Redis Elasticache stores for ActiveSupport

This gem is an extension to redis-activesupport that adds support for failover of an Elasticache Redis cluster. The project owes its existence to redis-cluster-activesupport that I found in a discussion about handling the LOADING Redis is loading the dataset in memory which is a message you will see when a replica is promoted to primary.

Usage

This gem is a small extension to redis-activesupport, so refer to their documentation for most configuration. Instead of specifying :redis_store you must now specify :redis_elasticache_store to load this extension.

module MyProject
  class Application < Rails::Application
    config.cache_store = :redis_elasticache_store, options
  end
end

Additionally, there's a new configuration option: :ignored_command_errors. This is useful if you're using a Redis Elasticache with automatic failover which will cause a Redis::CommandError with a message indicating the cluster is in READONLY mode. This extension allows you to whitelist certain ignored_command_errors that would normally be raised by redis-activesupport. By default this gem whitelists the following errors:

DEFAULT_IGNORED_COMMAND_ERRORS = [
  "READONLY You can't write against a read only slave.",
  'LOADING Redis is loading the dataset in memory',
  'A write operation was issued to an ELASTICACHE slave node.',
  'A write operation was issued to an ELASTICACHE slave node that is READONLY.',
  'A write operation was issued to an ELASTICACHE node that was previously READONLY and is now LOADING.'
]

• You get LOADING Redis is loading the dataset in memory when a replica becomes a primary.
• You get READONLY You can't write against a read only slave. if a failover has occured and you are talking to the new replica.
• You get A write operation was issued to an ELASTICACHE slave node. if you are using redis-elasticache to override the REAONLY error.
• You get the last two statements if you use redis-elasticache and this PR is accepted craigmcnamara/redis-elasticache#6.

If you need additional errors added to the whitelist, you can do this through your own configuration or open a pull request to add it to the default whitelist. NOTE: this list is turned into a Set to keep lookups fast, so feel free to make this list as big as you need. Example:

module MyProject
  class Application < Rails::Application
    config.cache_store = :redis_elasticache_store, {:ignored_command_errors => ["Uh oh", "Please, stop", "Fire emoji"]}
  end
end

With this change, your cache store will now silently fail if your Elasticache Redis experiences a failover event which means you won't knock your rails apps offline.

Installation

Add this line to your application's Gemfile:

gem 'redis-elasticache-activesupport'

And then execute:

$ bundle

Or install it yourself as:

$ gem install redis-elasticache-activesupport

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/UJET/redis-elasticache-activesupport.

License

The gem is available as open source under the terms of the MIT License.