Gitaly Cluster

Gitaly, the service that provides storage for Git repositories, can be run in a clustered configuration to increase fault tolerance. In this configuration, every Git repository is stored on every Gitaly node in the cluster. Multiple clusters (or shards), can be configured.

NOTE: Gitaly Clusters can be created using GitLab Core and higher tiers. However, technical support is limited to GitLab Premium and Ultimate customers only. Not available in GitLab.com.

Praefect is a router and transaction manager for Gitaly, and a required component for running a Gitaly Cluster.

Using a Gitaly Cluster increase fault tolerance by:

• Replicating write operations to warm standby Gitaly nodes.
• Detecting Gitaly node failures.
• Automatically routing Git requests to an available Gitaly node.

The availability objectives for Gitaly clusters are:

• Recovery Point Objective (RPO): Less than 1 minute.

  Writes are replicated asynchronously. Any writes that have not been replicated to the newly promoted primary are lost.

  Strong consistency can be used to avoid loss in some circumstances.

• Recovery Time Objective (RTO): Less than 10 seconds.

  Outages are detected by a health checks run by each Praefect node every second. Failover requires ten consecutive failed health checks on each Praefect node.

  Faster outage detection is planned to improve this to less than 1 second.

Gitaly Cluster supports:

• Strong consistency of the secondary replicas.
• Automatic failover from the primary to the secondary.
• Reporting of possible data loss if replication queue is non-empty.
• Marking repositories as read only if data loss is detected to prevent data inconsistencies.

Follow the HA Gitaly epic for improvements including horizontally distributing reads.

Gitaly Cluster compared to Geo

Gitaly Cluster and Geo both provide redundancy. However the redundancy of:

• Gitaly Cluster provides fault tolerance for data storage and is invisible to the user. Users are not aware when Gitaly Cluster is used.
• Geo provides replication and disaster recovery for an entire instance of GitLab. Users know when they are using Geo for replication. Geo replicates multiple datatypes, including Git data.

The following table outlines the major differences between Gitaly Cluster and Geo:

Tool	Nodes	Locations	Latency tolerance	Failover	Consistency	Provides redundancy for
Gitaly Cluster	Multiple	Single	Approximately 1 ms	Automatic	Strong	Data storage in Git
Geo	Multiple	Multiple	Up to one minute	Manual	Eventual	Entire GitLab instance

For more information, see:

• Gitaly architecture.
• Geo use cases and architecture.

Cluster or shard

Gitaly supports multiple models of scaling:

• Clustering using Gitaly Cluster, where each repository is stored on multiple Gitaly nodes in the cluster. Read requests are distributed between repository replicas and write requests are broadcast to repository replicas.
• Sharding using repository storage paths, where each repository is stored on the assigned Gitaly node. All requests are routed to this node.

Cluster	Shard

Generally, Gitaly Cluster can replace sharded configurations, at the expense of additional storage needed to store each repository on multiple Gitaly nodes. The benefit of using Gitaly Cluster over sharding is:

• Improved fault tolerance, because each Gitaly node has a copy of every repository.
• Improved resource utilization, reducing the need for over-provisioning for shard-specific peak loads, because read loads are distributed across replicas.
• Manual rebalancing for performance is not required, because read loads are distributed across replicas.
• Simpler management, because all Gitaly nodes are identical.

Under some workloads, CPU and memory requirements may require a large fleet of Gitaly nodes and it can be uneconomical to have one to one replication factor.

A hybrid approach can be used in these instances, where each shard is configured as a smaller cluster. Variable replication factor is planned to provide greater flexibility for extremely large GitLab instances.

Requirements for configuring a Gitaly Cluster

The minimum recommended configuration for a Gitaly Cluster requires:

• 1 load balancer
• 1 PostgreSQL server (PostgreSQL 11 or newer)
• 3 Praefect nodes
• 3 Gitaly nodes (1 primary, 2 secondary)

See the design document for implementation details.

Setup Instructions

If you installed GitLab using the Omnibus package (highly recommended), follow the steps below:

1. Preparation
2. Configuring the Praefect database
3. Configuring the Praefect proxy/router
4. Configuring each Gitaly node (once for each Gitaly node)
5. Configure the load balancer
6. Updating the GitLab server configuration
7. Configure Grafana

Preparation

Before beginning, you should already have a working GitLab instance. Learn how to install GitLab.

Provision a PostgreSQL server (PostgreSQL 11 or newer).

Prepare all your new nodes by installing GitLab.

• At least 1 Praefect node (minimal storage required)
• 3 Gitaly nodes (high CPU, high memory, fast storage)
• 1 GitLab server

You need the IP/host address for each node.

1. LOAD_BALANCER_SERVER_ADDRESS: the IP/host address of the load balancer
2. POSTGRESQL_SERVER_ADDRESS: the IP/host address of the PostgreSQL server
3. PRAEFECT_HOST: the IP/host address of the Praefect server
4. GITALY_HOST: the IP/host address of each Gitaly server
5. GITLAB_HOST: the IP/host address of the GitLab server

If you are using a cloud provider, you can look up the addresses for each server through your cloud provider's management console.

If you are using Google Cloud Platform, SoftLayer, or any other vendor that provides a virtual private cloud (VPC) you can use the private addresses for each cloud instance (corresponds to “internal address” for Google Cloud Platform) for PRAEFECT_HOST, GITALY_HOST, and GITLAB_HOST.

Secrets

The communication between components is secured with different secrets, which are described below. Before you begin, generate a unique secret for each, and make note of it. This makes it easy to replace these placeholder tokens with secure tokens as you complete the setup process.

