gitlab-org--gitlab-foss/doc/administration/postgresql/database_load_balancing.md

---
stage: Data Stores
group: Database
info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
---

# Database Load Balancing **(FREE SELF)**

> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/1283) in [GitLab Premium](https://about.gitlab.com/pricing/) 9.0.
> - [Moved](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/60894) from GitLab Premium to GitLab Free in 14.0.
> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/334494) for Sidekiq in GitLab 14.1.

With Database Load Balancing, read-only queries can be distributed across
multiple PostgreSQL nodes to increase performance.

This functionality is provided natively in GitLab Rails and Sidekiq where
they can be configured to balance their database read queries in a round-robin approach,
without any external dependencies:

```plantuml
@startuml
card "**Internal Load Balancer**" as ilb #9370DB
skinparam linetype ortho

together {
  collections "**GitLab Rails** x3" as gitlab #32CD32
  collections "**Sidekiq** x4" as sidekiq #ff8dd1
}

collections "**Consul** x3" as consul #e76a9b

card "Database" as database {
  collections "**PGBouncer x3**\n//Consul//" as pgbouncer #4EA7FF

  card "**PostgreSQL** //Primary//\n//Patroni//\n//PgBouncer//\n//Consul//" as postgres_primary #4EA7FF
  collections "**PostgreSQL** //Secondary// **x2**\n//Patroni//\n//PgBouncer//\n//Consul//" as postgres_secondary #4EA7FF

  pgbouncer -[#4EA7FF]-> postgres_primary
  postgres_primary .[#4EA7FF]r-> postgres_secondary
}

gitlab -[#32CD32]-> ilb
gitlab -[hidden]-> pgbouncer
gitlab .[#32CD32,norank]-> postgres_primary
gitlab .[#32CD32,norank]-> postgres_secondary

sidekiq -[#ff8dd1]-> ilb
sidekiq -[hidden]-> pgbouncer
sidekiq .[#ff8dd1,norank]-> postgres_primary
sidekiq .[#ff8dd1,norank]-> postgres_secondary

ilb -[#9370DB]-> pgbouncer

consul -[#e76a9b]r-> pgbouncer
consul .[#e76a9b,norank]r-> postgres_primary
consul .[#e76a9b,norank]r-> postgres_secondary
@enduml
```

## Requirements to enable Database Load Balancing

To enable Database Load Balancing, make sure that:

- The HA PostgreSQL setup has one or more secondary nodes replicating the primary.
- Each PostgreSQL node is connected with the same credentials and on the same port.

For Omnibus GitLab, you also need PgBouncer configured on each PostgreSQL node to pool
all load-balanced connections when [configuring a multi-node setup](replication_and_failover.md).

## Configuring Database Load Balancing

Database Load Balancing can be configured in one of two ways:

- (Recommended) [Hosts](#hosts): a list of PostgreSQL hosts.
- [Service Discovery](#service-discovery): a DNS record that returns a list of PostgreSQL hosts.

### Hosts

To configure a list of hosts, add the `gitlab_rails['db_load_balancing']` setting into the
`gitlab.rb` file in the GitLab Rails / Sidekiq nodes for each environment you want to balance.

For example, on an environment that has PostgreSQL running on the hosts `host1.example.com`,
`host2.example.com` and `host3.example.com` and reachable on the same port configured with
`gitlab_rails['db_port']`:

1. On each GitLab Rails / Sidekiq node, edit `/etc/gitlab/gitlab.rb` and add the following line:

  ```ruby
  gitlab_rails['db_load_balancing'] = { 'hosts' => ['host1.example.com', 'host2.example.com', `host3.example.com`] }
  ```

1. Save the file and [reconfigure GitLab](../restart_gitlab.md#omnibus-gitlab-reconfigure).

### Service Discovery

> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/5883) in GitLab 11.0.

Service discovery allows GitLab to automatically retrieve a list of PostgreSQL
hosts to use. It periodically
checks a DNS `A` record, using the IPs returned by this record as the addresses
for the secondaries. For service discovery to work, all you need is a DNS server
and an `A` record containing the IP addresses of your secondaries.

