Zero downtime deployment with cord file

When replacing a container currently we:
1. Boot the new container
2. Wait for it to become healthy
3. Stop the old container

Traefik will send requests to the old container until it notices that it
is unhealthy. But it may have stopped serving requests before that point
which can result in errors.

To get round that the new boot process is:

1. Create a directory with a single file on the host
2. Boot the new container, mounting the cord file into /tmp and
including a check for the file in the docker healthcheck
3. Wait for it to become healthy
4. Delete the healthcheck file ("cut the cord") for the old container
5. Wait for it to become unhealthy and give Traefik a couple of seconds
to notice
6. Stop the old container

The extra steps ensure that Traefik stops sending requests before the
old container is shutdown.

lib/kamal/cli/app.rb

@@ -18,8 +18,9 @@ def boot
             roles.each do |role|
               app = KAMAL.app(role: role)
               auditor = KAMAL.auditor(role: role)
+              role_config = KAMAL.config.role(role)
 
-              if capture_with_info(*app.container_id_for_version(version, only_running: true), raise_on_non_zero_exit: false).present?
+              if capture_with_info(*app.container_id_for_version(version), raise_on_non_zero_exit: false).present?
                 tmp_version = "#{version}_replaced_#{SecureRandom.hex(8)}"
                 info "Renaming container #{version} to #{tmp_version} as already deployed on #{host}"
                 execute *auditor.record("Renaming container #{version} to #{tmp_version}"), verbosity: :debug
@@ -29,11 +30,25 @@ def boot
               execute *auditor.record("Booted app version #{version}"), verbosity: :debug
 
               old_version = capture_with_info(*app.current_running_version, raise_on_non_zero_exit: false).strip
-              execute *app.start_or_run(hostname: "#{host}-#{SecureRandom.hex(6)}")
+
+              if role_config.uses_cord?
+                execute *app.tie_cord(role_config.cord_host_file)
+              end
+
+              execute *app.run(hostname: "#{host}-#{SecureRandom.hex(6)}")
 
               Kamal::Utils::HealthcheckPoller.wait_for_healthy(pause_after_ready: true) { capture_with_info(*app.status(version: version)) }
 
-              execute *app.stop(version: old_version), raise_on_non_zero_exit: false if old_version.present?
+              if old_version.present?
+                if role_config.uses_cord?
+                  cord = capture_with_info(*app.cord(version: old_version), raise_on_non_zero_exit: false).strip
+                  if cord.present?
+                    execute *app.cut_cord(cord)
+                    Kamal::Utils::HealthcheckPoller.wait_for_unhealthy(pause_after_ready: true) { capture_with_info(*app.status(version: old_version)) }
+                  end
+                end
+                execute *app.stop(version: old_version), raise_on_non_zero_exit: false
+              end
             end
           end
         end

lib/kamal/commands/app.rb

@@ -1,34 +1,29 @@
 class Kamal::Commands::App < Kamal::Commands::Base
   ACTIVE_DOCKER_STATUSES = [ :running, :restarting ]
 
-  attr_reader :role
+  attr_reader :role, :role_config
 
   def initialize(config, role: nil)
     super(config)
     @role = role
-  end
-
-  def start_or_run(hostname: nil)
-    combine start, run(hostname: hostname), by: "||"
+    @role_config = config.role(self.role)
   end
 
   def run(hostname: nil)
-    role = config.role(self.role)
-
     docker :run,
       "--detach",
       "--restart unless-stopped",
       "--name", container_name,
       *(["--hostname", hostname] if hostname),
       "-e", "KAMAL_CONTAINER_NAME=\"#{container_name}\"",
-      *role.env_args,
-      *role.health_check_args,
+      *role_config.env_args,
+      *role_config.health_check_args,
       *config.logging_args,
       *config.volume_args,
-      *role.label_args,
-      *role.option_args,
+      *role_config.label_args,
+      *role_config.option_args,
       config.absolute_image,
-      role.cmd
+      role_config.cmd
   end
 
   def start
@@ -76,14 +71,12 @@ def execute_in_existing_container(*command, interactive: false)
   end
 
