Thermos

Thermos is a library for caching in rails that re-warms caches in the background based on model changes.

Most cache strategies require either time-based or key-based expiration. These strategies have some downsides:

Time-based expiration:

• Stale data

Key-based expiration:

• Have to look up the record to determine whether the cache is warm, AND then might need to load more records in a cold cache scenario. Might have to balance cold vs warm cache performance as it pertains to eager loading records.
• Associated model dependencies need to 'touch' the primary model, meaning more database writes to other tables when changes are made.

Both:

• Potentially expensive cold-cache operations, people sometimes mitigate this with denormalization, which has it's own cache-related problems.

With Thermos, the cache-filling operation is performed in the background, by observing model (and dependent model) changes.

Thermos benefits:

• Always warm cache
• No need to 'touch' models to keep key-based cache up to date
• Cache is only as stale as your background workers' latency
• No need to worry about slow cold-cache operations (unless your cache store fails)

I just want to Thermos everything now!! Unbelievable improvement. It’s like every devs dream come true (@jono-booth)

Make sure that you have configured Rails' Cache Store to allow shared cache access across processes (i.e. not MemoryStore, and ideally not FileStore).

In these examples any changes to a category, it's category items, or it's products will trigger a rebuild of the cache for that category.

With keep_warm, the cached content is defined along with the cache block and dependencies definition. This is the simplest implementation, but is only compatible with the Active Job Inline Adapter. See the next section about fill/drink for compatibility with other Active Job Adapters.

API Controller

json = Thermos.keep_warm(key: "api_categories_show", model: Category, id: params[:id], deps: [:category_items, :products]) do |id|
  Category.find(id).to_json
end

render json: json

Frontend Controller

rendered_template = Thermos.keep_warm(key: "frontend_categories_show", model: Category, id: params[:id], deps: [:category_items, :products]) do |id|
  @category = Category.includes(category_items: :product).find(id)
  render_to_string :show
end

render rendered_template

With fill and drink the cache definition can be in one place, and the response can be used in multiple other places. This is useful if you share the same response in multiple controllers, and want to limit your number of cache keys. Even in the unlikely occurrence of a cache store failure and therefore cache miss, drink can still build up your desired response from the block that was originally defined in fill.

Rails Initializer

Thermos.fill(key: "api_categories_show", model: Category, deps: [:category_items, :products]) do |id|
  Category.find(id).to_json
end

API Controller

json = Thermos.drink(key: "api_categories_show", id: params[:id])
render json: json

If you want to be able to lookup by a key other than id (e.g. you use a slug in the params), you can specify the lookup_key as an argument to keep_warm or fill:

Thermos.keep_warm(key: "api_categories_show", model: Category, id: params[:slug], lookup_key: :slug) do |slug|
  Category.find_by(slug: slug).to_json
end

or

Thermos.fill(key: "api_categories_show", model: Category, lookup_key: :slug) do |slug|
  Category.find_by(slug: slug).to_json
end

If you want to specify a queue for the refill jobs to run other than the default queue, you can provide it to either way of using Thermos:

Thermos.keep_warm(key: "api_categories_show", model: Category, queue: "low_priority") do |id|
  Category.find(id).to_json
end

or

Thermos.fill(key: "api_categories_show", model: Category, queue: "low_priority") do |id|
  Category.find(id).to_json
end

Thermos.drink(key: "api_categories_show", id: params[:slug])

You can specify indirect relationships as dependencies as well. For example, if Store has_many categories, and Category has_many products, but there is no relationship specified on the Store model to Product:

Thermos.keep_warm(key: "api_stores_show", model: Store, id: params[:id], deps: [categories: [:products]]) do |id|
  Store.find(id).to_json
end

NOTE in this example, a change to any model in the association chain will trigger a refill of the cache.

You can provide a filter to restrict whether a record gets rebuilt on model changes:

filter = ->(model) { model.name.match("ball") }
Thermos.keep_warm(key: "api_categories_show", model: Category, id: params[:id], filter: filter) do |id|
  Category.find(id).to_json
end

This project uses MIT-LICENSE.