Implement heartbeat check-ins

Add an `Appsignal.CheckIn.heartbeat` helper that emits a single
heartbeat for the check-in identifier given.

When called with `continuous: true` as the second argument, it
starts and links a separate Elixir process that emits a heartbeat
every thirty seconds.

Unlike the equivalent functionality in the Ruby integration, which
spawns a thread that will stay alive for the lifetime of the Ruby
process, the Elixir process is linked to the process that spawned
it, meaning it will be shut down when its parent process is shut
down. This allows it to be used to track the lifetime of individual
Elixir processes.

Additionally, it is also possible to add `Appsignal.CheckIn.Heartbeat`
as a child process to a supervisor, meaning its lifetime will be tied
to that of the other processes supervised by it.

Finally, the functionality seen in the Ruby integration could also be
achieved by manually calling `Appsignal.CheckIn.Heartbeat.start/1`,
keeping the process unlinked and therefore alive for the entirety of
the Elixir node's lifetime, though this is unlikely to be useful in
the Elixir process model.

.changesets/add-heartbeat-check-ins.md

@@ -0,0 +1,39 @@
+---
+bump: minor
+type: add
+---
+
+Add support for heartbeat check-ins.
+
+Use the `Appsignal.CheckIn.heartbeat` method to send a single heartbeat check-in event from your application. This can be used, for example, in a `GenServer`'s callback:
+
+```elixir
+@impl true
+def handle_cast({:process_job, job}, jobs) do
+  Appsignal.CheckIn.heartbeat("job_processor")
+  {:noreply, [job | jobs], {:continue, :process_job}}
+end
+```
+
+Heartbeats are deduplicated and sent asynchronously, without blocking the current thread. Regardless of how often the `.heartbeat` method is called, at most one heartbeat with the same identifier will be sent every ten seconds.
+
+Pass `continuous: true` as the second argument to send heartbeats continuously during the entire lifetime of the current process. This can be used, for example, during a `GenServer`'s initialisation:
+
+```elixir
+@impl true
+def init(_arg) do
+  Appsignal.CheckIn.heartbeat("my_genserver", continuous: true)
+  {:ok, nil}
+end
+```
+
+You can also use `Appsignal.CheckIn.Heartbeat` as a supervisor's child process, in order for heartbeats to be sent continuously during the lifetime of the supervisor. This can be used, for example, during an `Application`'s start:
+
+```elixir
+@impl true
+def start(_type, _args) do
+  Supervisor.start_link([
+    {Appsignal.CheckIn.Heartbeat, "my_application"}
+  ], strategy: :one_for_one, name: MyApplication.Supervisor)
+end
+```

config/config.exs

@@ -20,7 +20,9 @@ if Mix.env() in [:bench, :test, :test_no_nif] do
   config :appsignal, appsignal_span: Appsignal.Test.Span
   config :appsignal, appsignal_tracer: Appsignal.Test.Tracer
   config :appsignal, appsignal_tracer_nif: Appsignal.Test.Nif
+
   config :appsignal, deletion_delay: 100
+  config :appsignal, appsignal_checkin_heartbeat_interval_milliseconds: 10
 
   config :appsignal, :config,
     otp_app: :appsignal,

lib/appsignal/check_in/check_in.ex

@@ -1,5 +1,12 @@
 defmodule Appsignal.CheckIn do
   alias Appsignal.CheckIn.Cron
+  alias Appsignal.CheckIn.Event
+
+  @scheduler Application.compile_env(
+               :appsignal,
+               :appsignal_checkin_scheduler,
+               Appsignal.CheckIn.Scheduler
+             )
 
   @spec cron(String.t()) :: :ok
   def cron(identifier) do
@@ -16,4 +23,18 @@ defmodule Appsignal.CheckIn do
 
     output
   end
+
+  @spec heartbeat(String.t()) :: :ok
+  @spec heartbeat(String.t(), continuous: boolean) :: :ok
+  def heartbeat(identifier) do
+    @scheduler.schedule(Event.heartbeat(identifier))
+    :ok
+  end
+
+  def heartbeat(identifier, continuous: true) do
+    Appsignal.CheckIn.Heartbeat.start_link(identifier)
+    :ok
+  end
+
+  def heartbeat(identifier, _), do: heartbeat(identifier)
 end

lib/appsignal/check_in/cron.ex

@@ -1,6 +1,6 @@
 defmodule Appsignal.CheckIn.Cron do
   alias __MODULE__
-  alias Appsignal.CheckIn.Cron.Event
+  alias Appsignal.CheckIn.Event
 