   def execute_in_new_container(*command, interactive: false)
-    role = config.role(self.role)
-
     docker :run,
       ("-it" if interactive),
       "--rm",
-      *role&.env_args,
+      *role_config&.env_args,
       *config.volume_args,
-      *role&.option_args,
+      *role_config&.option_args,
       config.absolute_image,
       *command
   end
@@ -112,7 +105,7 @@ def current_running_version
   def list_versions(*docker_args, statuses: nil)
     pipe \
       docker(:ps, *filter_args(statuses: statuses), *docker_args, "--format", '"{{.Names}}"'),
-      %(while read line; do echo ${line##{service_role_dest}-}; done) # Extract SHA from "service-role-dest-SHA"
+      %(while read line; do echo ${line##{role_config.full_name}-}; done) # Extract SHA from "service-role-dest-SHA"
   end
 
   def list_containers
@@ -150,16 +143,30 @@ def tag_current_as_latest
   end
 
   def make_env_directory
-    make_directory config.role(role).host_env_directory
+    make_directory role_config.host_env_directory
   end
 
   def remove_env_file
-    [:rm, "-f", config.role(role).host_env_file_path]
+    [:rm, "-f", role_config.host_env_file_path]
+  end
+
+  def cord(version:)
+    pipe \
+      docker(:inspect, "-f '{{ range .Mounts }}{{ .Source }} {{ .Destination }} {{ end }}'", container_name(version)),
+      [:awk, "'$2 == \"#{role_config.cord_container_directory}\" {print $1}'"]
+  end
+
+  def tie_cord(cord)
+    create_empty_file(cord)
+  end
+
+  def cut_cord(cord)
+    remove_directory(cord)
   end
 
   private
     def container_name(version = nil)
-      [ config.service, role, config.destination, version || config.version ].compact.join("-")
+      [ role_config.full_name, version || config.version ].compact.join("-")
     end
 
     def filter_args(statuses: nil)

lib/kamal/commands/base.rb

@@ -34,6 +34,10 @@ def make_directory(path)
       [ :mkdir, "-p", path ]
     end
 
+    def remove_directory(path)
+      [ :rm, "-r", path ]
+    end
+
     private
       def combine(*commands, by: "&&")
         commands
@@ -69,5 +73,11 @@ def docker(*args)
       def tags(**details)
         Kamal::Tags.from_config(config, **details)
       end
+
+      def create_empty_file(file)
+        chain \
+          make_directory_for(file),
+          [:touch, file]
+      end
   end
 end

lib/kamal/commands/healthcheck.rb

@@ -11,7 +11,7 @@ def run
       "--label", "service=#{container_name}",
       "-e", "KAMAL_CONTAINER_NAME=\"#{container_name}\"",
       *web.env_args,
-      *web.health_check_args,
+      *web.health_check_args(cord: false),
       *config.volume_args,
       *web.option_args,
       config.absolute_image,

lib/kamal/configuration.rb

@@ -61,6 +61,14 @@ def run_directory
     raw_config.run_directory || ".kamal"
   end
 
+  def run_directory_as_docker_volume
+    if Pathname.new(run_directory).absolute?
+      run_directory
+    else
+      File.join "$(pwd)", run_directory
+    end
+  end
+
 
   def roles
     @roles ||= role_names.collect { |role_name| Role.new(role_name, config: self) }
@@ -141,7 +149,7 @@ def sshkit
 
 
   def healthcheck
-    { "path" => "/up", "port" => 3000, "max_attempts" => 7 }.merge(raw_config.healthcheck || {})
+    { "path" => "/up", "port" => 3000, "max_attempts" => 7, "cord" => "/tmp/kamal-cord" }.merge(raw_config.healthcheck || {})
   end
 
   def readiness_delay
@@ -199,6 +207,10 @@ def host_env_directory
     "#{run_directory}/env"
   end
 
