Requirements for configuring a Gitaly Cluster
Setup Instructions
Enabling TLS support
Distributed reads
Strong consistency
Automatic failover and leader election
Primary Node Failure
Backend Node Recovery
Migrating existing repositories to Praefect
Debugging Praefect

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 improve this to “no loss”.
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.

The current version supports:

Eventual 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 the newly promoted primary read only if possible data loss is detected.

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

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:

Preparation
Configuring the Praefect database
Configuring the Praefect proxy/router
Configuring each Gitaly node (once for each Gitaly node)
Configure the load balancer
Updating the GitLab server configuration
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). Configuration through the Omnibus GitLab distribution is not yet supported. Follow this issue for updates.

Prepare all your new nodes by installing GitLab.

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

You will need the IP/host address for each node.

LOAD_BALANCER_SERVER_ADDRESS: the IP/host address of the load balancer
POSTGRESQL_SERVER_ADDRESS: the IP/host address of the PostgreSQL server
PRAEFECT_HOST: the IP/host address of the Praefect server
GITALY_HOST: the IP/host address of each Gitaly server
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 will make it easy to replace these placeholder tokens with secure tokens as you complete the setup process.

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.
PRAEFECT_EXTERNAL_TOKEN: repositories hosted on your Praefect cluster can only be accessed by Gitaly clients that carry this token.
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.
PRAEFECT_SQL_PASSWORD: this password is used by Praefect to connect to PostgreSQL.

We will 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.

To complete this section you will need:

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

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

SSH into the Praefect node and login as root:
```
sudo -i
```
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 which will 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';
```
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.

Praefect

To complete this section you will need:

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.

SSH into the Praefect node and login as root:
```
sudo -i
```

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
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

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'

Configure a strong auth_token for Praefect by editing /etc/gitlab/gitlab.rb. This will be needed by clients outside the cluster (like GitLab Shell) to communicate with the Praefect cluster :
```
praefect['auth_token'] = 'PRAEFECT_EXTERNAL_TOKEN'
```

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

You will 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'

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

# Connect to PostreSQL 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 will refuse to make an unencrypted connection to PostgreSQL. You can override this by uncommenting the following line:

# praefect['database_sslmode'] = 'disable'

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 will be replicas of each other.
CAUTION: 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 Praefect storage afterwards.

Replace PRAEFECT_INTERNAL_TOKEN with a strong secret, which will be 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.
Note: The gitaly-1 node is currently denoted the primary. This can be used to manually fail from one node to another. This will be removed in the future.
```
# 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',
      'primary' => true
    },
    'gitaly-2' => {
      'address' => 'tcp://GITALY_HOST:8075',
      'token'   => 'PRAEFECT_INTERNAL_TOKEN'
    },
    'gitaly-3' => {
      'address' => 'tcp://GITALY_HOST:8075',
      'token'   => 'PRAEFECT_INTERNAL_TOKEN'
    }
  }
}
```
Introduced in GitLab 13.1 and later, enable distribution of reads.
Save the changes to /etc/gitlab/gitlab.rb and reconfigure Praefect:
```
gitlab-ctl reconfigure
```
To ensure that Praefect has updated its Prometheus listen address, restart Gitaly:
```
gitlab-ctl restart praefect
```
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

Create certificates for Praefect servers.

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

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"

Save the file and reconfigure GitLab.
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/
```

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://praefect1.internal:3305' },
  'storage1' => { 'gitaly_address' => 'tls://praefect2.internal:3305' },
})

Save the file and reconfigure GitLab.

For installations from source

Create certificates for Praefect servers.

