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"])