Schemas by {feed, version}, as a value (spec/01 §1.12, visualize D-136).
A delivery names the version of its feed's schema (Macaw.Data.Delivery); a
version already held costs a lookup, and one not seen before is asked of the feed
once. The cache is not a process and not a table: whatever owns the feeds — a board's
process, or a page's sources — holds it in its own state, passes
it in and keeps the grown cache it gets back. A feed is any term that names one.
Examples
iex> alias Macaw.Data.Schema
iex> schema = Schema.new!(%{v: %{type: :number, unit: "ms"}})
iex> cache = Schema.Cache.new([{:latency, schema}])
iex> {:ok, ^schema, ^cache} = Schema.Cache.resolve(cache, :latency, Schema.version(schema), nil)
iex> Schema.Cache.resolve(cache, :latency, "0000", nil)
{:error, {:unknown_schema, :latency, "0000"}, cache}Summary
Types
The pull side of a feed: its present schema, or why it has none.
Functions
A feed's schema at a version, or :error when the cache does not hold it.
The empty cache.
A cache holding each {feed, schema} pair under the schema's own version.
The cache with a feed's schema added under its version.
A feed's schema at a version (spec/01 §1.12): the cached one without calling anything,
else describe's answer, asked once and cached.
Types
@type describe() :: (feed() -> {:ok, Macaw.Data.Schema.t()} | {:error, term()}) | nil
The pull side of a feed: its present schema, or why it has none.
@type feed() :: term()
@type reason() :: {:unknown_schema, feed(), String.t()} | {:describe, feed(), term()} | {:schema_version, feed(), String.t(), String.t()}
Why resolve/4 has no schema for a version (spec/01 §1.12).
@type t() :: %Macaw.Data.Schema.Cache{ entries: %{required({feed(), String.t()}) => Macaw.Data.Schema.t()} }
Functions
@spec fetch(t(), feed(), String.t()) :: {:ok, Macaw.Data.Schema.t()} | :error
A feed's schema at a version, or :error when the cache does not hold it.
@spec new() :: t()
The empty cache.
@spec new([{feed(), Macaw.Data.Schema.t()}]) :: t()
A cache holding each {feed, schema} pair under the schema's own version.
@spec put(t(), feed(), Macaw.Data.Schema.t()) :: t()
The cache with a feed's schema added under its version.
@spec resolve(t(), feed(), String.t(), describe()) :: {:ok, Macaw.Data.Schema.t(), t()} | {:error, reason(), t()}
A feed's schema at a version (spec/01 §1.12): the cached one without calling anything,
else describe's answer, asked once and cached.
describe answering a schema of another version is {:schema_version, feed, version, got}, with the answer cached under its own version; {:error, reason} from it is
{:describe, feed, reason}; no describe for a version the cache lacks is
{:unknown_schema, feed, version}. The cache comes back in every case.