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docs/snippets/elixir/plugins/11_stateful_plugin.exs

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+```elixir title="Elixir"
+# Define a stateful post-processor plugin using an Agent
+defmodule MyApp.Plugins.StatefulTextProcessor do
+  @behaviour Kreuzberg.Plugin.PostProcessor
+
+  @moduledoc """
+  A stateful post-processor that maintains a count of processed documents.
+  Demonstrates how to use an Agent to store state across multiple processing calls.
+  """
+
+  @impl true
+  def name, do: "stateful_text_processor"
+
+  @impl true
+  def version, do: "1.0.0"
+
+  @impl true
+  def processing_stage, do: :middle
+
+  # Start an Agent to maintain state
+  @impl true
+  def initialize do
+    case Agent.start_link(fn -> %{count: 0, errors: 0} end, name: __MODULE__) do
+      {:ok, _pid} -> :ok
+      {:error, {:already_started, _}} -> :ok
+      error -> error
+    end
+  end
+
+  @impl true
+  def shutdown do
+    case Agent.stop(__MODULE__) do
+      :ok -> :ok
+      error -> error
+    end
+  end
+
+  @impl true
+  def process(result, _config) do
+    # Increment the processed count
+    Agent.update(__MODULE__, fn state ->
+      %{state | count: state.count + 1}
+    end)
+
+    # Add metadata about processing
+    case normalize_content(result.content) do
+      {:ok, normalized} ->
+        Map.merge(result, %{
+          "content" => normalized,
+          "processed_count" => get_count(),
+          "processing_timestamp" => DateTime.utc_now() |> DateTime.to_iso8601()
+        })
+
+      {:error, reason} ->
+        Agent.update(__MODULE__, fn state ->
+          %{state | errors: state.errors + 1}
+        end)
+
+        {:error, "Failed to normalize content: #{reason}"}
+    end
+  end
+
+  # Retrieve the current processing count
+  defp get_count do
+    Agent.get(__MODULE__, fn state -> state.count end)
+  end
+
+  # Get error count
+  defp get_errors do
+    Agent.get(__MODULE__, fn state -> state.errors end)
+  end
+
+  # Normalize text content
+  defp normalize_content(content) when is_binary(content) do
+    {:ok,
+     content
+     |> String.trim()
+     |> String.replace(~r/\s+/, " ")}
+  end
+
+  defp normalize_content(_), do: {:error, "Content is not a string"}
+end
+
+# Register the stateful plugin
+:ok = Kreuzberg.Plugin.register_post_processor(:stateful, MyApp.Plugins.StatefulTextProcessor)
+
+# Initialize the plugin
+:ok = MyApp.Plugins.StatefulTextProcessor.initialize()
+
+# Process first document
+result1 = %{"content" => "  Example   text   with   spaces  "}
+processed1 = MyApp.Plugins.StatefulTextProcessor.process(result1, nil)
+IO.inspect(processed1, label: "First processing")
+
+# Process second document
+result2 = %{"content" => "Another  document\nwith\tmultiple\nlines"}
+processed2 = MyApp.Plugins.StatefulTextProcessor.process(result2, nil)
+IO.inspect(processed2, label: "Second processing")
+
+# The state persists across calls
+IO.puts("Documents processed: #{processed2["processed_count"]}")
+
+# Verify plugin is registered
+{:ok, processors} = Kreuzberg.Plugin.list_post_processors()
+IO.inspect(processors, label: "Registered processors")
+
+# Cleanup
+:ok = MyApp.Plugins.StatefulTextProcessor.shutdown()
+:ok = Kreuzberg.Plugin.unregister_post_processor(:stateful)
+```