+  def run_id
+    @run_id ||= SecureRandom.hex(16)
+  end
+
   private
     # Will raise ArgumentError if any required config keys are missing
     def ensure_required_keys_present

lib/kamal/configuration/role.rb

@@ -1,4 +1,5 @@
 class Kamal::Configuration::Role
+  CORD_FILE = "cord"
   delegate :argumentize, :env_file_with_secrets, :optionize, to: Kamal::Utils
 
   attr_accessor :name
@@ -47,28 +48,52 @@ def env_args
     argumentize "--env-file", host_env_file_path
   end
 
-  def health_check_args
+  def health_check_args(cord: true)
     if health_check_cmd.present?
-      optionize({ "health-cmd" => health_check_cmd, "health-interval" => health_check_interval })
+      if cord && uses_cord?
+        optionize({ "health-cmd" => health_check_cmd_with_cord, "health-interval" => health_check_interval })
+          .concat(["--volume", "#{cord_host_directory}:#{cord_container_directory}"])
+      else
+        optionize({ "health-cmd" => health_check_cmd, "health-interval" => health_check_interval })
+      end
     else
       []
     end
   end
 
   def health_check_cmd
-    options = specializations["healthcheck"] || {}
-    options = config.healthcheck.merge(options) if running_traefik?
+    health_check_options["cmd"] || http_health_check(port: health_check_options["port"], path: health_check_options["path"])
+  end
 
-    options["cmd"] || http_health_check(port: options["port"], path: options["path"])
+  def health_check_cmd_with_cord
+    "(#{health_check_cmd}) && (stat #{cord_container_file} > /dev/null || exit 1)"
   end
 
   def health_check_interval
-    options = specializations["healthcheck"] || {}
-    options = config.healthcheck.merge(options) if running_traefik?
+    health_check_options["interval"] || "1s"
+  end
+
+  def uses_cord?
+    running_traefik? && cord_container_directory.present? && health_check_cmd.present?
+  end
+
+  def cord_host_directory
+    File.join config.run_directory_as_docker_volume, "cords", [full_name, config.run_id].join("-")
+  end
+
+  def cord_host_file
+    File.join cord_host_directory, CORD_FILE
+  end
+
+  def cord_container_directory
+    health_check_options.fetch("cord", nil)
+  end
 
-    options["interval"] || "1s"
+  def cord_container_file
+    File.join cord_container_directory, CORD_FILE
   end
 
+
   def cmd
     specializations["cmd"]
   end
@@ -85,6 +110,10 @@ def running_traefik?
     name.web? || specializations["traefik"]
   end
 
+  def full_name
+    [ config.service, name, config.destination ].compact.join("-")
+  end
+
   private
     attr_accessor :config
 
@@ -164,4 +193,12 @@ def merged_env_with_secrets
     def http_health_check(port:, path:)
       "curl -f #{URI.join("http://localhost:#{port}", path)} || exit 1" if path.present? || port.present?
     end
+
+    def health_check_options
+      @health_check_options ||= begin
+        options = specializations["healthcheck"] || {}
+        options = config.healthcheck.merge(options) if running_traefik?
+        options
+      end
+    end
 end

lib/kamal/utils/healthcheck_poller.rb

@@ -1,5 +1,5 @@
 class Kamal::Utils::HealthcheckPoller
-  TRAEFIK_HEALTHY_DELAY = 2
+  TRAEFIK_UPDATE_DELAY = 2
 
   class HealthcheckError < StandardError; end
 
@@ -11,7 +11,7 @@ def wait_for_healthy(pause_after_ready: false, &block)
       begin
         case status = block.call
         when "healthy"
-          sleep TRAEFIK_HEALTHY_DELAY if pause_after_ready
+          sleep TRAEFIK_UPDATE_DELAY if pause_after_ready
         when "running" # No health check configured
           sleep KAMAL.config.readiness_delay if pause_after_ready
         else
@@ -31,6 +31,31 @@ def wait_for_healthy(pause_after_ready: false, &block)
       info "Container is healthy!"
     end
 
