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Merge branch 'gy-update-db-load-balancing-docs' into 'master' 

Move and update Database Load Balancing docs

See merge request gitlab-org/gitlab!74602
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doc/administration/database_load_balancing.md
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---
stage: Enablement
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
redirect_to: 'postgresql/database_load_balancing.md'
remove_date: '2022-02-19'
---
# Database Load Balancing **(FREE SELF)**
This file was moved to [another location](postgresql/database_load_balancing.md).
> - [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.
Distribute read-only queries among multiple database servers.
## Overview
Database load balancing improves the distribution of database workloads across
multiple computing resources. Load balancing aims to optimize resource use,
maximize throughput, minimize response time, and avoid overload of any single
resource. Using multiple components with load balancing instead of a single
component may increase reliability and availability through redundancy.
[_Wikipedia article_](https://en.wikipedia.org/wiki/Load_balancing_(computing))
When database load balancing is enabled in GitLab, the load is balanced using
a simple round-robin algorithm, without any external dependencies such as Redis.
In the following image, you can see the load is balanced rather evenly among
all the secondaries (`db4`, `db5`, `db6`). Because `SELECT` queries are not
sent to the primary (unless necessary), the primary (`db3`) hardly has any load.
![DB load balancing graph](img/db_load_balancing_postgres_stats.png)
## Requirements
For load balancing to work, you need at least PostgreSQL 11 or newer,
[**MySQL is not supported**](../install/requirements.md#database). You also need to make sure that you have
at least 1 secondary in [hot standby](https://www.postgresql.org/docs/11/hot-standby.html) mode.
Load balancing also requires that the configured hosts **always** point to the
primary, even after a database failover. Furthermore, the additional hosts to
balance load among must **always** point to secondary databases. This means that
you should put a load balancer in front of every database, and have GitLab connect
to those load balancers.
For example, say you have a primary (`db1.gitlab.com`) and two secondaries,
`db2.gitlab.com` and `db3.gitlab.com`. For this setup, you need to have 3
load balancers, one for every host. For example:
- `primary.gitlab.com` forwards to `db1.gitlab.com`
- `secondary1.gitlab.com` forwards to `db2.gitlab.com`
- `secondary2.gitlab.com` forwards to `db3.gitlab.com`
Now let's say that a failover happens and db2 becomes the new primary. This
means forwarding should now happen as follows:
- `primary.gitlab.com` forwards to `db2.gitlab.com`
- `secondary1.gitlab.com` forwards to `db1.gitlab.com`
- `secondary2.gitlab.com` forwards to `db3.gitlab.com`
GitLab does not take care of this for you, so you need to do so yourself.
Finally, load balancing requires that GitLab can connect to all hosts using the
same credentials and port as configured in the
[Enabling load balancing](#enabling-load-balancing) section. Using
different ports or credentials for different hosts is not supported.
## Use cases
- For GitLab instances with thousands of users and high traffic, you can use
database load balancing to reduce the load on the primary database and
increase responsiveness, thus resulting in faster page load inside GitLab.
## Enabling load balancing
For the environment in which you want to use load balancing, you'll need to add
the following. This balances the load between `host1.example.com` and
`host2.example.com`.
**In Omnibus installations:**
1. Edit `/etc/gitlab/gitlab.rb` and add the following line:
```ruby
gitlab_rails['db_load_balancing'] = { 'hosts' => ['host1.example.com', 'host2.example.com'] }
```
1. Save the file and [reconfigure GitLab](restart_gitlab.md#omnibus-gitlab-reconfigure) for the changes to take effect.
---
**In installations from source:**
1. Edit `/home/git/gitlab/config/database.yml` and add or amend the following lines:
```yaml
production:
username: gitlab
database: gitlab
encoding: unicode
load_balancing:
hosts:
- host1.example.com
- host2.example.com
```
1. Save the file and [restart GitLab](restart_gitlab.md#installations-from-source) for the changes to take effect.
### Load balancing for Sidekiq
> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/issues/334494) in GitLab 14.1, load balancing for Sidekick is enabled by default.
Sidekiq jobs mostly write to the primary database, but there are read-only jobs that can benefit
from the use of Sidekiq load balancing.
These jobs can use load balancing and database replicas to read the application state.
This allows to offload the primary database.
