2. Agent introduction

Section

Agent is a high-level wrapper for some typical message exchange between processes. In general we do not need to:

• Manually write receive/0 block to receive the message.
• Quickly send message to the target server without the need of using send/3.

Basically, Agent.update/2 and Agent.get/1 will:

• Prepare the message structure, including getting the sender process id using self().
• Send the message to the target Agent process.
• Receive message (if needed), then parse the response.

All messages format follow the convention from OTP and be hidden from public use.

defmodule AgentKeyValueStore do
  # Start an agent with an empty map as the initial state
  def start() do
    Agent.start(fn -> %{} end, name: __MODULE__)
  end

  # Function to store a key-value pair
  def put(key, value) do
    Agent.update(__MODULE__, fn state -> Map.put(state, key, value) end)
  end

  # Function to retrieve a value by key
  def get(key) do
    Agent.get(__MODULE__, fn state -> Map.get(state, key, :not_found) end)
  end
end

pid =
  case AgentKeyValueStore.start() do
    {:error, {:already_started, pid}} -> pid
    {:ok, pid} -> pid
  end

AgentKeyValueStore.put(:foo, "bar")

AgentKeyValueStore.get(:foo)

AgentKeyValueStore.get(:test)

:sys.get_state(pid)

:sys.replace_state(pid, fn state ->
  Map.put(state, :test, "test")
end)

AgentKeyValueStore.get(:test)

Process.exit(pid, :kill)