# Praefect: High Availability NOTE: **Note:** Praefect is a [beta](https://about.gitlab.com/handbook/product/#alpha-beta-ga) component that allows Gitaly to be run in a highly available configuration. While unexpected data loss is not likely, Praefect is not yet ready for production environments. [Gitaly](index.md) is the service that provides storage for Git repositories in the GitLab application. Praefect is an optional reverse proxy for Gitaly to manage multiple Gitaly nodes for high availability. High availability is currently implemented through **asynchronous replication**. If a Gitaly node becomes unavailable, Praefect will automatically route traffic to a warm Gitaly replica. - **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](https://gitlab.com/groups/gitlab-org/-/epics/1189) is planned 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](https://gitlab.com/gitlab-org/gitaly/-/issues/2608) 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. Follow the [HA Gitaly epic](https://gitlab.com/groups/gitlab-org/-/epics/1489) for improvements including [horizontally distributing reads](https://gitlab.com/groups/gitlab-org/-/epics/2013). ## Requirements for configuring Gitaly for High Availability A minimum highly available configuration requires: - 1 highly available load balancer - 1 highly available PostgreSQL server (PostgreSQL 9.6 or newer) - 2 Praefect nodes - 3 Gitaly nodes (1 primary, 2 secondary) ![Architecture diagram](img/praefect_architecture_v12_10.png) See the [design document](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/design_ha.md) for implementation details. ## Setup Instructions If you [installed](https://about.gitlab.com/install/) GitLab using the Omnibus package (highly recommended), follow the steps below: 1. [Preparation](#preparation) 1. [Configuring the Praefect database](#postgresql) 1. [Configuring the Praefect proxy/router](#praefect) 1. [Configuring each Gitaly node](#gitaly) (once for each Gitaly node) 1. [Configure the load balancer](#load-balancer) 1. [Updating the GitLab server configuration](#gitlab) 1. [Configure Grafana](#grafana) ### Preparation Before beginning, you should already have a working GitLab instance. [Learn how to install GitLab](https://about.gitlab.com/install/). Provision a PostgreSQL server (PostgreSQL 9.6 or newer). Configuration through the Omnibus GitLab distribution is not yet supported. Follow this [issue](https://gitlab.com/gitlab-org/gitaly/issues/2476) for updates. Prepare all your new nodes by [installing GitLab](https://about.gitlab.com/install/). - 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. 1. `LOAD_BALANCER_SERVER_ADDRESS`: the IP/hots address of the load balancer 1. `POSTGRESQL_SERVER_ADDRESS`: the IP/host address of the PostgreSQL server 1. `PRAEFECT_HOST`: the IP/host address of the Praefect server 1. `GITALY_HOST`: the IP/host address of each Gitaly server 1. `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. 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. 1. `PRAEFECT_EXTERNAL_TOKEN`: repositories hosted on your Praefect cluster can only be accessed by Gitaly clients that carry this token. 1. `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. 1. `PRAEFECT_SQL_PASSWORD`: this password is used by Praefect to connect to PostgreSQL. 1. `GRAFANA_PASSWORD`: this password is used to access the `admin` account in the Grafana dashboards. We will note in the instructions below where these secrets are required. ### PostgreSQL NOTE: **Note:** do not store the GitLab application database and the Praefect database on the same PostgreSQL server if using [Geo](../geo/replication/index.md). 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 9.6 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. 1. SSH into the **Praefect** node and login as root: ```shell sudo -i ``` 1. 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. ```shell /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. ```sql CREATE ROLE praefect WITH LOGIN CREATEDB PASSWORD 'PRAEFECT_SQL_PASSWORD'; ``` 1. Reconnect to the PostgreSQL server, this time as the `praefect` user: ```shell /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. ```sql 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](#postgresql), 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: ```shell sudo -i ``` 1. Disable all other services by editing `/etc/gitlab/gitlab.rb`: ```ruby # Disable all other services on the Praefect node postgresql['enable'] = false redis['enable'] = false nginx['enable'] = false prometheus['enable'] = false grafana['enable'] = false unicorn['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 ``` 1. Configure **Praefect** to listen on network interfaces by editing `/etc/gitlab/gitlab.rb`: You will need to replace: - `PRAEFECT_HOST` with the IP address or hostname of the Praefect node ```ruby praefect['listen_addr'] = 'PRAEFECT_HOST:2305' # Enable Prometheus metrics access to Praefect. You must use firewalls # to restrict access to this address/port. praefect['prometheus_listen_addr'] = 'PRAEFECT_HOST:9652' ``` 1. 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 : ```ruby praefect['auth_token'] = 'PRAEFECT_EXTERNAL_TOKEN' ``` 1. 