1. GITLAB_SHELL_SECRET_TOKEN: this is used by Git hooks to make callback HTTP API requests to GitLab when accepting a Git push. This secret is shared with GitLab Shell for legacy reasons.
2. PRAEFECT_EXTERNAL_TOKEN: repositories hosted on your Praefect cluster can only be accessed by Gitaly clients that carry this token.
3. PRAEFECT_INTERNAL_TOKEN: this token is used for replication traffic inside your Praefect cluster. This is distinct from PRAEFECT_EXTERNAL_TOKEN because Gitaly clients must not be able to access internal nodes of the Praefect cluster directly; that could lead to data loss.
4. PRAEFECT_SQL_PASSWORD: this password is used by Praefect to connect to PostgreSQL.

We note in the instructions below where these secrets are required.

PostgreSQL

NOTE: Do not store the GitLab application database and the Praefect database on the same PostgreSQL server if using Geo. The replication state is internal to each instance of GitLab and should not be replicated.

These instructions help set up a single PostgreSQL database, which creates a single point of failure. For greater fault tolerance, the following options are available:

• For non-Geo installations, use one of the fault-tolerant PostgreSQL setups.
• For Geo instances, either:
  • Set up a separate PostgreSQL instance.
  • Use a cloud-managed PostgreSQL service. AWS Relational Database Service is recommended.

To complete this section you need:

• 1 Praefect node
• 1 PostgreSQL server (PostgreSQL 11 or newer)
  • An SQL user with permissions to create databases

During this section, we configure the PostgreSQL server, from the Praefect node, using psql which is installed by Omnibus GitLab.

1. SSH into the Praefect node and login as root:

   sudo -i

2. Connect to the PostgreSQL server with administrative access. This is likely the postgres user. The database template1 is used because it is created by default on all PostgreSQL servers.

   /opt/gitlab/embedded/bin/psql -U postgres -d template1 -h POSTGRESQL_SERVER_ADDRESS

   Create a new user praefect to be used by Praefect. Replace PRAEFECT_SQL_PASSWORD with the strong password you generated in the preparation step.

   CREATE ROLE praefect WITH LOGIN CREATEDB PASSWORD 'PRAEFECT_SQL_PASSWORD';

3. Reconnect to the PostgreSQL server, this time as the praefect user:

   /opt/gitlab/embedded/bin/psql -U praefect -d template1 -h POSTGRESQL_SERVER_ADDRESS

   Create a new database praefect_production. By creating the database while connected as the praefect user, we are confident they have access.

   CREATE DATABASE praefect_production WITH ENCODING=UTF8;

The database used by Praefect is now configured.

PgBouncer

To reduce PostgreSQL resource consumption, we recommend setting up and configuring PgBouncer in front of the PostgreSQL instance. To do this, set the corresponding IP or host address of the PgBouncer instance in /etc/gitlab/gitlab.rb by changing the following settings:

• praefect['database_host'], for the address.
• praefect['database_port'], for the port.

Because PgBouncer manages resources more efficiently, Praefect still requires a direct connection to the PostgreSQL database because it uses LISTEN functionality that is not supported by PgBouncer with pool_mode = transaction.

Therefore, praefect['database_host_no_proxy'] and praefect['database_port_no_proxy'] should be set to a direct connection and not a PgBouncer connection.

Save the changes to /etc/gitlab/gitlab.rb and reconfigure Praefect.

This documentation doesn't provide PgBouncer installation instructions, but you can:

• Find instructions on the official website.
• Use a Docker image.

In addition to the base PgBouncer configuration options, set the following values in your pgbouncer.ini file:

• The Praefect PostgreSQL database in the [databases] section:

  [databases]
  * = host=POSTGRESQL_SERVER_ADDRESS port=5432 auth_user=praefect

• pool_mode and ignore_startup_parameters in the [pgbouncer] section:

  [pgbouncer]
  pool_mode = transaction
  ignore_startup_parameters = extra_float_digits

The praefect user and its password should be included in the file (default is userlist.txt) used by PgBouncer if the auth_file configuration option is set.

NOTE: By default PgBouncer uses port 6432 to accept incoming connections. You can change it by setting the listen_port configuration option. We recommend setting it to the default port value (5432) used by PostgreSQL instances. Otherwise you should change the configuration parameter praefect['database_port'] for each Praefect instance to the correct value.

Praefect

Introduced in GitLab 13.4, Praefect nodes can no longer be designated as primary.

NOTE: If there are multiple Praefect nodes, complete these steps for each node.

To complete this section you need a configured PostgreSQL server, including:

• IP/host address (POSTGRESQL_SERVER_ADDRESS)
• Password (PRAEFECT_SQL_PASSWORD)

Praefect should be run on a dedicated node. Do not run Praefect on the application server, or a Gitaly node.

