Dynamic Management Examples

Schedule Work

defmodule Counter do
  alias Tarearbol.DynamicManager
  use DynamicManager

  @impl DynamicManager
  def children_specs do
    now = DateTime.utc_now()
    %{
      second: [payload: now, timeout: 1_000],
      minute: [payload: now, timeout: 60_000]
    }
  end

  @impl DynamicManager
  def perform(:second, payload) do
    now = DateTime.utc_now()
    IO.inspect({payload, now}, label: "sec")
    {:ok, now} # do not replace a payload
  end

  def perform(:minute, payload) do
    now = DateTime.utc_now()
    IO.inspect({payload, now}, label: "min")
    {:replace, now} # replace a payload
  end
end

Once started with Counter.start_link(), it’ll output:

sec: {~U[2021-03-10 07:58:55.874591Z], ~U[2021-03-10 07:58:56.876633Z]}
...
sec: {~U[2021-03-10 07:58:55.874591Z], ~U[2021-03-10 07:59:54.938543Z]}
min: {~U[2021-03-10 07:58:55.874591Z], ~U[2021-03-10 07:59:55.876592Z]}
sec: {~U[2021-03-10 07:58:55.874591Z], ~U[2021-03-10 07:59:55.939661Z]}
...
sec: {~U[2021-03-10 07:58:55.874591Z], ~U[2021-03-10 08:00:54.998537Z]}
min: {~U[2021-03-10 07:59:55.876592Z], ~U[2021-03-10 08:00:55.877658Z]}
...

Note, that the payload of the :second one does not change, while the payload of minute gets updated to the latest value reported.

Workers Pool

One might create a workers pool with a help of Tarearbol.DynamicManager. Usually perform/2 is suppressed in workers with timeout: 0 and only Genserver.cast/2 and Genserver.call/2 are handled with helpers Tarearbol.Pool.defsynch/2 and Tarearbol.Pool.defasynch/2.

Written as a regular functions, they are wrapped during compilation time to be dispatched to the free instance of the pool behind. Inside the body of these functions, the following magic macros become available:

  • id! returning the id of the worker invoked
  • payload! returning the payload of the worker invoked
  • state! returning the state of the worker invoked as a tuple {id, payload}
defmodule Pool do
  use Tarearbol.Pool, init: &Pool.initializer/0, pool_size: 2

  def initializer, do: 0

  defsynch synch(),
    do: {:ok, payload!()}

  defsynch synch(n),
    do: {:ok, payload!() + n}

  defasynch asynch(n),
    do: {:replace, payload!() + n}
end

Now one might call Pool.synch/1 to perform a synchronized pooled state request, as well as Pool.asynch/1 to asynchronously update the state. Note, that in this contrived example, the state of the first free worker is going to be updated.

Pool.start_link()

Enum.map(1..3, &Pool.synch(&1))
#⇒ [ok: 1, ok: 2, ok: 3]
Pool.synch()
#⇒ 0 

But for asynch/1 function that needs some time to finish and updates the state, it’s different.

Pool.start_link()

Enum.map(1..3, &Pool.asynch(&1))
#⇒ [:ok, :ok, :ok]
Enum.reduce(Pool.state().children, 0, & &2 + elem(&1, 1).value)
#⇒ 6

Note, that there is no guarantee what worker would handle each call.

Multiple Stateful Processes

Consider we are building an online shop having buckets for each customer. Then we might back up the customer session with this kind of DynamicManager.

defmodule Bucket do
  alias Tarearbol.DynamicManager
  use DynamicManager

  def new(customer) do
    put(customer, payload: %{}, timeout: 10_000)
  end

  def do_smth(customer) do
    IO.inspect("Hey, #{customer}, we have a discount")
  end

  @impl DynamicManager
  def children_specs, do: %{}

  @impl DynamicManager
  def perform(id, payload) do
    if map_size(payload) > 0, do: do_smth(id)
    {:ok, DateTime.utc_now()}
  end

  @impl DynamicManager
  def call(:<, _from, {_id, payload}),
    do: {:ok, payload}

  @impl DynamicManager
  def cast({:+, item}, {_id, payload}),
    do: {:replace, Map.update(payload, item, 1, & &1+1)}
end

Then we can play with it.

iex|1 Bucket.start_link()
{:ok, #PID<0.325.0>}
iex|2 Bucket.new "Aleksei"  
:ok
iex|3 Bucket.synch_call "Aleksei", {:+, :tomato}
iex|4 Bucket.asynch_call "Aleksei", {:+, :tomato}
iex|5 Bucket.asynch_call "Aleksei", {:+, :cucumber}
# the below is printed from `perform/2`
"Hey, Aleksei, we have a discount"                   
iex|6 Bucket.synch_call "Aleksei", :<
{:ok, %{cucumber: 1, tomato: 2}}