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. ```ruby 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: ```ruby # 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: ```ruby # praefect['database_sslmode'] = 'disable' ``` 1. Configure the **Praefect** cluster to connect to each Gitaly node in the cluster by editing `/etc/gitlab/gitlab.rb`. In the example below we have configured one virtual storage (or shard) named `storage-1`. This cluster has three Gitaly nodes `gitaly-1`, `gitaly-2`, and `gitaly-3`, which will be replicas of each other. 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: **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](https://gitlab.com/gitlab-org/gitaly/-/issues/2634). ```ruby # 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'] = { 'storage-1' => { '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' } } } ``` 1. Enable the database replication queue: ```ruby praefect['postgres_queue_enabled'] = true ``` In the next release, database replication queue will be enabled by default. See [issue #2615](https://gitlab.com/gitlab-org/gitaly/-/issues/2615). 1. Enable automatic failover by editing `/etc/gitlab/gitlab.rb`: ```ruby praefect['failover_enabled'] = true praefect['failover_election_strategy'] = 'sql' ``` When automatic failover is enabled, Praefect checks the health of internal Gitaly nodes. If the primary has a certain amount of health checks fail, it will promote one of the secondaries to be primary, and demote the primary to be a secondary. NOTE: **Note:** Database leader election will be [enabled by default in the future](https://gitlab.com/gitlab-org/gitaly/-/issues/2682). Caution, **automatic failover** favors availability over consistency and will cause data loss if changes have not been replicated to the newly elected primary. In the next release, leader election will [prefer to promote up to date replicas](https://gitlab.com/gitlab-org/gitaly/-/issues/2642), and it will be an option to favor consistency by marking [out-of-date repositories read-only](https://gitlab.com/gitlab-org/gitaly/-/issues/2630). 1. Save the changes to `/etc/gitlab/gitlab.rb` and [reconfigure Praefect](../restart_gitlab.md#omnibus-gitlab-reconfigure): ```shell gitlab-ctl reconfigure ``` 1. Verify that Praefect can reach PostgreSQL: ```shell 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. ### Gitaly NOTE: **Note:** Complete these steps for **each** Gitaly node. To complete this section you will need: - [Configured Praefect node](#praefect) - 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](index.md), 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](#praefect). 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](#praefect). 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](index.md#3-gitaly-server-configuration). 1. SSH into the **Gitaly** node and login as root: ```shell sudo -i ``` 1. Disable all other services by editing `/etc/gitlab/gitlab.rb`: ```ruby # Disable all other services on the Praefect node postgresql['enable'] = false redis['enable'] = false nginx['enable'] = false prometheus['enable'] = false grafana['enable'] = false unicorn['enable'] = false sidekiq['enable'] = false gitlab_workhorse['enable'] = false prometheus_monitoring['enable'] = false # Enable only the Praefect service gitaly['enable'] = true # Prevent database connections during 'gitlab-ctl reconfigure' gitlab_rails['rake_cache_clear'] = false gitlab_rails['auto_migrate'] = false ``` 1. Configure **Gitaly** to listen on network interfaces by editing `/etc/gitlab/gitlab.rb`: You will need to replace: - `GITALY_HOST` with the IP address or hostname of the Gitaly node ```ruby # Make Gitaly accept connections on all network interfaces. # Use firewalls to restrict access to this address/port. gitaly['listen_addr'] = 'GITALY_HOST:8075' # Enable Prometheus metrics access to Gitaly. You must use firewalls # to restrict access to this address/port. gitaly['prometheus_listen_addr'] = 'GITALY_HOST:9236' ``` 1. 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. ```ruby gitaly['auth_token'] = 'PRAEFECT_INTERNAL_TOKEN' ``` 1. 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](index.md) ```ruby 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' ``` 1. 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 (eg `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 (eg `gitaly-1`) to a specific node, and requests are routed accordingly. This means every Gitaly node in your fleet can share the same configuration. ```ruby # 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" } }) ``` 1. Save the changes to `/etc/gitlab/gitlab.rb` and [reconfigure Gitaly](../restart_gitlab.md#omnibus-gitlab-reconfigure): ```shell gitlab-ctl reconfigure ``` 1. To ensure that Gitaly [has updated its Prometheus listen address](https://gitlab.com/gitlab-org/gitaly/-/issues/2521), [restart Gitaly](../restart_gitlab.md#omnibus-gitlab-restart): ```shell 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. 1. SSH into the **Praefect** node and run the Praefect connection checker: ```shell 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](https://www.haproxy.org/) (open-source), [Google Internal Load Balancer](https://cloud.google.com/load-balancing/docs/internal/), [AWS Elastic Load Balancer](https://aws.amazon.com/elasticloadbalancing/), 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](#praefect) - [Configured Gitaly nodes](#gitaly) 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](#praefect) section of this guide. This document uses `storage-1` as the Praefect storage name. 1. SSH into the **GitLab** node and login as root: ```shell sudo -i ``` 1. 