Rails Cache Migrator

A tool to migrate cache data between Rails 3 and 4

If you are not clearing your cache when you migrate to Rails 4 the cache entries may not be readable. This happens because Rails 3 used Marshal.dump to encode the entries and Rails 4 expects them just to be serialized/deserialize by the cache store. So your cached data from Rails 3 will end up being a Marshal string not an object.

See rails/rails#17923 for a description of the issue.

This resolves the issue by providing a migrator class that you can use to migrate your keys.

Installation

Add this line to your application's Gemfile:

gem 'rails_cache_migrator'

And then execute:

$ bundle

Or install it yourself as:

$ gem install rails_cache_migrator

Usage

Presuming you have an effective way of iterating your cache keys (see below), you can create an instance of the migrator and then migrate each key.

migrator = RailsCacheMigrator::Migrator.new(
    [:redis_store, 'redis://localhost:6379/0/cache', { expires_in: 90.minutes }],
    :mem_cache_store
)

cache_keys.each do |cache_key|
  migrator.migrate(cache_key)
end

ActiveSupport::Cache::Store does not provide a way to access the keys of the store directly, because this may be an intensive operation, like it is on Redis.

You could implement a system to store the keys separately, which is what we had in place. If you didn't do that then you could try something like this if your store supports it:

Rails.cache.instance_variable_get(:@data).keys

The two parameters to the Migrator constructor are cache specs as you would use in your config.cache_store line in your Rails config/application.rb file. To add parameters to one of the cache instances, you can make it an array.

The following are all valid options:

[:redis_store, 'redis://localhost:6379/0/cache', { expires_in: 90.minutes }]

:mem_cache_store

[:file_store, '/tmp/cache']

MyOwnCacheStore.new

See ActiveSupport::Cache.lookup_store for details.

If you are doing this on a production system where cache data is changing you may need to find a way to collect and process the changed keys as well. Although the migrator is pretty quick, so it may be OK to miss a few cache entries.

Security

Do not assign a Rail 4 cache to the old cache instance. Someone could inject data into your cache server that gets marshaled by this code when the data was not expected to be marshaled.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install.

Contributing