1. SSH into the Praefect node and login as root:

   sudo -i

2. Disable all other services by editing /etc/gitlab/gitlab.rb:

   # Disable all other services on the Praefect node
   postgresql['enable'] = false
   redis['enable'] = false
   nginx['enable'] = false
   alertmanager['enable'] = false
   prometheus['enable'] = false
   grafana['enable'] = false
   puma['enable'] = false
   sidekiq['enable'] = false
   gitlab_workhorse['enable'] = false
   gitaly['enable'] = false
   
   # Enable only the Praefect service
   praefect['enable'] = true
   
   # Prevent database connections during 'gitlab-ctl reconfigure'
   gitlab_rails['rake_cache_clear'] = false
   gitlab_rails['auto_migrate'] = false

3. Configure Praefect to listen on network interfaces by editing /etc/gitlab/gitlab.rb:

   praefect['listen_addr'] = '0.0.0.0:2305'
   
   # Enable Prometheus metrics access to Praefect. You must use firewalls
   # to restrict access to this address/port.
   praefect['prometheus_listen_addr'] = '0.0.0.0:9652'

4. Configure a strong auth_token for Praefect by editing /etc/gitlab/gitlab.rb. This is needed by clients outside the cluster (like GitLab Shell) to communicate with the Praefect cluster:

   praefect['auth_token'] = 'PRAEFECT_EXTERNAL_TOKEN'

5. Configure Praefect to connect to the PostgreSQL database by editing /etc/gitlab/gitlab.rb.

   You need to replace POSTGRESQL_SERVER_ADDRESS with the IP/host address of the database, and PRAEFECT_SQL_PASSWORD with the strong password set above.

   praefect['database_host'] = 'POSTGRESQL_SERVER_ADDRESS'
   praefect['database_port'] = 5432
   praefect['database_user'] = 'praefect'
   praefect['database_password'] = 'PRAEFECT_SQL_PASSWORD'
   praefect['database_dbname'] = 'praefect_production'
   praefect['database_host_no_proxy'] = 'POSTGRESQL_SERVER_ADDRESS'
   praefect['database_port_no_proxy'] = 5432

   If you want to use a TLS client certificate, the options below can be used:

   # Connect to PostgreSQL using a TLS client certificate
   # praefect['database_sslcert'] = '/path/to/client-cert'
   # praefect['database_sslkey'] = '/path/to/client-key'
   
   # Trust a custom certificate authority
   # praefect['database_sslrootcert'] = '/path/to/rootcert'

   By default, Praefect refuses to make an unencrypted connection to PostgreSQL. You can override this by uncommenting the following line:

   # praefect['database_sslmode'] = 'disable'

6. Configure the Praefect cluster to connect to each Gitaly node in the cluster by editing /etc/gitlab/gitlab.rb.

   The virtual storage's name must match the configured storage name in GitLab configuration. In a later step, we configure the storage name as default so we use default here as well. This cluster has three Gitaly nodes gitaly-1, gitaly-2, and gitaly-3, which are intended to be replicas of each other.

   WARNING: If you have data on an already existing storage called default, you should configure the virtual storage with another name and migrate the data to the Gitaly Cluster storage afterwards.

   Replace PRAEFECT_INTERNAL_TOKEN with a strong secret, which is used by Praefect when communicating with Gitaly nodes in the cluster. This token is distinct from the PRAEFECT_EXTERNAL_TOKEN.

   Replace GITALY_HOST with the IP/host address of the each Gitaly node.

   More Gitaly nodes can be added to the cluster to increase the number of replicas. More clusters can also be added for very large GitLab instances.

   # Name of storage hash must match storage name in git_data_dirs on GitLab
   # server ('praefect') and in git_data_dirs on Gitaly nodes ('gitaly-1')
   praefect['virtual_storages'] = {
     'default' => {
       'gitaly-1' => {
         'address' => 'tcp://GITALY_HOST:8075',
         'token'   => 'PRAEFECT_INTERNAL_TOKEN',
       },
       'gitaly-2' => {
         'address' => 'tcp://GITALY_HOST:8075',
         'token'   => 'PRAEFECT_INTERNAL_TOKEN'
       },
       'gitaly-3' => {
         'address' => 'tcp://GITALY_HOST:8075',
         'token'   => 'PRAEFECT_INTERNAL_TOKEN'
       }
     }
   }

7. Introduced in GitLab 13.1 and later, enable distribution of reads.

8. Save the changes to /etc/gitlab/gitlab.rb and reconfigure Praefect:

   gitlab-ctl reconfigure

9. To ensure that Praefect has updated its Prometheus listen address, restart Praefect:

   gitlab-ctl restart praefect

10. Verify that Praefect can reach PostgreSQL:

    sudo -u git /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml sql-ping

    If the check fails, make sure you have followed the steps correctly. If you edit /etc/gitlab/gitlab.rb, remember to run sudo gitlab-ctl reconfigure again before trying the sql-ping command.

The steps above must be completed for each Praefect node!

Enabling TLS support

Introduced in GitLab 13.2.

Praefect supports TLS encryption. To communicate with a Praefect instance that listens for secure connections, you must:

• Use a tls:// URL scheme in the gitaly_address of the corresponding storage entry in the GitLab configuration.
• Bring your own certificates because this isn't provided automatically. The certificate corresponding to each Praefect server must be installed on that Praefect server.

Additionally the certificate, or its certificate authority, must be installed on all Gitaly servers and on all Praefect clients that communicate with it following the procedure described in GitLab custom certificate configuration (and repeated below).

Note the following:

• The certificate must specify the address you use to access the Praefect server. If addressing the Praefect server by:

  • Hostname, you can either use the Common Name field for this, or add it as a Subject Alternative Name.
  • IP address, you must add it as a Subject Alternative Name to the certificate.

• You can configure Praefect servers with both an unencrypted listening address listen_addr and an encrypted listening address tls_listen_addr at the same time. This allows you to do a gradual transition from unencrypted to encrypted traffic, if necessary.