   @scheduler Application.compile_env(
                :appsignal,
@@ -25,40 +25,11 @@ defmodule Appsignal.CheckIn.Cron do
 
   @spec start(Cron.t()) :: :ok
   def start(cron) do
-    @scheduler.schedule(Event.new(cron, :start))
+    @scheduler.schedule(Event.cron(cron, :start))
   end
 
   @spec finish(Cron.t()) :: :ok
   def finish(cron) do
-    @scheduler.schedule(Event.new(cron, :finish))
-  end
-end
-
-defmodule Appsignal.CheckIn.Cron.Event do
-  alias __MODULE__
-  alias Appsignal.CheckIn.Cron
-
-  @derive Jason.Encoder
-
-  @type kind :: :start | :finish
-  @type t :: %Event{
-          identifier: String.t(),
-          digest: String.t(),
-          kind: kind,
-          timestamp: integer,
-          check_in_type: :cron
-        }
-
-  defstruct [:identifier, :digest, :kind, :timestamp, :check_in_type]
-
-  @spec new(Cron.t(), kind) :: t
-  def new(%Cron{identifier: identifier, digest: digest}, kind) do
-    %Event{
-      identifier: identifier,
-      digest: digest,
-      kind: kind,
-      timestamp: System.system_time(:second),
-      check_in_type: :cron
-    }
+    @scheduler.schedule(Event.cron(cron, :finish))
   end
 end

lib/appsignal/check_in/event.ex

@@ -0,0 +1,92 @@
+defmodule Appsignal.CheckIn.Event do
+  alias __MODULE__
+  alias Appsignal.CheckIn.Cron
+
+  @type kind :: :start | :finish
+  @type check_in_type :: :cron | :heartbeat
+  @type t :: %Event{
+          identifier: String.t(),
+          digest: String.t() | nil,
+          kind: kind | nil,
+          timestamp: integer,
+          check_in_type: check_in_type
+        }
+
+  defstruct [:identifier, :digest, :kind, :timestamp, :check_in_type]
+
+  @spec cron(Cron.t(), kind) :: t
+  def cron(%Cron{identifier: identifier, digest: digest}, kind) do
+    %Event{
+      identifier: identifier,
+      digest: digest,
+      kind: kind,
+      timestamp: System.system_time(:second),
+      check_in_type: :cron
+    }
+  end
+
+  @spec heartbeat(String.t()) :: t
+  def heartbeat(identifier) do
+    %Event{
+      identifier: identifier,
+      timestamp: System.system_time(:second),
+      check_in_type: :heartbeat
+    }
+  end
+
+  @spec describe([t]) :: String.t()
+  def describe([]) do
+    # This shouldn't happen.
+    "no check-in events"
+  end
+
+  def describe([%Event{check_in_type: :cron} = event]) do
+    "cron check-in `#{event.identifier || "unknown"}` " <>
+      "#{event.kind || "unknown"} event (digest #{event.digest || "unknown"})"
+  end
+
+  def describe([%Event{check_in_type: :heartbeat} = event]) do
+    "heartbeat check-in `#{event.identifier || "unknown"}` event"
+  end
+
+  def describe([_event]) do
+    # This shouldn't happen.
+    "unknown check-in event"
+  end
+
+  def describe(events) do
+    "#{Enum.count(events)} check-in events"
+  end
+
+  @spec redundant?(t, t) :: boolean
+  def redundant?(
+        %Event{check_in_type: :cron} = event,
+        %Event{check_in_type: :cron} = new_event
+      ) do
+    # Consider any existing cron check-in event redundant if it has the
+    # same identifier, digest and kind as the one we're adding.
+    event.identifier == new_event.identifier &&
+      event.kind == new_event.kind &&
+      event.digest == new_event.digest
+  end
+
+  def redundant?(
+        %Event{check_in_type: :heartbeat} = event,
+        %Event{check_in_type: :heartbeat} = new_event
+      ) do
+    # Consider any existing heartbeat check-in event redundant if it has
+    # the same identifier as the one we're adding.
+    event.identifier == new_event.identifier
+  end
+
+  def redundant?(_event, _new_event), do: false
+end
+
+defimpl Jason.Encoder, for: Appsignal.CheckIn.Event do
+  def encode(%Appsignal.CheckIn.Event{} = event, opts) do
+    event
+    |> Map.from_struct()
+    |> Map.reject(fn {k, v} -> is_nil(v) end)
+    |> Jason.Encode.map(opts)
+  end
+end

