feat: allow setting Datadog syslog.hostname attribute (#87)

* Remove syslog hostname from datadog formatter

* feat: support init/1 callback and formatter_state

* feat: allow setting datadog syslog.hostname attribute

* convert formatter_opts to a map

lib/logger_json.ex

@@ -10,12 +10,18 @@ defmodule LoggerJSON do
   LoggerJSON provides three JSON formatters out of the box.
 
   You can change this structure by implementing `LoggerJSON.Formatter` behaviour and passing module
-  name to `:formatter` config option. Example implementations can be found in `LoggerJSON.Formatters.GoogleCloudLogger`
-  and `LoggerJSON.Formatters.BasicLogger`.
+  name to `:formatter` config option. Example implementations can be found in `LoggerJSON.Formatters.GoogleCloudLogger`,
+  `LoggerJSON.Formatters.DatadogLogger`, and `LoggerJSON.Formatters.BasicLogger`.
 
       config :logger_json, :backend,
         formatter: MyFormatterImplementation
 
+  If your formatter supports different options, you can specify them with `:formatter_opts`.
+
+      config :logger_json, :backend,
+        formatter: LoggerJSON.Formatters.DatadogLogger,
+        formatter_opts: [hostname: "example.com"]
+
   ## Encoders support
 
   You can replace default Jason encoder with other module that supports `encode!/1` function. This can be even used
@@ -55,7 +61,8 @@ defmodule LoggerJSON do
             output: nil,
             json_encoder: nil,
             on_init: nil,
-            formatter: nil
+            formatter: nil,
+            formatter_state: %{}
 
   @doc """
   Configures Logger log level at runtime by using value from environment variable.
@@ -197,14 +204,18 @@ defmodule LoggerJSON do
       |> Keyword.get(:metadata, [])
       |> configure_metadata()
 
+    formatter_opts = Keyword.get(config, :formatter_opts, %{})
+    formatter_state = apply(formatter, :init, [formatter_opts])
+
     %{
       state
       | metadata: metadata,
         level: level,
         device: device,
         max_buffer: max_buffer,
         json_encoder: json_encoder,
-        formatter: formatter
+        formatter: formatter,
+        formatter_state: formatter_state
     }
   end
 
@@ -259,15 +270,15 @@ defmodule LoggerJSON do
   end
 
   defp format_event(level, msg, ts, md, state) do
-    %{json_encoder: json_encoder, formatter: formatter, metadata: md_keys} = state
+    %{json_encoder: json_encoder, formatter: formatter, formatter_state: formatter_state, metadata: md_keys} = state
 
     unless formatter do
       raise ArgumentError,
             "invalid :formatter option for :logger_json application. " <>
               "Expected module name that implements LoggerJSON.Formatter behaviour, " <> "got: #{inspect(json_encoder)}"
     end
 
-    event = formatter.format_event(level, msg, ts, md, md_keys)
+    event = formatter.format_event(level, msg, ts, md, md_keys, formatter_state)
 
     case json_encoder do
       nil ->

lib/logger_json/formatter.ex

@@ -6,6 +6,13 @@ defmodule LoggerJSON.Formatter do
   `LoggerJSON.Formatters.BasicLogger`.
   """
 
+  @doc """
+  Initialization callback. Ran on startup with the given `formatter_opts` list.
+
+  Returned list will be used as formatter_state in `format_event/6`.
+  """
+  @callback init(Keyword.t()) :: Keyword.t()
+
   @doc """
   Format event callback.
 
@@ -16,6 +23,7 @@ defmodule LoggerJSON.Formatter do
               msg :: Logger.message(),
               ts :: Logger.Formatter.time(),
               md :: [atom] | :all,
-              state :: map
+              state :: map,
+              formatter_state :: map
             ) :: map | iodata() | %Jason.Fragment{}
 end

lib/logger_json/formatters/basic_logger.ex

@@ -12,7 +12,10 @@ defmodule LoggerJSON.Formatters.BasicLogger do
   @processed_metadata_keys ~w[pid file line function module application]a
 
