diff --git a/llms.txt b/llms.txt
index 35064b09..062182fc 100644
--- a/llms.txt
+++ b/llms.txt
@@ -250,7 +250,7 @@ Documentation for the end-of-life v0.12 series is at https://docs.fluentd.org/0.
 - [How to Write Output Plugin](https://docs.fluentd.org/plugin-development/api-plugin-output.md): Extend `Fluent::Plugin::Output` class and implement its methods.
 - [How to Write Parser Plugin](https://docs.fluentd.org/plugin-development/api-plugin-parser.md): Fluentd supports pluggable and customizable formats for input plugins.
 - [How to Write Storage Plugin](https://docs.fluentd.org/plugin-development/api-plugin-storage.md): Fluentd supports pluggable storage which lets a plugin keep its internal state as key-value pairs.
-- [How to Write Service Discovery Plugin](https://docs.fluentd.org/plugin-development/api-plugin-service_discovery.md)
+- [How to Write Service Discovery Plugin](https://docs.fluentd.org/plugin-development/api-plugin-service_discovery.md): Fluentd supports pluggable service discovery which lets an output plugin resolve its destinations at run time.
 - [How to Write Tests for Plugin](https://docs.fluentd.org/plugin-development/plugin-test-code.md): This article explains how to write Fluentd plugin test code using `test-unit`.
 - [Configuration Parameter Types](https://docs.fluentd.org/plugin-development/api-config-types.md): The `config_param` data types: string, regexp, integer, float, size, time, bool, enum, array and hash.
 - [Upgrade Plugin from v0.12](https://docs.fluentd.org/plugin-development/plugin-update-from-v0.12.md): This guide is for plugin authors to show how to update input/output/filter plugins written for Fluentd v0.12 or earlier.
diff --git a/plugin-development/api-plugin-service_discovery.md b/plugin-development/api-plugin-service_discovery.md
index 5f1ae31c..ddc0a59c 100644
--- a/plugin-development/api-plugin-service_discovery.md
+++ b/plugin-development/api-plugin-service_discovery.md
@@ -1,3 +1,280 @@
 # How to Write Service Discovery Plugin
 
-TODO: Write
+Fluentd supports [pluggable service discovery](../service_discovery/) which lets an output plugin resolve its destinations at run time. The plugin filenames prefixed `sd_` are registered as Service Discovery Plugins.
+
+See [Plugin Base Class API](api-plugin-base.md) for details on the common APIs for all the plugin types.
+
+Here is an example of a custom service discovery plugin that reads a list of servers from a plain text file, one `host:port` per line, and re-reads it on a timer. It takes a required parameter called `path`, which is the pathname of that file, and an optional `interval`.
+
+```ruby
+require 'fluent/plugin/service_discovery'
+require 'fluent/plugin_helper'
+
+module Fluent::Plugin
+  class MyListServiceDiscovery < ServiceDiscovery
+    include Fluent::PluginHelper::Mixin
+
+    # Register MyListServiceDiscovery as 'my_list'.
+    Fluent::Plugin.register_sd('my_list', self)
+
+    helpers :timer
+
+    config_param :path, :string
+    config_param :interval, :time, default: 10
+
+    def configure(conf)
+      super
+
+      unless File.readable?(@path)
+        raise Fluent::ConfigError, "my_list: path=#{@path} is not readable"
+      end
+
+      @services = fetch_services
+    end
+
+    def start(queue)
+      timer_execute(:sd_my_list, @interval) do
+        refresh(queue)
+      end
+
+      super()
+    end
+
+    private
+
+    # Reads one "host:port" per line. Blank lines and # comments are skipped.
+    def fetch_services
+      File.readlines(@path).filter_map do |line|
+        line = line.sub(/#.*/, '').strip
+        next if line.empty?
+
+        host, port = line.split(':', 2)
+        Service.new(:my_list, host, port.to_i, line, 60, false, nil, nil, nil)
+      end
+    end
+
+    # Announces the additions before the removals, so that the owner plugin is
+    # never left without a destination.
+    def refresh(queue)
+      services = begin
+                   fetch_services
+                 rescue => e
+                   log.error "my_list: failed to read the list", path: @path, error: e
+                   return
+                 end
+
+      (services - @services).each { |s| queue.push(ServiceDiscovery.service_in_msg(s)) }
+      (@services - services).each { |s| queue.push(ServiceDiscovery.service_out_msg(s)) }
+      @services = services
+    end
+  end
+end
+```
+
+Save this as `sd_my_list.rb` in a loadable plugin path.