When using Omnibus GitLab the provided [Consul](../consul.md) service works as
a DNS server and returns PostgreSQL addresses via the `postgresql-ha.service.consul`
record. For example:

1. On each GitLab Rails / Sidekiq node, edit `/etc/gitlab/gitlab.rb` and add the following:

  ```ruby
  gitlab_rails['db_load_balancing'] = { 'discover' => {
      'nameserver' => 'localhost'
      'record' => 'postgresql-ha.service.consul'
      'record_type' => 'A'
      'port' => '8600'
      'interval' => '60'
      'disconnect_timeout' => '120'
    }
  }
  ```

1. Save the file and [reconfigure GitLab](../restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to take effect.

| Option               | Description                                                                                       | Default   |
|----------------------|---------------------------------------------------------------------------------------------------|-----------|
| `nameserver`         | The nameserver to use for looking up the DNS record.                                              | localhost |
| `record`             | The record to look up. This option is required for service discovery to work.                     |           |
| `record_type`        | Optional record type to look up, this can be either `A` or `SRV` (GitLab 12.3 and later)          | `A`       |
| `port`               | The port of the nameserver.                                                                       | 8600      |
| `interval`           | The minimum time in seconds between checking the DNS record.                                      | 60        |
| `disconnect_timeout` | The time in seconds after which an old connection is closed, after the list of hosts was updated. | 120       |
| `use_tcp`            | Lookup DNS resources using TCP instead of UDP                                                     | false     |

If `record_type` is set to `SRV`, then GitLab continues to use round-robin algorithm
and ignores the `weight` and `priority` in the record. Since `SRV` records usually
return hostnames instead of IPs, GitLab needs to look for the IPs of returned hostnames
in the additional section of the `SRV` response. If no IP is found for a hostname, GitLab
needs to query the configured `nameserver` for `ANY` record for each such hostname looking for `A` or `AAAA`
records, eventually dropping this hostname from rotation if it can't resolve its IP.

The `interval` value specifies the _minimum_ time between checks. If the `A`
record has a TTL greater than this value, then service discovery honors said
TTL. For example, if the TTL of the `A` record is 90 seconds, then service
discovery waits at least 90 seconds before checking the `A` record again.

When the list of hosts is updated, it might take a while for the old connections
to be terminated. The `disconnect_timeout` setting can be used to enforce an
upper limit on the time it takes to terminate all old database connections.

### Handling stale reads

> [Moved](https://gitlab.com/gitlab-org/gitlab/-/issues/327902) from GitLab Premium to GitLab Free in 14.0.

To prevent reading from an outdated secondary the load balancer checks if it
is in sync with the primary. If the data is recent enough, the
secondary is used, otherwise it is ignored. To reduce the overhead of
these checks we only perform them at certain intervals.

There are three configuration options that influence this behavior:

| Option                       | Description                                                                                                    | Default    |
|------------------------------|----------------------------------------------------------------------------------------------------------------|------------|
| `max_replication_difference` | The amount of data (in bytes) a secondary is allowed to lag behind when it hasn't replicated data for a while. | 8 MB       |
| `max_replication_lag_time`   | The maximum number of seconds a secondary is allowed to lag behind before we stop using it.                    | 60 seconds |
| `replica_check_interval`     | The minimum number of seconds we have to wait before checking the status of a secondary.                       | 60 seconds |

The defaults should be sufficient for most users.

To configure these options with a hosts list, use the following example:

```ruby
gitlab_rails['db_load_balancing'] = {
  'hosts' => ['host1.example.com', 'host2.example.com', `host3.example.com`]
  'max_replication_difference' => 16777216 # 16 MB
  'max_replication_lag_time' => 30
  'replica_check_interval' => 30
}
```

## Logging

The load balancer logs various events in
[`database_load_balancing.log`](../logs.md#database_load_balancinglog), such as

- When a host is marked as offline
- When a host comes back online
- When all secondaries are offline
- When a read is retried on a different host due to a query conflict

The log is structured with each entry a JSON object containing at least:

- An `event` field useful for filtering.
- A human-readable `message` field.
- Some event-specific metadata. For example, `db_host`
- Contextual information that is always logged. For example, `severity` and `time`.

For example:

```json
{"severity":"INFO","time":"2019-09-02T12:12:01.728Z","correlation_id":"abcdefg","event":"host_online","message":"Host came back online","db_host":"111.222.333.444","db_port":null,"tag":"rails.database_load_balancing","environment":"production","hostname":"web-example-1","fqdn":"gitlab.example.com","path":null,"params":null}
```

## Implementation Details

### Balancing queries

Read-only `SELECT` queries balance among all the given hosts.
Everything else (including transactions) executes on the primary.
Queries such as `SELECT ... FOR UPDATE` are also executed on the primary.

### Prepared statements

Prepared statements don't work well with load balancing and are disabled
automatically when load balancing is enabled. This shouldn't impact
response timings.

### Primary sticking

After a write has been performed, GitLab sticks to using the primary for a
certain period of time, scoped to the user that performed the write. GitLab
reverts back to using secondaries when they have either caught up, or after 30
seconds.

### Failover handling

In the event of a failover or an unresponsive database, the load balancer
tries to use the next available host. If no secondaries are available the
operation is performed on the primary instead.

If a connection error occurs while writing data, the
operation retries up to 3 times using an exponential back-off.

When using load balancing, you should be able to safely restart a database server
without it immediately leading to errors being presented to the users.