Pin accessories to tags

lib/kamal/configuration/accessory.rb

@@ -32,7 +32,7 @@ class Kamal::Configuration::Accessory
   end
 
   def hosts
-    hosts_from_host || hosts_from_hosts || hosts_from_roles
+    hosts_from_host || hosts_from_hosts || hosts_from_roles || hosts_from_tags
   end
 
   def port
@@ -206,6 +206,12 @@ class Kamal::Configuration::Accessory
       end
     end
 
+    def hosts_from_tags
+      if accessory_config.key?("tags")
+        accessory_config["tags"].flat_map { |tag| config.tag(tag)&.hosts }
+      end
+    end
+
     def network
       accessory_config["network"] || DEFAULT_NETWORK
     end

lib/kamal/configuration/docs/accessory.yml

@@ -46,13 +46,15 @@ accessories:
 
     # Accessory hosts
     #
-    # Specify one of `host`, `hosts`, or `roles`:
+    # Specify one of `host`, `hosts`, `roles`, or `tags`:
     host: mysql-db1
     hosts:
       - mysql-db1
       - mysql-db2
     roles:
       - mysql
+    tags:
+      - writer
 
     # Custom command
     #

lib/kamal/configuration/validator/accessory.rb

@@ -2,8 +2,8 @@ class Kamal::Configuration::Validator::Accessory < Kamal::Configuration::Validat
   def validate!
     super
 
-    if (config.keys & [ "host", "hosts", "roles" ]).size != 1
-      error "specify one of `host`, `hosts` or `roles`"
+    if (config.keys & [ "host", "hosts", "roles", "tags" ]).size != 1
+      error "specify one of `host`, `hosts`, `roles` or `tags`"
     end
 
     validate_docker_options!(config["options"])

test/configuration/accessory_test.rb

@@ -7,7 +7,7 @@ class ConfigurationAccessoryTest < ActiveSupport::TestCase
       image: "dhh/app",
       registry: { "username" => "dhh", "password" => "secret" },
       servers: {
-        "web" => [ "1.1.1.1", "1.1.1.2" ],
+        "web" => [ { "1.1.1.1" => "writer" }, { "1.1.1.2" => "reader" } ],
         "workers" => [ "1.1.1.3", "1.1.1.4" ]
       },
       builder: { "arch" => "amd64" },
@@ -70,6 +70,10 @@ class ConfigurationAccessoryTest < ActiveSupport::TestCase
           "proxy" => {
             "host" => "monitoring.example.com"
           }
+        },
+        "proxy" => {
+          "image" => "proxy:latest",
+          "tags" => [ "writer" ]
         }
       }
     }
@@ -117,14 +121,14 @@ class ConfigurationAccessoryTest < ActiveSupport::TestCase
     end
   end
 
-  test "setting host, hosts and roles" do
+  test "setting host, hosts, roles and tags" do
     @deploy[:accessories]["mysql"]["hosts"] = [ "mysql-db1" ]
     @deploy[:accessories]["mysql"]["roles"] = [ "db" ]
 
     exception = assert_raises(Kamal::ConfigurationError) do
       Kamal::Configuration.new(@deploy)
     end
-    assert_equal "accessories/mysql: specify one of `host`, `hosts` or `roles`", exception.message
+    assert_equal "accessories/mysql: specify one of `host`, `hosts`, `roles` or `tags`", exception.message
   end
 
   test "all hosts" do