+    def wait_for_unhealthy(pause_after_ready: false, &block)
+      attempt = 1
+      max_attempts = KAMAL.config.healthcheck["max_attempts"]
+
+      begin
+        case status = block.call
+        when "unhealthy"
+          sleep TRAEFIK_UPDATE_DELAY if pause_after_ready
+        else
+          raise HealthcheckError, "container not unhealthy (#{status})"
+        end
+      rescue HealthcheckError => e
+        if attempt <= max_attempts
+          info "#{e.message}, retrying in #{attempt}s (attempt #{attempt}/#{max_attempts})..."
+          sleep attempt
+          attempt += 1
+          retry
+        else
+          raise
+        end
+      end
+
+      info "Container is unhealthy!"
+    end
+
     private
       def info(message)
         SSHKit.config.output.info(message)

test/cli/app_test.rb

@@ -11,10 +11,11 @@ class CliAppTest < CliTestCase
   end
 
   test "boot will rename if same version is already running" do
+    Object.any_instance.stubs(:sleep)
     run_command("details") # Preheat Kamal const
 
     SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
-      .with(:docker, :container, :ls, "--filter", "name=^app-web-latest$", "--quiet", raise_on_non_zero_exit: false)
+      .with(:docker, :container, :ls, "--all", "--filter", "name=^app-web-latest$", "--quiet", raise_on_non_zero_exit: false)
       .returns("12345678") # running version
 
     SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
@@ -25,6 +26,14 @@ class CliAppTest < CliTestCase
       .with(:docker, :ps, "--filter", "label=service=app", "--filter", "label=role=web", "--filter", "status=running", "--filter", "status=restarting", "--latest", "--format", "\"{{.Names}}\"", "|", "while read line; do echo ${line#app-web-}; done", raise_on_non_zero_exit: false)
       .returns("123") # old version
 
+    SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+      .with(:docker, :inspect, "-f '{{ range .Mounts }}{{ .Source }} {{ .Destination }} {{ end }}'", "app-web-123", "|", :awk, "'$2 == \"/tmp/kamal-cord\" {print $1}'", :raise_on_non_zero_exit => false)
+      .returns("cordfile") # old version
+
+    SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+      .with(:docker, :container, :ls, "--all", "--filter", "name=^app-web-123$", "--quiet", "|", :xargs, :docker, :inspect, "--format", "'{{if .State.Health}}{{.State.Health.Status}}{{else}}{{.State.Status}}{{end}}'")
+      .returns("unhealthy") # old version unhealthy
+
     run_command("boot").tap do |output|
       assert_match /Renaming container .* to .* as already deployed on 1.1.1.1/, output # Rename
       assert_match /docker rename app-web-latest app-web-latest_replaced_[0-9a-f]{16}/, output
@@ -180,10 +189,16 @@ def run_command(*command, config: :with_accessories)
     end
 
     def stub_running
+      Object.any_instance.stubs(:sleep)
+
       SSHKit::Backend::Abstract.any_instance.stubs(:capture_with_info).returns("123") # old version
 
       SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
         .with(:docker, :container, :ls, "--all", "--filter", "name=^app-web-latest$", "--quiet", "|", :xargs, :docker, :inspect, "--format", "'{{if .State.Health}}{{.State.Health.Status}}{{else}}{{.State.Status}}{{end}}'")
         .returns("running") # health check
+
+      SSHKit::Backend::Abstract.any_instance.expects(:capture_with_info)
+        .with(:docker, :container, :ls, "--all", "--filter", "name=^app-web-123$", "--quiet", "|", :xargs, :docker, :inspect, "--format", "'{{if .State.Health}}{{.State.Health.Status}}{{else}}{{.State.Status}}{{end}}'")
+        .returns("unhealthy") # health check
     end
 end

test/cli/healthcheck_test.rb

@@ -6,6 +6,7 @@ class CliHealthcheckTest < CliTestCase
     Thread.report_on_exception = false
 
     Kamal::Utils::HealthcheckPoller.stubs(:sleep) # No sleeping when retrying
+    Kamal::Configuration.any_instance.stubs(:run_id).returns("12345678901234567890123456789012")
 
     SSHKit::Backend::Abstract.any_instance.stubs(:execute)
       .with(:docker, :container, :ls, "--all", "--filter", "name=^healthcheck-app-999$", "--quiet", "|", :xargs, :docker, :stop, raise_on_non_zero_exit: false)