+
+With `out_forward` output plugin:
+
+```text
+<match test>
+  @type forward
+  <service_discovery>
+    @type my_list
+    path /path/to/servers.txt
+    interval 2
+  </service_discovery>
+</match>
+```
+
+`/path/to/servers.txt` holds one server per line:
+
+```text
+# host:port
+127.0.0.1:24230
+127.0.0.1:24231
+```
+
+`out_forward` picks up both of them at startup:
+
+```text
+adding forwarding server '127.0.0.1:24230' host="127.0.0.1" port=24230 weight=60
+adding forwarding server '127.0.0.1:24231' host="127.0.0.1" port=24231 weight=60
+```
+
+Replacing the second line of the file with another server swaps the destination without a restart:
+
+```text
+Service in: name=127.0.0.1:24232 127.0.0.1:24232
+Service out: name=127.0.0.1:24231 127.0.0.1:24231
+```
+
+`Fluent::Plugin::ServiceDiscovery` does not include the plugin helper mixin, so a plugin which needs a helper includes `Fluent::PluginHelper::Mixin` itself, as the example above does. The built-in [`sd_file`](https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/sd_file.rb) and [`sd_srv`](https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/sd_srv.rb) do the same.
+
+## How To Use Service Discovery From Plugins
+
+Service discovery plugins are designed to be used from output plugins. The service discovery plugin helper is there for this purpose:
+
+```ruby
+# in class definition
+helpers :service_discovery
+
+# in #configure
+service_discovery_configure(:my_output_sd_watcher, static_default_service_directive: 'server')
+
+# in #write, or wherever a destination is picked
+service_discovery_select_service do |node|
+  send_data(node, chunk)
+end
+```
+
+The helper owns the lifecycle of the configured plugins. It creates one instance per `<service_discovery>` section, starts them with the queue they push their updates onto, keeps the current set of services, and shuts them down with the owner plugin.
+
+See [Service Discovery Plugin Helper API](../plugin-helper-overview/api-plugin-helper-service_discovery.md) for details.
+
+## Methods
+
+`Fluent::Plugin::ServiceDiscovery` keeps the current destinations in `@services`, and gives the plugin two class methods to announce a change. A plugin implements `#configure` for the initial list, and `#start` for the updates.
+
+#### `#configure(conf)`
+
+It sets `@services` to the destinations the plugin knows at startup. The owner plugin reads them while it configures itself, so a destination missing here does not exist for the owner plugin until the plugin announces it later.
+
+Raise `Fluent::ConfigError` here for anything the plugin cannot work with, so that a misconfiguration stops Fluentd at startup instead of showing up later as a lost destination.
+
+#### `#start(queue)`
+
+It starts watching wherever the plugin discovers destinations from. `queue` is a `Thread::Queue` owned by the owner plugin. A plugin which never changes its list, like [`sd_static`](https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/sd_static.rb), does not have to override this method.
+
+The plugin schedules its own updates. The example above and `sd_srv` use the [timer plugin helper](../plugin-helper-overview/api-plugin-helper-timer.md), and `sd_file` watches the file with the [event loop plugin helper](../plugin-helper-overview/api-plugin-helper-event_loop.md).
+
+#### `.service_in_msg(service)`
+
+It builds the message which adds `service` to the destinations of the owner plugin.
+
+#### `.service_out_msg(service)`
+
+It builds the message which removes `service` from the destinations of the owner plugin. The built-in plugins push the additions before the removals, so that the owner plugin always has at least one destination left.
+
+## Services
+
+A destination is a `Fluent::Plugin::ServiceDiscovery::Service`, a `Struct` with these members in this order:
+
+| Member | Description |
+| :--- | :--- |
+| `plugin_name` | The name of the plugin which found the service. The built-in plugins use `:static`, `:file` and `:srv`. |
+| `host` | The IP address or the host name of the server. |
+| `port` | The port number of the server. |
+| `name` | The name of the server. The owner plugin uses it for logging, and `out_forward` also uses it to verify the certificate when `host` is an address. |
+| `weight` | The load balancing weight. The built-in plugins default to `60`. |
+| `standby` | `true` marks the server as a standby node. |
+| `username` | The username for authentication. |
+| `password` | The password for authentication. |
+| `shared_key` | The shared key for the server. |
+
+`#discovery_id` identifies a service, and it is built from every member. Two services which differ in any one of them, `weight` included, are different services, so a changed weight is announced as one removal and one addition of the same server.