For Sidekiq, we can define
[data consistency](../development/sidekiq_style_guide.md#job-data-consistency-strategies)
requirements for a specific job.
## Service Discovery **(PREMIUM SELF)**
> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/5883) in GitLab 11.0.
Service discovery allows GitLab to automatically retrieve a list of secondary
databases to use, instead of having to manually specify these in the
`database.yml` configuration file. Service discovery works by periodically
checking 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.
To use service discovery you need to change your `database.yml` configuration
file so it looks like the following:
```yaml
production:
username: gitlab
database: gitlab
encoding: unicode
load_balancing:
discover:
nameserver: localhost
record: secondary.postgresql.service.consul
record_type: A
port: 8600
interval: 60
disconnect_timeout: 120
```
Here, the `discover:` section specifies the configuration details to use for
service discovery.
### Configuration
The following options can be set:
| 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.
Some nameservers (like [Consul](https://www.consul.io/docs/discovery/dns#udp-based-dns-queries)) can return a truncated list of hosts when
queried over UDP. To overcome this issue, you can use TCP for querying by setting
`use_tcp` to `true`.
## Balancing queries
Read-only `SELECT` queries balance among all the secondary 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 should have no impact on
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 is retried 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.
## 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}
```
## Handling Stale Reads **(PREMIUM SELF)**
> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/3526) in GitLab 10.3.
To prevent reading from an outdated secondary the load balancer checks if it
is in sync with the primary. If the data is determined to be recent enough the
secondary is used, otherwise it is ignored. To reduce the overhead of
these checks we only perform these checks 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. Should you want to change them
you can specify them in `config/database.yml` like so:
```yaml
production:
username: gitlab
database: gitlab
encoding: unicode
load_balancing:
hosts:
- host1.example.com
- host2.example.com
max_replication_difference: 16777216 # 16 MB
max_replication_lag_time: 30
replica_check_interval: 30
```
<!-- This redirect file can be deleted after <2022-02-19>. -->
<!-- Before deletion, see: https://docs.gitlab.com/ee/development/documentation/#move-or-rename-a-page -->
doc/administration/index.md
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......@@ -31,8 +31,6 @@ Learn how to install, configure, update, and maintain your GitLab instance.
### Installing GitLab
- [Install](../install/index.md): Requirements, directory structures, and installation methods.
- [Database load balancing](database_load_balancing.md): Distribute database queries among multiple database servers.
- [Omnibus support for log forwarding](https://docs.gitlab.com/omnibus/settings/logs.html#udp-log-shipping-gitlab-enterprise-edition-only).
- [Reference architectures](reference_architectures/index.md): Add additional resources to support more users.
- [Installing GitLab on Amazon Web Services (AWS)](../install/aws/index.md): Set up GitLab on Amazon AWS.
- [Geo](geo/index.md): Replicate your GitLab instance to other geographic locations as a read-only fully operational version.
......@@ -79,6 +77,8 @@ Learn how to install, configure, update, and maintain your GitLab instance.
- [Enabling and disabling features flags](feature_flags.md): how to enable and
disable GitLab features deployed behind feature flags.
- [Application settings cache expiry interval](application_settings_cache.md)
- [Database Load Balancing](postgresql/database_load_balancing.md): Distribute database queries among multiple database servers.
- [Omnibus support for log forwarding](https://docs.gitlab.com/omnibus/settings/logs.html#udp-log-shipping-gitlab-enterprise-edition-only).
#### Customizing GitLab appearance
......
doc/administration/logs.md
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......@@ -830,7 +830,7 @@ are generated in a location based on your installation method:
> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/15442) in GitLab 12.3.
Contains details of GitLab [Database Load Balancing](database_load_balancing.md).
Contains details of GitLab [Database Load Balancing](postgresql/database_load_balancing.md).
Depending on your installation method, this file is located at:
- Omnibus GitLab: `/var/log/gitlab/gitlab-rails/database_load_balancing.log`
......
doc/administration/monitoring/performance/performance_bar.md
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......@@ -26,11 +26,10 @@ From left to right, the performance bar displays:
details for each query:
- **In a transaction**: shows up below the query if it was executed in
the context of a transaction
- **Role**: shows up when [database load
balancing](../../database_load_balancing.md) is enabled. It shows
which server role was used for the query. "Primary" means that the query
was sent to the read/write primary server. "Replica" means it was sent
to a read-only replica.