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. - `GITLAB_HOST` with the IP address or hostname of the GitLab server - `PRAEFECT_EXTERNAL_TOKEN` with the real secret ```ruby git_data_dirs({ "default" => { "gitaly_address" => "tcp://GITLAB_HOST:8075" }, "storage-1" => { "gitaly_address" => "tcp://LOAD_BALANCER_SERVER_ADDRESS:2305", "gitaly_token" => 'PRAEFECT_EXTERNAL_TOKEN' } }) ``` 1. Allow Gitaly to listen on a tcp port by editing `/etc/gitlab/gitlab.rb` ```ruby gitaly['listen_addr'] = 'GITLAB_HOST:8075' ``` 1. 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. ```ruby gitlab_shell['secret_token'] = 'GITLAB_SHELL_SECRET_TOKEN' ``` 1. 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: ```ruby external_url 'GITLAB_SERVER_URL' ``` 1. 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 ```ruby prometheus['scrape_configs'] = [ { 'job_name' => 'praefect', 'static_configs' => [ 'targets' => [ 'PRAEFECT_HOST:9652' # praefect ] ] }, { 'job_name' => 'praefect-gitaly', 'static_configs' => [ 'targets' => [ 'GITALY_HOST:9236', # gitaly-1 'GITALY_HOST:9236', # gitaly-2 'GITALY_HOST:9236', # gitaly-3 ] ] } ] ``` 1. Save the changes to `/etc/gitlab/gitlab.rb` and [reconfigure GitLab](../restart_gitlab.md#omnibus-gitlab-reconfigure): ```shell gitlab-ctl reconfigure ``` 1. Verify each `gitlab-shell` on each Gitaly instance can reach GitLab. On each Gitaly instance run: ```shell /opt/gitlab/embedded/service/gitlab-shell/bin/check -config /opt/gitlab/embedded/service/gitlab-shell/config.yml ``` 1. Verify that GitLab can reach Praefect: ```shell gitlab-rake gitlab:gitaly:check ``` 1. Update the **Repository storage** settings from **Admin Area > Settings > Repository > Repository storage** to make the newly configured Praefect cluster the storage location for new Git repositories. - Deselect the **default** storage location - Select the **praefect** storage location ![Update repository storage](img/praefect_storage_v12_10.png) 1. Verify everything is still 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](https://docs.gitlab.com/omnibus/settings/grafana.html) for detailed documentation. To get started quickly: 1. SSH into the **GitLab** node and login as root: ```shell sudo -i ``` 1. Enable the Grafana login form by editing `/etc/gitlab/gitlab.rb`. ```ruby grafana['disable_login_form'] = false ``` 1. Save the changes to `/etc/gitlab/gitlab.rb` and [reconfigure GitLab](../restart_gitlab.md#omnibus-gitlab-reconfigure): ```shell gitlab-ctl reconfigure ``` 1. Set the Grafana admin password. This command will prompt you to enter a new password: ```shell gitlab-ctl set-grafana-password ``` 1. 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`. 1. 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. ## 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 setting `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. - **Memory:** Enabled by setting `praefect['failover_enabled'] = true` 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. NOTE: **Note:**: Praefect does not yet account for replication lag on the secondaries during the election process, so data loss can occur during a failover. Follow issue [#2642](https://gitlab.com/gitlab-org/gitaly/-/issues/2642) for updates. It is likely that we will implement support for Consul, and a cloud native strategy in the future. ## Identifying Impact of a Primary Node Failure When a primary Gitaly node fails, there is a chance of dataloss. Dataloss can occur if there were outstanding replication jobs the secondaries did not manage to process before the failure. The Praefect `dataloss` subcommand helps identify these cases by counting the number of dead replication jobs for each repository within a given timeframe. ```shell sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss -from -to ``` If the timeframe is not specified, dead replication jobs from the last six hours are counted: ```shell sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss Failed replication jobs between [2020-01-02 00:00:00 +0000 UTC, 2020-01-02 06:00:00 +0000 UTC): example/repository-1: 1 jobs example/repository-2: 4 jobs example/repository-3: 2 jobs ``` To specify a timeframe in UTC, run: ```shell sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml dataloss -from 2020-01-02T00:00:00+00:00 -to 2020-01-02T00:02:00+00:00 ``` ### Checking repository checksums To check a project's checksums across all nodes, the Praefect replicas Rake task can be used: ```shell sudo gitlab-rake "gitlab:praefect:replicas[project_id]" ``` ## 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` subcommand allows for the manual reconciliation between a backend node and the current primary. Run the following command on the Praefect server after all placeholders (`` and ``) have been replaced: ```shell sudo /opt/gitlab/embedded/bin/praefect -config /var/opt/gitlab/praefect/config.toml reconcile -virtual -target ``` - Replace the placeholder `` with the virtual storage containing the backend node storage to be checked. - Replace the placeholder `` 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](../../api/projects.html#edit-project): ```shell curl --request PUT --header "PRIVATE-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.