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

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
```

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://praefect1.internal:3305
        path: /some/dummy/path
      storage1:
        gitaly_address: tls://praefect2.internal:3305
        path: /some/dummy/path

Note: /some/dummy/path should be set to a local folder that exists, however no data will be stored in this folder. This will no longer be necessary after this issue is resolved.

Save the file and restart GitLab.
Copy all Praefect server certificates, or their certificate authority, to the system trusted certificates on each Gitaly server so the Praefect server will trust the certificate when called by Gitaly servers:
```
sudo cp cert.pem /usr/local/share/ca-certificates/praefect.crt
sudo update-ca-certificates
```

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'

Save the file and restart GitLab.

Gitaly

Note: Complete these steps for each Gitaly node.

To complete this section you will 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.

SSH into the Gitaly node and login as root:
```
sudo -i
```

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

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'

Configure a strong auth_token for Gitaly by editing /etc/gitlab/gitlab.rb. This will be needed by clients to communicate with this Gitaly nodes. Typically, this token will be the same for all Gitaly nodes.
```
gitaly['auth_token'] = 'PRAEFECT_INTERNAL_TOKEN'
```

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://example.gitlab.com', 'http://1.2.3.4'
gitlab_rails['internal_api_url'] = 'http://GITLAB_HOST'

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"
  }
})
```
Save the changes to /etc/gitlab/gitlab.rb and reconfigure Gitaly:
```
gitlab-ctl reconfigure
```
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 config.

SSH into the 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.

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 will outline what ports and protocols you need configure.

LB Port	Backend Port	Protocol
2305	2305	TCP

GitLab

To complete this section you will 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. This was set in the Praefect section of this guide. This document uses storage-1 as the Praefect storage name.

SSH into the GitLab node and login as root:
```
sudo -i
```
Configure the external_url so that files could be served by GitLab by proper endpoint access by editing /etc/gitlab/gitlab.rb:
You will need to replace GITLAB_SERVER_URL with the real external facing URL on which current GitLab instance is serving:
```
external_url 'GITLAB_SERVER_URL'
```
Disable the default Gitaly service running on the GitLab host. It won’t be needed as GitLab will connect to the configured cluster.
CAUTION If you have existing data stored on the default Gitaly storage, you should migrate the data your Praefect storage first.
```
gitaly['enable'] = false
```
Add the Praefect cluster as a storage location by editing /etc/gitlab/gitlab.rb.
You will need to replace:
- LOAD_BALANCER_SERVER_ADDRESS with the IP address or hostname of the load balancer.
- PRAEFECT_EXTERNAL_TOKEN with the real secret
```
git_data_dirs({
  "default" => {
    "gitaly_address" => "tcp://LOAD_BALANCER_SERVER_ADDRESS:2305",
    "gitaly_token" => 'PRAEFECT_EXTERNAL_TOKEN'
  }
})
```
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 will need to replace GITLAB_SHELL_SECRET_TOKEN with the real secret.
```
gitlab_shell['secret_token'] = 'GITLAB_SHELL_SECRET_TOKEN'
```

Add Prometheus monitoring settings by editing /etc/gitlab/gitlab.rb.

You will 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
      ]
    ]
  }
]

Save the changes to /etc/gitlab/gitlab.rb and reconfigure GitLab:
```
gitlab-ctl reconfigure
```

Verify each gitlab-shell on each Gitaly instance can reach GitLab. On each Gitaly instance run:

/opt/gitlab/embedded/service/gitlab-shell/bin/check -config /opt/gitlab/embedded/service/gitlab-shell/config.yml

Verify that GitLab can reach Praefect:
```
gitlab-rake gitlab:gitaly:check
```
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.
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:

SSH into the GitLab node and login as root:
```
sudo -i
```
Enable the Grafana login form by editing /etc/gitlab/gitlab.rb.
```
grafana['disable_login_form'] = false
```
Save the changes to /etc/gitlab/gitlab.rb and reconfigure GitLab:
```
gitlab-ctl reconfigure
```
Set the Grafana admin password. This command will prompt you to enter a new password:
```
gitlab-ctl set-grafana-password
```
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.
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.

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

To allow for performance testing, distributed reads are currently in beta and 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 will be 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.

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. For more information, see the strong consistency epic.

To enable strong consistency:

In GitLab 13.2 and later, enable the :gitaly_reference_transactions feature flag.
In GitLab 13.1, enable the :gitaly_reference_transactions and :gitaly_hooks_rpc feature flags.

Enabling 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)

To monitor strong consistency, use the gitaly_praefect_transactions_total and gitaly_praefect_transactions_delay_seconds Prometheus counter metrics.