+
+## How The Services Reach The Owner Plugin
+
+The service discovery plugin helper owns a manager which holds the plugins configured in the `<service_discovery>` sections. The manager builds the initial set of destinations from `#services` of every plugin, then polls the queue every 3 seconds by default and applies the messages it finds there. This is where the `Service in:` and `Service out:` log lines come from.
+
+The manager skips the polling when every configured plugin is `static`, since a static list never changes. A plugin registered under any other name is polled.
+
+The owner plugin may keep an object of its own per service instead of the struct. `out_forward` builds a node which owns the connection to the server, which is why `service_discovery_select_service` yields that node rather than the `Service`.
+
+## Writing Tests
+
+Fluentd does not provide a test driver for service discovery plugins, so tests instantiate the plugin and drive it directly. The tests of the built-in plugins do the same.
+
+```ruby
+# test/plugin/test_sd_my_list.rb
+
+require 'test/unit'
+require 'fileutils'
+require 'timeout'
+require 'fluent/test'
+require 'fluent/test/helpers'
+
+# Your own plugin
+require 'fluent/plugin/sd_my_list'
+
+class ServiceDiscoveryMyListTest < Test::Unit::TestCase
+  include Fluent::Test::Helpers
+
+  TMP_DIR = File.expand_path(File.dirname(__FILE__) + '/tmp/sd_my_list')
+  LIST_PATH = File.join(TMP_DIR, 'list.txt')
+
+  Service = Fluent::Plugin::ServiceDiscovery::Service
+
+  def setup
+    Fluent::Test.setup
+    FileUtils.mkdir_p(TMP_DIR)
+  end
+
+  def teardown
+    FileUtils.rm_rf(TMP_DIR)
+  end
+
+  def create_plugin(conf)
+    sd = Fluent::Plugin::MyListServiceDiscovery.new
+    sd.configure(config_element('service_discovery', '', conf))
+    sd
+  end
+
+  sub_test_case 'configure' do
+    test 'reads the list' do
+      File.write(LIST_PATH, "# host:port\n127.0.0.1:24224\n")
+      sd = create_plugin('path' => LIST_PATH)
+      assert_equal([Service.new(:my_list, '127.0.0.1', 24224, '127.0.0.1:24224', 60, false, nil, nil, nil)], sd.services)
+    end
+
+    test 'raises ConfigError for an unreadable path' do
+      assert_raise(Fluent::ConfigError) do
+        create_plugin('path' => File.join(TMP_DIR, 'no-such-file'))
+      end
+    end
+  end
+
+  sub_test_case 'start' do
+    test 'pushes the difference onto the queue' do
+      File.write(LIST_PATH, "127.0.0.1:24224\n")
+      sd = create_plugin('path' => LIST_PATH, 'interval' => '1')
+      queue = []
+      sd.start(queue)
+      begin
+        File.write(LIST_PATH, "127.0.0.1:24225\n")
+        Timeout.timeout(10) { sleep 0.1 until queue.size >= 2 }
+      ensure
+        sd.stop; sd.shutdown; sd.close; sd.terminate
+      end
+      assert_equal([:service_in, :service_out], queue.map(&:type))
+      assert_equal(24225, queue[0].service.port)
+      assert_equal(24224, queue[1].service.port)
+    end
+  end
+end
+```
+
+`config_element` comes from `Fluent::Test::Helpers`, which `fluent/test` does not load by itself. A plain `Array` works as the queue in tests, since the plugin only pushes onto it.
+
+### Overview of Tests
+
+Testing for service discovery plugins is mainly for:
+
+* Validation of configuration \(i.e. `#configure`\)
+* Validation of the services built from the configuration
+* Validation of the messages pushed while the plugin is running
+
+The lifecycle of the plugin in a test is:
+
+1. Instantiate the plugin
+2. Configure the plugin
+3. Start the plugin with a queue, and run test code
+4. Assert the services and the messages on the queue
+5. Shut the plugin down
+
+For:
+
+* configuration tests, repeat steps \# 1-2
+* full feature tests, repeat steps \# 1-5
+
+See [Testing API for Plugins](plugin-test-code.md) for details.