GenServer Drills

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Drills

Drills help you develop familiarity and muscle memory with syntax through repeated exercises. Unlike usual problems, Drills are not intended to develop problem solving skills, they are purely for developing comfort and speed.

This set of drills is for GenServers.

A GenServer is a process like any other Elixir process and it can be used to keep state, execute code asynchronously and so on. The advantage of using a generic server process (GenServer) implemented using this module is that it will have a standard set of interface functions and include functionality for tracing and error reporting. It will also fit into a supervision tree.

• HexDocs: GenServer

GenServer

Create a Zero Genserver that does nothing other than store the integer 0 in its state. Use GenServer.start_link/3 to start the Zero process. Use :sys.get_state/1 to view the state of your Zero process.

Example Solution ```elixir defmodule Zero do use GenServer @impl true def init(_opts) do {:ok, 0} end end {:ok, pid} = GenServer.start_link(Zero, "init_arg") :sys.get_state(pid) ```

Use GenServer.start_link/3 to start your Zero GenServer as a named process.

Example Solution ```elixir GenServer.start_link(Zero, "init_arg", [name: :my_name]) ```

Create a SimpleCounter GenServer whose state starts as 0. Implement a GenServer.handle_call/3 callback function which accepts the :increment message and increments the state by 1 and returns :ok.

Use GenServer.start_link/3 and GenServer.call/3 to spawn a SimpleCounter process and send it an :increment message.

Use :sys.get_state/1 to see that the state of the counter has incremented.

Example Solution ```elixir defmodule SimpleCounter do use GenServer do def init(_opts) do {:ok, 0} end def handle_call(:increment, _from, state) do {:reply, :ok, state + 1} end end {:ok, pid} = GenServer.start_link(SimpleCounter, []) GenServer.call(pid, :increment) :sys.get_state(pid) ```

Create an InitialState GenServer whose initial state can be configured. Call GenServer.start_link/3 to spawn a State GenServer with an initial state. Use :sys.get_state/1 to confirm the state matches your configured state.

Example Solution ```elixir defmodule InitialState do use GenServer @impl true def init(state) do {:ok, state} end end {:ok, pid} = GenServer.start_link(InitialState, "my initial state") :sys.get_state(pid) ```

Create a State module.

It should:

• Define a State.get/1 function that uses GenServer.call/3 and a GenServer.handle_call/3 callback function to retrieve state.
• Define a State.set/2 function that uses GenServer.cast/2 and a GenServer.handle_cast/2 callback function to update state.
• Define a State.start_link/1 function with a GenServer.init/1 callback function to initialize the GenServer with a configurable state.

Manually test each function (State.set/2, State.get/1 and State.start_link/1) to confirm they work as expected.

Example Solution ```elixir defmodule State do use GenServer def start_link(state) do GenServer.start_link(__MODULE__, state) end def set(pid, new_state) do GenServer.cast(pid, {:set, new_state}) end def get(pid) do GenServer.call(pid, :get) end @impl true def init(state) do {:ok, state} end @impl true def handle_cast({:set, new_state}, state) do {:noreply, new_state} end @impl true def handle_call(:get, _from, state) do # response is not specified {:reply, state, state} end end {:ok, pid} = State.start_link("initial state") "initial state" = State.get(pid) State.set(pid, "updated state") "updated state" = State.get(pid) ```

Create a minimal Named GenServer that can be started and configured as a named process using Named.start_link/1. Start a Named process and use GenServer.whereis/1 and/or Process.whereis/1 to find the pid of the named process.

Example Solution ```elixir defmodule Named do use GenServer def start_link(opts) do name = Keyword.get(opts, :name, __MODULE__) GenServer.start_link(__MODULE__, [], name: name) end def init(_opts) do {:ok, "any state!"} end end Named.start_link([name: :my_configured_name]) Process.whereis(:my_configured_name) ```

Create a minimal NamedState GenServer that can be started and configured as a named process with a configurable state using NamedState.start_link. Use :sys.get_state/1 to confirm the initial state is as expected and GenServer.whereis/1 and/or Process.whereis/1 to find the pid of the named process.

Example Solution There are many ways to configure `NamedState.start_link/1` Using multiple parameters: ```elixir defmodule NamedState do use GenServer def start_link(state, opts \\ []) do name = Keyword.get(opts, :name, __MODULE__) GenServer.start_link(__MODULE__, state, name: name) end def init(state) do {:ok, state} end end NamedState.start_link("initial state", name: :multi_arg_example) Process.whereis(:multi_arg_example) ``` Using a keyword list with different keys: ```elixir defmodule NamedState do use GenServer def start_link(opts) do name = Keyword.get(opts, :name, __MODULE__) state = Keyword.get(opts, :state) GenServer.start_link(__MODULE__, state, name: name) end def init(state) do {:ok, state} end end NamedState.start_link(state: "initial state", name: :keyword_list_example) Process.whereis(:keyword_list_example) ```

Commit Your Progress

DockYard Academy now recommends you use the latest Release rather than forking or cloning our repository.

Run git status to ensure there are no undesirable changes. Then run the following in your command line from the curriculum folder to commit your progress.

$ git add .
$ git commit -m "finish GenServer Drills exercise"
$ git push

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