In memory cache for static font_metrics assets.

Assets such as font_metrics tend to be relatively large compared to other data. These assets are often used across multiple scenes and may need to be shared with multiple drivers.

Goals

Given this situation, the Cache module has multiple goals.

• Reuse - assets used by multiple scenes should only be stored in memory once
• Load Time- loading cost should only be paid once
• Copy time - assets are stored in ETS, so they don't need to be copied as they are used
• Pub/Sub - Consumers of static assets (drivers...) should be notified when an asset is loaded or changed. They should not poll the system.
• Security - Base assets can become an attack vector. Helper modules are provided to assist in verifying these files.

Scope

When a font_metrics term is loaded into the cache, it is assigned a scope. The scope is used to determine how long to hold the asset in memory before it is unloaded. Scope is either the atom :global, or a pid.

The typical flow is that a scene will load a font into the cache. A scope is automatically defined that tracks the asset against the pid of the scene that loaded it. When the scene is closed, the scope becomes empty and the asset is unloaded.

If, while that scene is loaded, another scene (or any process...) attempts to load the same asset into the cache, a second scope is added and the duplicate load is skipped. When the first scene closes, the asset stays in memory as long as the second scope remains valid.

When a scene closes, it's scope stays valid for a short time in order to give the next scene a chance to load its assets (or claim a scope) and possibly re-use the already loaded assets.

This is also useful in the event of a scene crashing and being restarted. The delay in unloading the scope means that the replacement scene will use already loaded assets instead of loading the same files again for no real benefit.

When you load assets you can alternately provide your own scope instead of taking the default, which is your processes pid. If you provide :global, then the asset will stay in memory until you explicitly release it.

Hashes

At its simplest, accessing the cache is a key-value store. This cache is meant to be static in nature, so the key is be a hash of the data.

Why? Read below...

Security

A lesson learned the hard way is that static assets (fonts, images, etc.) that your app loads out of storage can easily become attack vectors.

These formats are complicated! There is no guarantee (on any system) that a malformed asset will not cause an error in the C code that interprets it. Again - these are complicated and the renderer needs to be fast...

The solution is to compute a SHA hash of these files during build-time of your and to store the result in your applications code itself. Then during run time, you compare then pre-computed hash against the run-time of the asset being loaded.

Please take advantage of the helper modules Cache.Support.File and Cache.Support.Hash to build the hashes.

These scheme is much stronger when the application code itself is also signed and verified, but that is an exercise for the packaging tools.

Full Example:

defmodule MyApp.MyScene do
  use Scenic.Scene
  import Scenic.Primitives

  # build the path to the static asset file (compile time)
  @font_metrics :code.priv_dir(:my_app) |> Path.join("/static/fonts/my_font.ttf.metrics")
  @font_metrics_hash Scenic.Cache.Hash.file!( @font_metrics, :sha )

  def init( _, _ ) do
    # load the asset into the cache (run time)
    Scenic.Cache.Static.FontMetrics.load(@font_metrics, @font_metrics_hash)

    {:ok, :some_state}
  end

end

Pub/Sub

Drivers (or any process...) listen to the font Cache via a simple pub/sub api.

Because the graph, may be computed during compile time and pushed at some other time than the assets are loaded, the drivers need to know when the assets become available.

Whenever any asset is loaded into the cache, messages are sent to any subscribing processes along with the affected keys. This allows them to react in a loosely-coupled way to how the assets are managed in your scene.