To configure Praefect with TLS:

For Omnibus GitLab

1. Create certificates for Praefect servers.

2. On the Praefect servers, create the /etc/gitlab/ssl directory and copy your key and certificate there:

   sudo mkdir -p /etc/gitlab/ssl
   sudo chmod 755 /etc/gitlab/ssl
   sudo cp key.pem cert.pem /etc/gitlab/ssl/
   sudo chmod 644 key.pem cert.pem

3. Edit /etc/gitlab/gitlab.rb and add:

   praefect['tls_listen_addr'] = "0.0.0.0:3305"
   praefect['certificate_path'] = "/etc/gitlab/ssl/cert.pem"
   praefect['key_path'] = "/etc/gitlab/ssl/key.pem"

4. Save the file and reconfigure.

5. On the Praefect clients (including each Gitaly server), copy the certificates, or their certificate authority, into /etc/gitlab/trusted-certs:

   sudo cp cert.pem /etc/gitlab/trusted-certs/

6. On the Praefect clients (except Gitaly servers), edit git_data_dirs in /etc/gitlab/gitlab.rb as follows:

   git_data_dirs({
     "default" => {
       "gitaly_address" => 'tls://LOAD_BALANCER_SERVER_ADDRESS:2305',
       "gitaly_token" => 'PRAEFECT_EXTERNAL_TOKEN'
     }
   })

7. Save the file and reconfigure GitLab.

For installations from source

1. Create certificates for Praefect servers.

2. On the Praefect servers, create the /etc/gitlab/ssl directory and copy your key and certificate there:

   sudo mkdir -p /etc/gitlab/ssl
   sudo chmod 755 /etc/gitlab/ssl
   sudo cp key.pem cert.pem /etc/gitlab/ssl/
   sudo chmod 644 key.pem cert.pem

3. On the Praefect clients (including each Gitaly server), copy the certificates, or their certificate authority, into the system trusted certificates:

   sudo cp cert.pem /usr/local/share/ca-certificates/praefect.crt
   sudo update-ca-certificates

4. On the Praefect clients (except Gitaly servers), edit storages in /home/git/gitlab/config/gitlab.yml as follows:

   gitlab:
     repositories:
       storages:
         default:
           gitaly_address: tls://LOAD_BALANCER_SERVER_ADDRESS:3305
           path: /some/local/path

   NOTE: /some/local/path should be set to a local folder that exists, however no data is stored in this folder. This requirement is scheduled to be removed when this issue is resolved.

5. Save the file and restart GitLab.

6. Copy all Praefect server certificates, or their certificate authority, to the system trusted certificates on each Gitaly server so the Praefect server trusts the certificate when called by Gitaly servers:

   sudo cp cert.pem /usr/local/share/ca-certificates/praefect.crt
   sudo update-ca-certificates

7. Edit /home/git/praefect/config.toml and add:

   tls_listen_addr = '0.0.0.0:3305'
   
   [tls]
   certificate_path = '/etc/gitlab/ssl/cert.pem'
   key_path = '/etc/gitlab/ssl/key.pem'

8. Save the file and restart GitLab.

Gitaly

NOTE: Complete these steps for each Gitaly node.

To complete this section you need:

• Configured Praefect node
• 3 (or more) servers, with GitLab installed, to be configured as Gitaly nodes. These should be dedicated nodes, do not run other services on these nodes.

Every Gitaly server assigned to the Praefect cluster needs to be configured. The configuration is the same as a normal standalone Gitaly server, except:

• The storage names are exposed to Praefect, not GitLab
• The secret token is shared with Praefect, not GitLab

The configuration of all Gitaly nodes in the Praefect cluster can be identical, because we rely on Praefect to route operations correctly.

Particular attention should be shown to:

• The gitaly['auth_token'] configured in this section must match the token value under praefect['virtual_storages'] on the Praefect node. This was set in the previous section. This document uses the placeholder PRAEFECT_INTERNAL_TOKEN throughout.
• The storage names in git_data_dirs configured in this section must match the storage names under praefect['virtual_storages'] on the Praefect node. This was set in the previous section. This document uses gitaly-1, gitaly-2, and gitaly-3 as Gitaly storage names.

For more information on Gitaly server configuration, see our Gitaly documentation.

1. SSH into the Gitaly node and login as root:

   sudo -i

2. Disable all other services by editing /etc/gitlab/gitlab.rb:

   # Disable all other services on the Praefect node
   postgresql['enable'] = false
   redis['enable'] = false
   nginx['enable'] = false
   grafana['enable'] = false
   puma['enable'] = false
   sidekiq['enable'] = false
   gitlab_workhorse['enable'] = false
   prometheus_monitoring['enable'] = false
   
   # Enable only the Gitaly service
   gitaly['enable'] = true
   
   # Enable Prometheus if needed
   prometheus['enable'] = true
   
   # Prevent database connections during 'gitlab-ctl reconfigure'
   gitlab_rails['rake_cache_clear'] = false
   gitlab_rails['auto_migrate'] = false

3. Configure Gitaly to listen on network interfaces by editing /etc/gitlab/gitlab.rb:

   # Make Gitaly accept connections on all network interfaces.
   # Use firewalls to restrict access to this address/port.
   gitaly['listen_addr'] = '0.0.0.0:8075'
   
   # Enable Prometheus metrics access to Gitaly. You must use firewalls
   # to restrict access to this address/port.
   gitaly['prometheus_listen_addr'] = '0.0.0.0:9236'

4. Configure a strong auth_token for Gitaly by editing /etc/gitlab/gitlab.rb. This is needed by clients to communicate with this Gitaly nodes. Typically, this token is the same for all Gitaly nodes.

   gitaly['auth_token'] = 'PRAEFECT_INTERNAL_TOKEN'

5. Configure the GitLab Shell secret_token, and internal_api_url which are needed for git push operations.

   If you have already configured Gitaly on its own server

   gitlab_shell['secret_token'] = 'GITLAB_SHELL_SECRET_TOKEN'
   
   # Configure the gitlab-shell API callback URL. Without this, `git push` will
   # fail. This can be your front door GitLab URL or an internal load balancer.
   # Examples: 'https://gitlab.example.com', 'http://1.2.3.4'
   gitlab_rails['internal_api_url'] = 'http://GITLAB_HOST'

6. Configure the storage location for Git data by setting git_data_dirs in /etc/gitlab/gitlab.rb. Each Gitaly node should have a unique storage name (such as gitaly-1).

