Claudio Ortolina
b8b3b24ffa
Expand @moduledoc with cache key structure, 7-day TTL via @one_week_seconds, periodic pruning by PruneAssetCache (every 12h), and rationale for TTL-only invalidation (content-addressable immutable assets). Update architecture.md entry accordingly.
80 lines
2.5 KiB
Elixir
80 lines
2.5 KiB
Elixir
defmodule MusicLibrary.Assets.Cache do
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@moduledoc """
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ETS-based asset cache with TTL for serving frequently accessed images.
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## Cache key
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Each entry is keyed by `{payload, format}` where `payload` is a transform
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parameter string (encoding width and asset hash) and `format` is the output
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MIME type (e.g. `"image/webp"`).
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## TTL and pruning
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The TTL is configured via the `@one_week_seconds` module attribute
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(`60 * 60 * 24 * 7` = 7 days). The `prune/0` function deletes all entries
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whose `inserted_at` timestamp is older than this threshold.
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Pruning runs automatically every 12 hours via the `PruneAssetCache` Oban
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cron worker.
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## Invalidation strategy
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This cache uses **TTL-based expiry only** — there is no explicit
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invalidation. This is sufficient because:
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1. Assets are content-addressable (SHA256 hashes) and immutable once
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written. When an image is replaced (e.g. a new cover is uploaded),
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the new asset gets a new hash, producing a different cache key. The
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old entry is never requested again and naturally expires.
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2. The ETS table is in-memory and cleared on application restart.
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Explicit invalidation would add complexity (tracking which cache entries
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derive from which original asset hash) with no practical benefit given
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the above properties.
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"""
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@spec new() :: :ets.table()
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def new do
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:ets.new(__MODULE__, [:named_table, :public, :compressed, read_concurrency: true])
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end
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@spec set(String.t(), String.t(), binary()) :: true
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def set(payload, format, content) do
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inserted_at = DateTime.utc_now() |> DateTime.to_unix()
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:ets.insert(__MODULE__, {{payload, format}, inserted_at, content})
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end
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@spec get(String.t(), String.t()) :: {:found, binary()} | :not_found
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def get(payload, format) do
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case :ets.lookup(__MODULE__, {payload, format}) do
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[{{^payload, ^format}, _inserted_at, content}] -> {:found, content}
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[] -> :not_found
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end
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end
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@spec total_content_size() :: non_neg_integer()
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def total_content_size do
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:ets.foldl(
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fn {_key, _inserted_at, content}, acc -> acc + byte_size(content) end,
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0,
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__MODULE__
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)
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end
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@one_week_seconds 60 * 60 * 24 * 7
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@spec prune(non_neg_integer()) :: non_neg_integer()
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def prune(older_than_seconds \\ @one_week_seconds) do
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threshold =
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DateTime.utc_now()
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|> DateTime.add(older_than_seconds * -1, :second)
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|> DateTime.to_unix()
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:ets.select_delete(
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__MODULE__,
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[{{:"$1", :"$2", :"$3"}, [{:<, :"$2", threshold}], [true]}]
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)
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end
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end
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