lib/appsignal/check_in/heartbeat.ex

@@ -0,0 +1,39 @@
+defmodule Appsignal.CheckIn.Heartbeat do
+  use GenServer, shutdown: :brutal_kill
+
+  @interval_milliseconds Application.compile_env(
+                           :appsignal,
+                           :appsignal_checkin_heartbeat_interval_milliseconds,
+                           30_000
+                         )
+
+  @impl true
+  def init(identifier) do
+    {:ok, identifier, {:continue, :heartbeat}}
+  end
+
+  def start(identifier) do
+    GenServer.start(__MODULE__, identifier)
+  end
+
+  def start_link(identifier) do
+    GenServer.start_link(__MODULE__, identifier)
+  end
+
+  def heartbeat(identifier) do
+    GenServer.cast(__MODULE__, {:heartbeat, identifier})
+    :ok
+  end
+
+  @impl true
+  def handle_continue(:heartbeat, identifier) do
+    Appsignal.CheckIn.heartbeat(identifier)
+    Process.send_after(self(), :heartbeat, @interval_milliseconds)
+    {:noreply, identifier}
+  end
+
+  @impl true
+  def handle_info(:heartbeat, identifier) do
+    {:noreply, identifier, {:continue, :heartbeat}}
+  end
+end

lib/appsignal/check_in/scheduler.ex

@@ -26,7 +26,7 @@ end
 defmodule Appsignal.CheckIn.Scheduler do
   use GenServer
 
-  alias Appsignal.CheckIn.Cron
+  alias Appsignal.CheckIn.Event
 
   @debounce Application.compile_env(
               :appsignal,
@@ -68,17 +68,15 @@ defmodule Appsignal.CheckIn.Scheduler do
     if Appsignal.Config.active?() do
       GenServer.cast(__MODULE__, {:schedule, event})
     else
-      @integration_logger.debug(
-        "AppSignal not active, not scheduling #{describe_events([event])}"
-      )
+      @integration_logger.debug("AppSignal not active, not scheduling #{Event.describe([event])}")
     end
 
     :ok
   end
 
   @impl true
   def handle_cast({:schedule, event}, state) do
-    @integration_logger.trace("Scheduling #{describe_events([event])} to be transmitted")
+    @integration_logger.trace("Scheduling #{Event.describe([event])} to be transmitted")
 
     schedule_transmission(state)
 
@@ -95,7 +93,7 @@ defmodule Appsignal.CheckIn.Scheduler do
 
   @impl true
   def handle_continue({:transmit, events}, state) do
-    description = describe_events(events)
+    description = Event.describe(events)
 
     config = Appsignal.Config.config()
     endpoint = "#{config[:logging_endpoint]}/check_ins/json"
@@ -150,42 +148,17 @@ defmodule Appsignal.CheckIn.Scheduler do
   defp add_event(events, event) do
     # Remove redundant events, keeping the newly added one, which
     # should be the one with the most recent timestamp.
-    [event | Enum.reject(events, &redundant_event?(&1, event))]
-  end
-
-  defp redundant_event?(%Cron.Event{} = event, %Cron.Event{} = new_event) do
-    # Consider any existing cron check-in event redundant if it has the
-    # same identifier, digest and kind as the one we're adding.
-    is_redundant =
-      event.identifier == new_event.identifier &&
-        event.kind == new_event.kind &&
-        event.digest == new_event.digest
-
-    if is_redundant do
-      @integration_logger.debug("Replacing previously scheduled #{describe_events([event])}")
-    end
-
-    is_redundant
-  end
-
-  defp redundant_event?(_event, _new_event), do: false
-
-  defp describe_events([]) do
-    # This shouldn't happen.
-    "no check-in events"
-  end
-
-  defp describe_events(events) when length(events) > 1 do
-    "#{Enum.count(events)} check-in events"
-  end
-
-  defp describe_events([%Cron.Event{} = event]) do
-    "cron check-in `#{event.identifier || "unknown"}` " <>
-      "#{event.kind || "unknown"} event (digest #{event.digest || "unknown"})"
-  end
-
-  defp describe_events([_event]) do
-    # This shouldn't happen.
-    "unknown check-in event"
+    [
+      event
+      | Enum.reject(events, fn existing_event ->
+          is_redundant = Event.redundant?(existing_event, event)
+
+          if is_redundant do
+            @integration_logger.debug("Replacing previously scheduled #{Event.describe([event])}")
+          end
+
+          is_redundant
+        end)
+    ]
   end
 end