   Instead of configuring git_data_dirs uniquely for each Gitaly node, it is often easier to have include the configuration for all Gitaly nodes on every Gitaly node. This is supported because the Praefect virtual_storages configuration maps each storage name (such as gitaly-1) to a specific node, and requests are routed accordingly. This means every Gitaly node in your fleet can share the same configuration.

   # You can include the data dirs for all nodes in the same config, because
   # Praefect will only route requests according to the addresses provided in the
   # prior step.
   git_data_dirs({
     "gitaly-1" => {
       "path" => "/var/opt/gitlab/git-data"
     },
     "gitaly-2" => {
       "path" => "/var/opt/gitlab/git-data"
     },
     "gitaly-3" => {
       "path" => "/var/opt/gitlab/git-data"
     }
   })

7. Save the changes to /etc/gitlab/gitlab.rb and reconfigure Gitaly:

   gitlab-ctl reconfigure

8. To ensure that Gitaly has updated its Prometheus listen address, restart Gitaly:

   gitlab-ctl restart gitaly

The steps above must be completed for each Gitaly node!

After all Gitaly nodes are configured, you can run the Praefect connection checker to verify Praefect can connect to all Gitaly servers in the Praefect configuration.

1. SSH into each Praefect node and run the Praefect connection checker:

   sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dial-nodes

Load Balancer

In a highly available Gitaly configuration, a load balancer is needed to route internal traffic from the GitLab application to the Praefect nodes. The specifics on which load balancer to use or the exact configuration is beyond the scope of the GitLab documentation.

NOTE: The load balancer must be configured to accept traffic from the Gitaly nodes in addition to the GitLab nodes. Some requests handled by gitaly-ruby sidecar processes call into the main Gitaly process. gitaly-ruby uses the Gitaly address set in the GitLab server's git_data_dirs setting to make this connection.

We hope that if you’re managing HA systems like GitLab, you have a load balancer of choice already. Some examples include HAProxy (open-source), Google Internal Load Balancer, AWS Elastic Load Balancer, F5 Big-IP LTM, and Citrix Net Scaler. This documentation outlines what ports and protocols you need configure.

LB Port	Backend Port	Protocol
2305	2305	TCP

GitLab

To complete this section you need:

• Configured Praefect node
• Configured Gitaly nodes

The Praefect cluster needs to be exposed as a storage location to the GitLab application. This is done by updating the git_data_dirs.

Particular attention should be shown to:

• the storage name added to git_data_dirs in this section must match the storage name under praefect['virtual_storages'] on the Praefect node(s). This was set in the Praefect section of this guide. This document uses default as the Praefect storage name.

1. SSH into the GitLab node and login as root:

   sudo -i

2. Configure the external_url so that files could be served by GitLab by proper endpoint access by editing /etc/gitlab/gitlab.rb:

   You need to replace GITLAB_SERVER_URL with the real external facing URL on which current GitLab instance is serving:

   external_url 'GITLAB_SERVER_URL'

3. Disable the default Gitaly service running on the GitLab host. It isn't needed because GitLab connects to the configured cluster.

   WARNING: If you have existing data stored on the default Gitaly storage, you should migrate the data your Gitaly Cluster storage first.

   gitaly['enable'] = false

4. Add the Praefect cluster as a storage location by editing /etc/gitlab/gitlab.rb.

   You need to replace:

   • LOAD_BALANCER_SERVER_ADDRESS with the IP address or hostname of the load balancer.
   • PRAEFECT_EXTERNAL_TOKEN with the real secret

   If you are using TLS, the gitaly_address should begin with tls://.

   git_data_dirs({
     "default" => {
       "gitaly_address" => "tcp://LOAD_BALANCER_SERVER_ADDRESS:2305",
       "gitaly_token" => 'PRAEFECT_EXTERNAL_TOKEN'
     }
   })

5. Configure the gitlab_shell['secret_token'] so that callbacks from Gitaly nodes during a git push are properly authenticated by editing /etc/gitlab/gitlab.rb:

   You need to replace GITLAB_SHELL_SECRET_TOKEN with the real secret.

   gitlab_shell['secret_token'] = 'GITLAB_SHELL_SECRET_TOKEN'

6. Add Prometheus monitoring settings by editing /etc/gitlab/gitlab.rb. If Prometheus is enabled on a different node, make edits on that node instead.

   You need to replace:

   • PRAEFECT_HOST with the IP address or hostname of the Praefect node
   • GITALY_HOST with the IP address or hostname of each Gitaly node

   prometheus['scrape_configs'] = [
     {
       'job_name' => 'praefect',
       'static_configs' => [
         'targets' => [
           'PRAEFECT_HOST:9652', # praefect-1
           'PRAEFECT_HOST:9652', # praefect-2
           'PRAEFECT_HOST:9652', # praefect-3
         ]
       ]
     },
     {
       'job_name' => 'praefect-gitaly',
       'static_configs' => [
         'targets' => [
           'GITALY_HOST:9236', # gitaly-1
           'GITALY_HOST:9236', # gitaly-2
           'GITALY_HOST:9236', # gitaly-3
         ]
       ]
     }
   ]

7. Save the changes to /etc/gitlab/gitlab.rb and reconfigure GitLab:

   gitlab-ctl reconfigure

8. Verify on each Gitaly node the Git Hooks can reach GitLab. On each Gitaly node run:

   /opt/gitlab/embedded/bin/gitaly-hooks check /var/opt/gitlab/gitaly/config.toml

9. Verify that GitLab can reach Praefect:

   gitlab-rake gitlab:gitaly:check

10. Check in Admin Area > Settings > Repository > Repository storage that the Praefect storage is configured to store new repositories. Following this guide, the default storage should have weight 100 to store all new repositories.