   @impl true
-  def format_event(level, msg, ts, md, md_keys) do
+  def init(_formatter_opts), do: []
+
+  @impl true
+  def format_event(level, msg, ts, md, md_keys, _formatter_state) do
     json_map(
       time: FormatterUtils.format_timestamp(ts),
       severity: Atom.to_string(level),

lib/logger_json/formatters/datadog_logger.ex

@@ -1,19 +1,51 @@
 defmodule LoggerJSON.Formatters.DatadogLogger do
   @moduledoc """
-  DataDog formatter.
+  [DataDog](https://www.datadoghq.com) formatter. This will adhere to the
+  [default standard attribute list](https://docs.datadoghq.com/logs/processing/attributes_naming_convention/#default-standard-attribute-list)
+  as much as possible.
+
+  ## Options
+
+  This formatter has a couple of options to fine tune the logging output for
+  your deployed environment.
+
+  ### `hostname`
+
+  By setting the `hostname` value, you can change how the `syslog.hostname` is
+  set in logs. In most cases, you can leave this unset and it will use default
+  to `:system`, which uses `:inet.gethostname/0` to resolve the value.
+
+  If you are running in an environment where the hostname is not correct, you
+  can hard code it by setting `hostname` to a string. In places where the
+  hostname is inaccurate but also dynamic (like Kubernetes), you can set
+  `hostname` to `:unset` to exclude it entirely. You'll then be relying on
+  [`dd-agent`](https://docs.datadoghq.com/agent/) to determine the hostname.
 
-  Adhere to the
-  [default standard attribute list](https://docs.datadoghq.com/logs/processing/attributes_naming_convention/#default-standard-attribute-list).
   """
   import Jason.Helpers, only: [json_map: 1]
 
   alias LoggerJSON.{FormatterUtils, JasonSafeFormatter}
 
   @behaviour LoggerJSON.Formatter
 
+  @default_opts %{hostname: :system}
   @processed_metadata_keys ~w[pid file line function module application span_id trace_id]a
 
-  def format_event(level, msg, ts, md, md_keys) do
+  @impl true
+  def init(formatter_opts \\ %{}) do
+    opts = Map.merge(@default_opts, formatter_opts)
+
+    unless is_binary(opts.hostname) or opts.hostname in [:system, :unset] do
+      raise ArgumentError,
+            "invalid :hostname option for :formatter_opts logger_json backend. " <>
+              "Expected :system, :unset, or string, " <> "got: #{inspect(opts.hostname)}"
+    end
+
+    opts
+  end
+
+  @impl true
+  def format_event(level, msg, ts, md, md_keys, formatter_state) do
     Map.merge(
       %{
         logger:
@@ -24,12 +56,7 @@ defmodule LoggerJSON.Formatters.DatadogLogger do
             line: Keyword.get(md, :line)
           ),
         message: IO.chardata_to_string(msg),
-        syslog:
-          json_map(
-            hostname: node_hostname(),
-            severity: Atom.to_string(level),
-            timestamp: FormatterUtils.format_timestamp(ts)
-          )
+        syslog: syslog(level, ts, formatter_state.hostname)
       },
       format_metadata(md, md_keys)
     )
@@ -62,8 +89,28 @@ defmodule LoggerJSON.Formatters.DatadogLogger do
     FormatterUtils.format_function(module, function)
   end
 
-  defp node_hostname do
+  defp syslog(level, ts, :system) do
     {:ok, hostname} = :inet.gethostname()
-    to_string(hostname)
+
+    json_map(
+      hostname: to_string(hostname),
+      severity: Atom.to_string(level),
+      timestamp: FormatterUtils.format_timestamp(ts)
+    )
+  end
+
+  defp syslog(level, ts, :unset) do
+    json_map(
+      severity: Atom.to_string(level),
+      timestamp: FormatterUtils.format_timestamp(ts)
+    )
+  end
+
+  defp syslog(level, ts, hostname) do
+    json_map(
+      hostname: hostname,
+      severity: Atom.to_string(level),
+      timestamp: FormatterUtils.format_timestamp(ts)
+    )
   end
 end

lib/logger_json/formatters/google_cloud_logger.ex

@@ -19,14 +19,18 @@ defmodule LoggerJSON.Formatters.GoogleCloudLogger do
     {:error, "ERROR"}
   ]
 
+  @impl true
+  def init(_formatter_opts), do: []
+
   @doc """
   Builds structured payload which is mapped to Google Cloud Logger
   [`LogEntry`](https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry) format.
 