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 will allow multiple Praefect nodes to coordinate via the PostgreSQL database to elect a primary Gitaly node. This configuration will cause 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.
Manual: Automatic failover is disabled. The primary node can be reconfigured in /etc/gitlab/gitlab.rb on the Praefect node. Modify the praefect['virtual_storages'] field by moving the primary = true to promote a different Gitaly node to primary. In the steps above, gitaly-1 was set to the primary. Requires praefect['failover_enabled'] = false in the configuration.
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.

It is likely that we will implement support for Consul, and a cloud native strategy in the future.

Primary Node Failure

Praefect recovers from a failing primary Gitaly node by promoting a healthy secondary as the new primary. To minimize data loss, Praefect elects the secondary with the least unreplicated writes from the primary. There can still be some unreplicated writes, leading to data loss.

Praefect switches a virtual storage in to read-only mode after a failover event. This eases data recovery efforts by preventing new, possibly conflicting writes to the newly elected primary. This allows the administrator to attempt recovering the lost data before allowing new writes.

If you prefer write availability over consistency, this behavior can be turned off by setting praefect['failover_read_only_after_failover'] = false in /etc/gitlab/gitlab.rb and reconfiguring Praefect.

Checking for data loss

The Praefect dataloss sub-command helps identify lost writes by checking for uncompleted replication jobs. This is useful for identifying possible data loss cases after a failover. This command must be executed on a Praefect node.

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

If the virtual storage is not specified, every configured virtual storage is checked for data loss.

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

Virtual storage: default
  Current read-only primary: gitaly-2
  Previous write-enabled primary: gitaly-1
    Nodes with data loss from failing over from gitaly-1:
      @hashed/2c/62/2c624232cdd221771294dfbb310aca000a0df6ac8b66b696d90ef06fdefb64a3.git: gitaly-0
      @hashed/4b/22/4b227777d4dd1fc61c6f884f48641d02b4d121d3fd328cb08b5531fcacdabf8a.git: gitaly-0, gitaly-2

Currently dataloss only considers a repository up to date if it has been directly replicated to from the previous write-enabled primary. While reconciling from an up to date secondary can recover the data, this is not visible in the data loss report. This is due for improvement via Gitaly#2866.

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

Checking repository checksums

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

Recovering lost writes

The Praefect reconcile sub-command can be used to recover lost writes from the previous primary once it is back online. This is only possible when the virtual storage is still in read-only mode.

sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml reconcile -virtual <virtual-storage> -reference <previous-primary> -target <current-primary> -f

Refer to Backend Node Recovery section for more details on the reconcile sub-command.

Enabling Writes

Any data recovery attempts should have been made before enabling writes to eliminate any chance of conflicting writes. Virtual storage can be re-enabled for writes by using the Praefect enable-writes sub-command.

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

Backend Node Recovery

When a Praefect backend node fails and is no longer able to replicate changes, the backend node will start to drift from the primary. If that node eventually recovers, it will need to be reconciled with the current primary. The primary node is considered the single source of truth for the state of a shard. The Praefect reconcile sub-command allows for the manual reconciliation between a backend node and the current primary.

Run the following command on the Praefect server after all placeholders (<virtual-storage> and <target-storage>) have been replaced:

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

Replace the placeholder <virtual-storage> with the virtual storage containing the backend node storage to be checked.
Replace the placeholder <target-storage> with the backend storage name.

The command will return a list of repositories that were found to be inconsistent against the current primary. Each of these inconsistencies will also be logged with an accompanying replication job ID.

Migrating existing repositories to Praefect

If your GitLab instance already has repositories, these won’t be migrated automatically.

Repositories may be moved from one storage location using the Repository API:

curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" --data "repository_storage=praefect" https://example.gitlab.com/api/v4/projects/123

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.

Gitaly Cluster

Gitaly Cluster

Requirements for configuring a Gitaly Cluster

Setup Instructions

Preparation

Secrets

PostgreSQL

Praefect

Enabling TLS support

Gitaly

Load Balancer

GitLab

Grafana

Distributed reads

Strong consistency

Automatic failover and leader election

Primary Node Failure

Checking for data loss

Checking repository checksums

Recovering lost writes

Enabling Writes

Backend Node Recovery

Migrating existing repositories to Praefect

Debugging Praefect

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