11. Verify everything is working by creating a new project. Check the "Initialize repository with a README" box so that there is content in the repository that viewed. If the project is created, and you can see the README file, it works!

Grafana

Grafana is included with GitLab, and can be used to monitor your Praefect cluster. See Grafana Dashboard Service for detailed documentation.

To get started quickly:

1. SSH into the GitLab node (or whichever node has Grafana enabled) and login as root:

   sudo -i

2. Enable the Grafana login form by editing /etc/gitlab/gitlab.rb.

   grafana['disable_login_form'] = false

3. Save the changes to /etc/gitlab/gitlab.rb and reconfigure GitLab:

   gitlab-ctl reconfigure

4. Set the Grafana admin password. This command prompts you to enter a new password:

   gitlab-ctl set-grafana-password

5. In your web browser, open /-/grafana (e.g. https://gitlab.example.com/-/grafana) on your GitLab server.

   Login using the password you set, and the username admin.

6. Go to Explore and query gitlab_build_info to verify that you are getting metrics from all your machines.

Congratulations! You've configured an observable highly available Praefect cluster.

Distributed reads

• Introduced in GitLab 13.1 in beta with feature flag gitaly_distributed_reads set to disabled.
• Made generally available and enabled by default in GitLab 13.3.
• Disabled by default in GitLab 13.5.

Praefect supports distribution of read operations across Gitaly nodes that are configured for the virtual node.

The feature is disabled by default. To enable distributed reads, the gitaly_distributed_reads feature flag must be enabled in a Ruby console:

Feature.enable(:gitaly_distributed_reads)

If enabled, all RPCs marked with ACCESSOR option like GetBlob are redirected to an up to date and healthy Gitaly node.

Up to date in this context means that:

• There is no replication operations scheduled for this node.
• The last replication operation is in completed state.

If there is no such nodes, or any other error occurs during node selection, the primary node is chosen to serve the request.

To track distribution of read operations, you can use the gitaly_praefect_read_distribution Prometheus counter metric. It has two labels:

• virtual_storage.
• storage.

They reflect configuration defined for this instance of Praefect.

Strong consistency

• Introduced in GitLab 13.1 in alpha, disabled by default.
• Entered beta in GitLab 13.2, disabled by default.
• From GitLab 13.3, disabled unless primary-wins reference transactions strategy is disabled.
• From GitLab 13.4, enabled by default.

Praefect guarantees eventual consistency by replicating all writes to secondary nodes after the write to the primary Gitaly node has happened.

Praefect can instead provide strong consistency by creating a transaction and writing changes to all Gitaly nodes at once. Strong consistency is currently in alpha and not enabled by default. If enabled, transactions are only available for a subset of RPCs. For more information, see the strong consistency epic.

To enable strong consistency:

• In GitLab 13.5, you must use Git v2.28.0 or higher on Gitaly nodes to enable strong consistency.
• In GitLab 13.4 and later, the strong consistency voting strategy has been improved. Instead of requiring all nodes to agree, only the primary and half of the secondaries need to agree. This strategy is enabled by default. To disable it and continue using the primary-wins strategy, enable the :gitaly_reference_transactions_primary_wins feature flag.
• In GitLab 13.3, reference transactions are enabled by default with a primary-wins strategy. This strategy causes all transactions to succeed for the primary and thus does not ensure strong consistency. To enable strong consistency, disable the :gitaly_reference_transactions_primary_wins feature flag.
• In GitLab 13.2, enable the :gitaly_reference_transactions feature flag.
• In GitLab 13.1, enable the :gitaly_reference_transactions and :gitaly_hooks_rpc feature flags.

Changing feature flags requires access to the Rails console. In the Rails console, enable or disable the flags as required. For example:

Feature.enable(:gitaly_reference_transactions)
Feature.disable(:gitaly_reference_transactions_primary_wins)

To monitor strong consistency, you can use the following Prometheus metrics:

• gitaly_praefect_transactions_total: Number of transactions created and voted on.
• gitaly_praefect_subtransactions_per_transaction_total: Number of times nodes cast a vote for a single transaction. This can happen multiple times if multiple references are getting updated in a single transaction.
• gitaly_praefect_voters_per_transaction_total: Number of Gitaly nodes taking part in a transaction.
• gitaly_praefect_transactions_delay_seconds: Server-side delay introduced by waiting for the transaction to be committed.
• gitaly_hook_transaction_voting_delay_seconds: Client-side delay introduced by waiting for the transaction to be committed.

Replication factor

Replication factor is the number of copies Praefect maintains of a given repository. A higher replication factor offers better redundancy and distribution of read workload, but also results in a higher storage cost. By default, Praefect replicates repositories to every storage in a virtual storage.

Variable replication factor

WARNING: The feature is not production ready yet. After you set a replication factor, you can't unset it without manually modifying database state. Variable replication factor requires you to enable repository-specific primaries by configuring the per_repository primary election strategy. The election strategy is not production ready yet.