   See: https://cloud.google.com/logging/docs/agent/configuration#special_fields_in_structured_payloads
   """
   for {level, gcp_level} <- @severity_levels do
-    def format_event(unquote(level), msg, ts, md, md_keys) do
+    @impl true
+    def format_event(unquote(level), msg, ts, md, md_keys, _formatter_state) do
       Map.merge(
         %{
           time: FormatterUtils.format_timestamp(ts),
@@ -38,7 +42,8 @@ defmodule LoggerJSON.Formatters.GoogleCloudLogger do
     end
   end
 
-  def format_event(_level, msg, ts, md, md_keys) do
+  @impl true
+  def format_event(_level, msg, ts, md, md_keys, _formatter_state) do
     Map.merge(
       %{
         time: FormatterUtils.format_timestamp(ts),

test/unit/logger_json/ecto_test.exs

@@ -12,7 +12,8 @@ defmodule LoggerJSON.EctoTest do
         metadata: [],
         json_encoder: Jason,
         on_init: :disabled,
-        formatter: LoggerJSON.Formatters.GoogleCloudLogger
+        formatter: LoggerJSON.Formatters.GoogleCloudLogger,
+        formatter_state: %{}
       )
 
     diff = :erlang.convert_time_unit(1, :microsecond, :native)

test/unit/logger_json/plug_test.exs

@@ -24,7 +24,8 @@ defmodule LoggerJSON.PlugTest do
         metadata: :all,
         json_encoder: Jason,
         on_init: :disabled,
-        formatter: LoggerJSON.Formatters.GoogleCloudLogger
+        formatter: LoggerJSON.Formatters.GoogleCloudLogger,
+        formatter_state: %{}
       )
   end
 

test/unit/logger_json_basic_test.exs

@@ -14,7 +14,8 @@ defmodule LoggerJSONBasicTest do
         metadata: [],
         json_encoder: Jason,
         on_init: :disabled,
-        formatter: BasicLogger
+        formatter: BasicLogger,
+        formatter_state: %{}
       )
   end
 

test/unit/logger_json_datadog_test.exs

@@ -15,7 +15,8 @@ defmodule LoggerJSONDatadogTest do
         metadata: [],
         json_encoder: Jason,
         on_init: :disabled,
-        formatter: DatadogLogger
+        formatter: DatadogLogger,
+        formatter_state: %{}
       )
 
     :ok = Logger.reset_metadata([])
@@ -280,6 +281,40 @@ defmodule LoggerJSONDatadogTest do
            end) == ""
   end
 
+  test "logs hostname when set to :system" do
+    Logger.configure_backend(LoggerJSON, formatter_opts: %{hostname: :system})
+    {:ok, hostname} = :inet.gethostname()
+
+    log =
+      fn -> Logger.debug("hello") end
+      |> capture_log()
+      |> Jason.decode!()
+
+    assert log["syslog"]["hostname"] == to_string(hostname)
+  end
+
+  test "does not log hostname when set to :unset" do
+    Logger.configure_backend(LoggerJSON, formatter_opts: %{hostname: :unset})
+
+    log =
+      fn -> Logger.debug("hello") end
+      |> capture_log()
+      |> Jason.decode!()
+
+    assert log["syslog"]["hostname"] == nil
+  end
+
+  test "logs hostname when set to string" do
+    Logger.configure_backend(LoggerJSON, formatter_opts: %{hostname: "testing"})
+
+    log =
+      fn -> Logger.debug("hello") end
+      |> capture_log()
+      |> Jason.decode!()
+
+    assert log["syslog"]["hostname"] == "testing"
+  end
+
   test "logs severity" do
     log =
       fn -> Logger.debug("hello") end

test/unit/logger_json_google_error_reporter_test.exs

@@ -12,7 +12,8 @@ defmodule LoggerJSONGoogleErrorReporterTest do
         metadata: :all,
         json_encoder: Jason,
         on_init: :disabled,
-        formatter: GoogleCloudLogger
+        formatter: GoogleCloudLogger,
+        formatter_state: %{}
       )
 
     :ok = Logger.reset_metadata([])

test/unit/logger_json_google_test.exs

@@ -15,7 +15,8 @@ defmodule LoggerJSONGoogleTest do
         metadata: [],
         json_encoder: Jason,
         on_init: :disabled,
-        formatter: GoogleCloudLogger
+        formatter: GoogleCloudLogger,
+        formatter_state: %{}
       )
 
     :ok = Logger.reset_metadata([])