Cache invalidation during database transactions

Suppose we have CachedUser implemented like this:

def fetch
  Rails.cache.fetch("user:#{user_id}") do
    User.find(user_id) # load from the database on cache-miss
  end
end

then we will run into issues when using it during database transactions.

Mid-transaction invalidation

If the cache is invalidated immediately after a not-yet-committed write, the following cache read would fill the cache with data that could be rolled back later.

user # actual: #<User id: 1, name: "Old Name">
CachedUser.new(user.id).fetch # cache: #<User id: 1, name: "Old Name">
User.transaction do
  user.update!(name: "✨ New Name ✨")
  CachedUser.new(user.id).invalidate # mid-transaction invalidation 🗑
  CachedUser.new(user.id).fetch # cache: #<User id: 1, name: "✨ New Name 
  raise ActiveRecord::Rollback
end
User.find(user.id) # actual: #<User id: 1, name: "Old Name">
CachedUser.new(user.id).fetch # cache: #<User id: 1, name: "✨ New Name ✨">

Post-transaction invalidation

If the cache is invalidated after committing the transaction, the following cache read would use the stale data from the cache as if the changes did not occur.

user # actual: #<User id: 1, name: "Old Name">
CachedUser.new(user.id).fetch # cache: #<User id: 1, name: "Old Name">
User.transaction do
  user.update!(name: "✨ New Name ✨")
  User.find(user.id) # actual: #<User id: 1, name: "✨ New Name ✨">
  CachedUser.new(user.id).fetch # cache: #<User id: 1, name: "Old Name">
  raise ActiveRecord::Rollback
end
CachedUser.new(user.id).invalidate # post-transaction invalidation 🗑

With RedisMemo

RedisMemo resolves this issue by implementing MVCC.