Praefect supports configuring a replication factor on a per-repository basis, by assigning specific storage nodes to host a repository.

In an upcoming release, we intend to support configuring a default replication factor for a virtual storage. The default replication factor is applied to every newly-created repository.

Prafect does not store the actual replication factor, but assigns enough storages to host the repository so the desired replication factor is met. If a storage node is later removed from the virtual storage, the replication factor of repositories assigned to the storage is decreased accordingly.

The only way to configure a repository's replication factor is the set-replication-factor sub-command. set-replication-factor automatically assigns or unassigns random storage nodes as necessary to reach the desired replication factor. The repository's primary node is always assigned first and is never unassigned.

sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml set-replication-factor -virtual-storage <virtual-storage> -repository <relative-path> -replication-factor <replication-factor>

• -virtual-storage is the virtual storage the repository is located in.
• -repository is the repository's relative path in the storage.
• -replication-factor is the desired replication factor of the repository. The minimum value is 1, as the primary needs a copy of the repository. The maximum replication factor is the number of storages in the virtual storage.

On success, the assigned host storages are printed. For example:

$ sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml set-replication-factor -virtual-storage default -repository @hashed/3f/db/3fdba35f04dc8c462986c992bcf875546257113072a909c162f7e470e581e278.git -replication-factor 2

current assignments: gitaly-1, gitaly-2

Automatic failover and leader election

Praefect regularly checks the health of each backend Gitaly node. This information can be used to automatically failover to a new primary node if the current primary node is found to be unhealthy.

• PostgreSQL (recommended): Enabled by default, and equivalent to: praefect['failover_election_strategy'] = sql. This configuration option allows multiple Praefect nodes to coordinate via the PostgreSQL database to elect a primary Gitaly node. This configuration causes Praefect nodes to elect a new primary, monitor its health, and elect a new primary if the current one has not been reachable in 10 seconds by a majority of the Praefect nodes.
• Memory: Enabled by setting praefect['failover_election_strategy'] = 'local' in /etc/gitlab/gitlab.rb on the Praefect node. If a sufficient number of health checks fail for the current primary backend Gitaly node, and new primary will be elected. Do not use with multiple Praefect nodes! Using with multiple Praefect nodes is likely to result in a split brain.

We are likely to implement support for Consul, and a cloud native, strategy in the future.

Primary Node Failure

Gitaly Cluster recovers from a failing primary Gitaly node by promoting a healthy secondary as the new primary.

To minimize data loss, Gitaly Cluster:

• Switches repositories that are outdated on the new primary to read-only mode.
• Elects the secondary with the least unreplicated writes from the primary to be the new primary. Because there can still be some unreplicated writes, data loss can occur.

Read-only mode

• Introduced in GitLab 13.0 as generally available.
• Between GitLab 13.0 and GitLab 13.2, read-only mode applied to the whole virtual storage and occurred whenever failover occurred.
• In GitLab 13.3 and later, read-only mode applies on a per-repository basis and only occurs if a new primary is out of date.

When Gitaly Cluster switches to a new primary, repositories enter read-only mode if they are out of date. This can happen after failing over to an outdated secondary. Read-only mode eases data recovery efforts by preventing writes that may conflict with the unreplicated writes on other nodes.

To enable writes again, an administrator can:

1. Check for data loss.
2. Attempt to recover missing data.
3. Either enable writes in the virtual storage or accept data loss if necessary, depending on the version of GitLab.

Check for data loss

The Praefect dataloss sub-command identifies replicas that are likely to be outdated. This is useful for identifying potential data loss after a failover. The following parameters are available:

• -virtual-storage that specifies which virtual storage to check. The default behavior is to display outdated replicas of read-only repositories as they might require administrator action.
• In GitLab 13.3 and later, -partially-replicated that specifies whether to display a list of outdated replicas of writable repositories.

NOTE: dataloss is still in beta and the output format is subject to change.

To check for repositories with outdated primaries, run:

sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss [-virtual-storage <virtual-storage>]

Every configured virtual storage is checked if none is specified:

sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss

Repositories which have assigned storage nodes that contain an outdated copy of the repository are listed in the output. A number of useful information is printed for each repository:

• A repository's relative path to the storage directory identifies each repository and groups the related information.
• The repository's current status is printed in parentheses next to the disk path. If the repository's primary is outdated, the repository is in read-only mode and can't accept writes. Otherwise, the mode is writable.
• The primary field lists the repository's current primary. If the repository has no primary, the field shows No Primary.
• The In-Sync Storages lists replicas which have replicated the latest successful write and all writes preceding it.
• The Outdated Storages lists replicas which contain an outdated copy of the repository. Replicas which have no copy of the repository but should contain it are also listed here. The maximum number of changes the replica is missing is listed next to replica. It's important to notice that the outdated replicas may be fully up to date or contain later changes but Praefect can't guarantee it.

Whether a replica is assigned to host the repository is listed with each replica's status. assigned host is printed next to replicas which are assigned to store the repository. The text is omitted if the replica contains a copy of the repository but is not assigned to store the repository. Such replicas won't be kept in-sync by Praefect but may act as replication sources to bring assigned replicas up to date.

Example output:

Virtual storage: default
  Outdated repositories:
    @hashed/3f/db/3fdba35f04dc8c462986c992bcf875546257113072a909c162f7e470e581e278.git (read-only):
      Primary: gitaly-1
      In-Sync Storages:
        gitaly-2, assigned host
      Outdated Storages:
        gitaly-1 is behind by 3 changes or less, assigned host
        gitaly-3 is behind by 3 changes or less

A confirmation is printed out when every repository is writable. For example:

Virtual storage: default
  All repositories are writable!

