/
 
 EE releases are available not long after CE releases. To obtain the GitLab EE there is a [repository at gitlab.com](https://gitlab.com/subscribers/gitlab-ee). For more information about the release process see the section 'New versions and upgrading' in the readme.
 
-Both EE and CE require an add-on component called gitlab-shell. It is obtained from the [gitlab-shell repository](https://gitlab.com/gitlab-org/gitlab-shell/tree/master). 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.
+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.
 
 ## Physical office analogy
 
@@ -35,8 +35,10 @@ Their job description:
  - make tasks for Sidekiq;
  - fetch stuff from the warehouse or move things around in there;
 
-**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.
+**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 Enterprise Edition (the application)** is the collection of processes and business practices that the office is run by.
 
@@ -62,7 +64,9 @@ The GitLab web app uses MySQL or PostgreSQL for persistent database information
 
 When serving repositories over HTTP/HTTPS GitLab utilizes the GitLab API to resolve authorization and access as well as serving git objects.
 
-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 directly 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.
+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 Workhorse, 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)
 
 ### Installation Folder Summary