- **Role**: shows up when [Database Load Balancing](../../postgresql/database_load_balancing.md)
is enabled. It shows which server role was used for the query.
"Primary" means that the query was sent to the read/write primary server.
"Replica" means it was sent to a read-only replica.
- **Config name**: shows up only when the
`GITLAB_MULTIPLE_DATABASE_METRICS` environment variable is set. This is
used to distinguish between different databases configured for different
......
doc/administration/postgresql/database_load_balancing.md 0 → 100644
View file @ f290a6b5
---
stage: Enablement
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 Postgres setup has one or more secondary nodes replicating the primary.
- Each Postgres 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 **(PREMIUM SELF)**
> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/3526) in GitLab 10.3.
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.
doc/development/licensed_feature_availability.md
View file @ f290a6b5
......@@ -29,7 +29,7 @@ project.feature_available?(:feature_symbol)
## Restricting global features (instance)
However, for features such as [Geo](../administration/geo/index.md) and
[Load balancing](../administration/database_load_balancing.md), which cannot be restricted
[Database Load Balancing](../administration/postgresql/database_load_balancing.md), which cannot be restricted
to only a subset of projects or namespaces, the check is made directly in
the instance license.
......
doc/development/merge_request_performance_guidelines.md
View file @ f290a6b5
......@@ -160,10 +160,10 @@ query. This in turn makes it much harder for this code to overload a database.
## Use read replicas when possible
In a DB cluster we have many read replicas and one primary. A classic use of scaling the DB is to have read-only actions be performed by the replicas. We use [load balancing](../administration/database_load_balancing.md) to distribute this load. This allows for the replicas to grow as the pressure on the DB grows.
In a DB cluster we have many read replicas and one primary. A classic use of scaling the DB is to have read-only actions be performed by the replicas. We use [load balancing](../administration/postgresql/database_load_balancing.md) to distribute this load. This allows for the replicas to grow as the pressure on the DB grows.
By default, queries use read-only replicas, but due to
[primary sticking](../administration/database_load_balancing.md#primary-sticking), GitLab uses the
[primary sticking](../administration/postgresql/database_load_balancing.md#primary-sticking), GitLab uses the
primary for some time and reverts to secondaries after they have either caught up or after 30 seconds.
Doing this can lead to a considerable amount of unnecessary load on the primary.
To prevent switching to the primary [merge request 56849](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/56849) introduced the
......@@ -187,7 +187,7 @@ Internally, our database load balancer classifies the queries based on their mai
- Sidekiq background jobs
After the above queries are executed, GitLab
[sticks to the primary](../administration/database_load_balancing.md#primary-sticking).
[sticks to the primary](../administration/postgresql/database_load_balancing.md#primary-sticking).
To make the inside queries prefer using the replicas,
[merge request 59086](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/59086) introduced
`fallback_to_replicas_for_ambiguous_queries`. This MR is also an example of how we redirected a
......
doc/development/scalability.md
View file @ f290a6b5
......@@ -123,8 +123,7 @@ the read replicas. [Omnibus ships with Patroni](../administration/postgresql/rep
#### Load-balancing
GitLab EE has [application support for load balancing using read
replicas](../administration/database_load_balancing.md). This load balancer does
GitLab EE has [application support for load balancing using read replicas](../administration/postgresql/database_load_balancing.md). This load balancer does
some actions that aren't traditionally available in standard load balancers. For
example, the application considers a replica only if its replication lag is low
(for example, WAL data behind by less than 100 MB).
......
doc/development/sidekiq_style_guide.md
View file @ f290a6b5
......@@ -574,7 +574,7 @@ of reading a stale record is non-zero due to replicas potentially lagging behind
When the number of jobs that rely on the database increases, ensuring immediate data consistency
can put unsustainable load on the primary database server. We therefore added the ability to use
[database load balancing for Sidekiq workers](../administration/database_load_balancing.md#load-balancing-for-sidekiq).
[Database Load Balancing for Sidekiq workers](../administration/postgresql/database_load_balancing.md).
By configuring a worker's `data_consistency` field, we can then allow the scheduler to target read replicas
under several strategies outlined below.
......
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