Outdated replicas of writable repositories

Introduced in GitLab 13.3.

To also list information of repositories whose primary is up to date but one or more assigned replicas are outdated, use the -partially-replicated flag.

A repository is writable if the primary has the latest changes. Secondaries might be temporarily outdated while they are waiting to replicate the latest changes.

sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss [-virtual-storage <virtual-storage>] [-partially-replicated]

Example output:

Virtual storage: default
  Outdated repositories:
    @hashed/3f/db/3fdba35f04dc8c462986c992bcf875546257113072a909c162f7e470e581e278.git (writable):
      Primary: gitaly-1
      In-Sync Storages:
        gitaly-1, assigned host
      Outdated Storages:
        gitaly-2 is behind by 3 changes or less, assigned host
        gitaly-3 is behind by 3 changes or less

With the -partially-replicated flag set, a confirmation is printed out if every assigned replica is fully up to date.

For example:

Virtual storage: default
  All repositories are up to date!

Check repository checksums

To check a project's repository checksums across on all Gitaly nodes, run the replicas Rake task on the main GitLab node.

Enable writes or accept data loss

Praefect provides the following subcommands to re-enable writes:

• In GitLab 13.2 and earlier, enable-writes to re-enable virtual storage for writes after data recovery attempts.

  sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml enable-writes -virtual-storage <virtual-storage>

• In GitLab 13.3 and later, accept-dataloss to accept data loss and re-enable writes for repositories after data recovery attempts have failed. Accepting data loss causes current version of the repository on the authoritative storage to be considered latest. Other storages are brought up to date with the authoritative storage by scheduling replication jobs.

  sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml accept-dataloss -virtual-storage <virtual-storage> -repository <relative-path> -authoritative-storage <storage-name>

WARNING: accept-dataloss causes permanent data loss by overwriting other versions of the repository. Data recovery efforts must be performed before using it.

Data recovery

If a Gitaly node fails replication jobs for any reason, it ends up hosting outdated versions of the affected repositories. Praefect provides tools for:

• Automatic reconciliation, for GitLab 13.4 and later.
• Manual reconciliation, for:
  • GitLab 13.3 and earlier.
  • Repositories upgraded to GitLab 13.4 and later without entries in the repositories table. A migration tool is planned.

These tools reconcile the outdated repositories to bring them fully up to date again.

Automatic reconciliation

Introduced in GitLab 13.4.

Praefect automatically reconciles repositories that are not up to date. By default, this is done every five minutes. For each outdated repository on a healthy Gitaly node, the Praefect picks a random, fully up to date replica of the repository on another healthy Gitaly node to replicate from. A replication job is scheduled only if there are no other replication jobs pending for the target repository.

The reconciliation frequency can be changed via the configuration. The value can be any valid Go duration value. Values below 0 disable the feature.

Examples:

praefect['reconciliation_scheduling_interval'] = '5m' # the default value

praefect['reconciliation_scheduling_interval'] = '30s' # reconcile every 30 seconds

praefect['reconciliation_scheduling_interval'] = '0' # disable the feature

Manual reconciliation

The Praefect reconcile sub-command allows for the manual reconciliation between two Gitaly nodes. The command replicates every repository on a later version on the reference storage to the target storage.

sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml reconcile -virtual <virtual-storage> -reference <up-to-date-storage> -target <outdated-storage> -f

• Replace the placeholder <virtual-storage> with the virtual storage containing the Gitaly node storage to be checked.
• Replace the placeholder <up-to-date-storage> with the Gitaly storage name containing up to date repositories.
• Replace the placeholder <outdated-storage> with the Gitaly storage name containing outdated repositories.

Migrate existing repositories to Gitaly Cluster

If your GitLab instance already has repositories on single Gitaly nodes, these aren't migrated to Gitaly Cluster automatically.

Repositories may be moved from one storage location using the Project repository storage moves API:

NOTE: The Project repository storage moves API cannot move all repository types.

To move repositories to Gitaly Cluster:

1. Schedule repository storage moves for all projects on a storage shard using the API. For example:

   curl --request POST --header "Private-Token: <your_access_token>" --header "Content-Type: application/json" \
   --data '{"source_storage_name":"gitaly","destination_storage_name":"praefect"}' "https://gitlab.example.com/api/v4/project_repository_storage_moves"

2. Query the most recent repository moves using the API. The query indicates either:

   • The moves have completed successfully. The state field is finished.
   • The moves are in progress. Re-query the repository move until it completes successfully.
   • The moves have failed. Most failures are temporary and are solved by rescheduling the move.

3. Once the moves are complete, query projects using the API to confirm that all projects have moved. No projects should be returned with repository_storage field set to the old storage.

Debugging Praefect

If you receive an error, check /var/log/gitlab/gitlab-rails/production.log.

Here are common errors and potential causes:

• 500 response code
  • ActionView::Template::Error (7:permission denied)
    • praefect['auth_token'] and gitlab_rails['gitaly_token'] do not match on the GitLab server.
  • Unable to save project. Error: 7:permission denied
    • Secret token in praefect['storage_nodes'] on GitLab server does not match the value in gitaly['auth_token'] on one or more Gitaly servers.
• 503 response code
  • GRPC::Unavailable (14:failed to connect to all addresses)
    • GitLab was unable to reach Praefect.
  • GRPC::Unavailable (14:all SubCons are in TransientFailure...)
    • Praefect cannot reach one or more of its child Gitaly nodes. Try running the Praefect connection checker to diagnose.