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https://github.com/kenn/standby

Read from standby databases for ActiveRecord
https://github.com/kenn/standby

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Read from standby databases for ActiveRecord

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# Standby - Read from standby databases for ActiveRecord (formerly Slavery)



![Build Status](https://github.com/kenn/standby/actions/workflows/ci.yml/badge.svg)



Standby is a simple, easy to use gem for ActiveRecord that enables conservative reading from standby databases, which means it won't automatically redirect all SELECTs to standbys.



Instead, you can do `Standby.on_standby { User.count }` to send a particular query to a standby.



Background: Probably your app started off with one single database. As it grows, you would upgrade to a primary-standby (or master-slave) replication for redundancy. At this point, all queries still go to the primary and standbys are just backups. With that configuration, it's tempting to run some long-running queries on one of the standbys. And that's exactly what Standby does.



* Conservative - Safe by default. Installing Standby won't change your app's current behavior.

* Future proof - No dirty hacks. Simply works as a proxy for `ActiveRecord::Base.connection`.

* Simple code - Intentionally small. You can read the entire source and completely stay in control.



Standby works with ActiveRecord 3 or later.



## Install



Add this line to your application's Gemfile:



```ruby

gem 'standby'

```



And create standby configs for each environment.



```yaml

development:

  database: myapp_development



development_standby:

  database: myapp_development

```



By convention, config keys with `[env]_standby` are automatically used for standby reads.



Notice that we just copied the settings of `development` to `development_standby`. For `development` and `test`, it's actually recommended as probably you don't want to have replicating multiple databases on your machine. Two connections to the same identical database should be fine for testing purpose.



In case you prefer DRYer definition, YAML's aliasing and key merging might help.



```yaml

common: &common

  adapter: mysql2

  username: root

  database: myapp_development



development:

  <<: *common



development_standby:

  <<: *common

```



Optionally, you can use a database url for your connections:



```yaml

development: postgres://root:@localhost:5432/myapp_development

development_standby: postgres://root:@localhost:5432/myapp_development_standby

```



At this point, Standby does nothing. Run tests and confirm that nothing is broken.



## Usage



To start using Standby, you need to add `Standby.on_standby` in your code. Queries in the `Standby.on_standby` block run on the standby.



```ruby

Standby.on_standby { User.count }   # => runs on standby

Standby.on_standby(:two) { User.count }  # => runs on another standby configured as `development_standby_two`

```



You can nest `on_standby` and `on_primary` interchangeably. The following code works as expected.



```ruby

Standby.on_standby do

  ...

  Standby.on_primary do

    ...

  end

  ...

end

```



Alternatively, you may call `on_standby` directly on the scope, so that the query will be read from standby when it's executed.



```ruby

User.on_standby.where(active: true).count

```



Caveat: `pluck` is not supported by the scope syntax, you still need `Standby.on_standby` in this case.



## Read-only user



For an extra safeguard, it is recommended to use a read-only user for standby access.



```yaml

development_standby:

  <<: *common

  username: readonly

```



With MySQL, `GRANT SELECT` creates a read-only user.



```SQL

GRANT SELECT ON *.* TO 'readonly'@'localhost';

```



With this user, writes on a standby should raise an exception.



```ruby

Standby.on_standby { User.create }  # => ActiveRecord::StatementInvalid: Mysql2::Error: INSERT command denied...

```



With Postgres you can set the entire database to be readonly:



```SQL

ALTER DATABASE myapp_development_standby SET default_transaction_read_only = true;

```



It is a good idea to confirm this behavior in your test code as well.



## Disable temporarily



You can quickly disable standby reads by dropping the following line in `config/initializers/standby.rb`.



```ruby

Standby.disabled = true

```



With this line, Standby stops connection switching and all queries go to the primary.



This may be useful when one of the primary or the standby goes down. You would rewrite `database.yml` to make all queries go to the surviving database, until you restore or rebuild the failed one.



## Transactional fixtures



When `use_transactional_fixtures` is set to `true`, it's NOT recommended to

write to the database besides fixtures, since the standby connection is not aware

of changes performed in the primary connection due to [transaction isolation](https://en.wikipedia.org/wiki/Isolation_(database_systems)).



In that case, you are suggested to disable Standby in the test environment by

putting the following in `test/test_helper.rb`

(or `spec/spec_helper.rb` for RSpec users):



```ruby

Standby.disabled = true

```



## Upgrading from version 3 to version 4



The gem name has been changed from `slavery` to `standby`.



Update your Gemfile



```ruby

gem 'standby'

```



Then



* Replace `Slavery` with `Standby`, `on_slave` with `on_standby`, and `on_master` with `on_primary`

* Update keys in `database.yml` (e.g. `development_slave` to `development_standby`)



## Upgrading from version 2 to version 3



Please note that `Standby.spec_key=` method has been removed from version 3.



## Support for non-Rails apps



If you're using ActiveRecord in a non-Rails app (e.g. Sinatra), be sure to set `RACK_ENV` environment variable in the boot sequence, then:



```ruby

require 'standby'



ActiveRecord::Base.configurations = {

  'development' =>          { adapter: 'mysql2', ... },

  'development_standby' =>  { adapter: 'mysql2', ... }

}

ActiveRecord::Base.establish_connection(:development)

```



## Changelog



* v4.0.0: Rename gem from Slavery to Standby

* v3.0.0: Support for multiple standby targets ([@punchh](https://github.com/punchh))

* v2.1.0: Debug log support / Database URL support / Rails 3.2 & 4.0 compatibility (Thanks to [@citrus](https://github.com/citrus))

* v2.0.0: Rails 5 support