feat(server): metrics schema + context with record/latest/prune

server/lib/server/metrics.ex

@@ -0,0 +1,48 @@
+defmodule Server.Metrics do
+  @moduledoc "Metric sample storage and retrieval."
+
+  import Ecto.Query
+
+  alias Server.Repo
+  alias Server.Schema.Metric
+
+  @spec record_sample(integer(), String.t(), DateTime.t(), map()) ::
+          {:ok, Metric.t()} | {:error, Ecto.Changeset.t()}
+  def record_sample(host_id, interval_type, collected_at, payload) do
+    changeset =
+      Metric.changeset(%Metric{}, %{
+        host_id: host_id,
+        interval_type: interval_type,
+        collected_at: collected_at,
+        payload: payload
+      })
+
+    with %Ecto.Changeset{valid?: true} = cs <- changeset,
+         true <- host_exists?(host_id) || {:host_missing, cs} do
+      Repo.insert(cs)
+    else
+      %Ecto.Changeset{} = cs -> {:error, cs}
+      {:host_missing, cs} -> {:error, Ecto.Changeset.add_error(cs, :host, "does not exist")}
+    end
+  end
+
+  defp host_exists?(host_id) do
+    Repo.exists?(from(h in Server.Schema.Host, where: h.id == ^host_id))
+  end
+
+  @spec latest_sample(integer(), String.t()) :: Metric.t() | nil
+  def latest_sample(host_id, interval_type) do
+    from(m in Metric,
+      where: m.host_id == ^host_id and m.interval_type == ^interval_type,
+      order_by: [desc: m.collected_at],
+      limit: 1
+    )
+    |> Repo.one()
+  end
+
+  @spec delete_older_than(DateTime.t()) :: {non_neg_integer(), nil}
+  def delete_older_than(%DateTime{} = cutoff) do
+    from(m in Metric, where: m.collected_at < ^cutoff)
+    |> Repo.delete_all()
+  end
+end

server/lib/server/schema/metric.ex

@@ -0,0 +1,22 @@
+defmodule Server.Schema.Metric do
+  use Ecto.Schema
+  import Ecto.Changeset
+
+  @intervals ~w(fast medium slow)
+
+  schema "metrics" do
+    belongs_to :host, Server.Schema.Host
+    field :collected_at, :utc_datetime_usec
+    field :interval_type, :string
+    field :payload, :map
+
+    timestamps(type: :utc_datetime_usec, updated_at: false)
+  end
+
+  def changeset(metric, attrs) do
+    metric
+    |> cast(attrs, [:host_id, :collected_at, :interval_type, :payload])
+    |> validate_required([:host_id, :collected_at, :interval_type, :payload])
+    |> validate_inclusion(:interval_type, @intervals)
+  end
+end