Handle loadbalancing for multiple web hosts

lib/kamal/cli/main.rb

@@ -41,6 +41,11 @@ def deploy(boot_accessories: false)
 
         invoke "kamal:cli:app:boot", [], invoke_options
 
+        if KAMAL.config.proxy.load_balancing?
+          say "Updating loadbalancer configuration...", :magenta
+          invoke "kamal:cli:proxy:loadbalancer", [ "deploy" ], invoke_options
+        end
+
         say "Prune old containers and images...", :magenta
         invoke "kamal:cli:prune:all", [], invoke_options
       end
@@ -70,6 +75,11 @@ def redeploy
         invoke "kamal:cli:app:stale_containers", [], invoke_options.merge(stop: true)
 
         invoke "kamal:cli:app:boot", [], invoke_options
+
+        if KAMAL.config.proxy.load_balancing?
+          say "Updating loadbalancer configuration...", :magenta
+          invoke "kamal:cli:proxy:loadbalancer", [ "deploy" ], invoke_options
+        end
       end
     end
 

lib/kamal/cli/proxy.rb

@@ -19,6 +19,14 @@ def boot
         execute *KAMAL.proxy.ensure_apps_config_directory
         execute *KAMAL.proxy.start_or_run
       end
+
+      if KAMAL.config.proxy.load_balancing?
+        on(KAMAL.config.proxy.effective_loadbalancer) do |host|
+          info "Starting loadbalancer on #{host}..."
+          execute *KAMAL.registry.login
+          execute *KAMAL.loadbalancer.start_or_run
+        end
+      end
     end
   end
 
@@ -114,7 +122,7 @@ def reboot
             execute *KAMAL.auditor.record("Rebooted proxy"), verbosity: :debug
             execute *KAMAL.registry.login
 
-            "Stopping and removing kamal-proxy on #{host}, if running..."
+            info "Stopping and removing kamal-proxy on #{host}, if running..."
             execute *KAMAL.proxy.stop, raise_on_non_zero_exit: false
             execute *KAMAL.proxy.remove_container
             execute *KAMAL.proxy.ensure_apps_config_directory
@@ -123,6 +131,24 @@ def reboot
           end
           run_hook "post-proxy-reboot", hosts: host_list
         end
+
+        if KAMAL.config.proxy.load_balancing?
+          lb_host = KAMAL.config.proxy.effective_loadbalancer
+          run_hook "pre-loadbalancer-reboot", hosts: lb_host
+
+          on(lb_host) do |host|
+            execute *KAMAL.auditor.record("Rebooted loadbalancer"), verbosity: :debug
+            execute *KAMAL.registry.login
+
+            info "Stopping and removing load-balancer on #{host}, if running..."
+            execute *KAMAL.loadbalancer.stop, raise_on_non_zero_exit: false
+            execute *KAMAL.loadbalancer.remove_container
+
+            execute *KAMAL.loadbalancer.run
+          end
+
+          run_hook "post-loadbalancer-reboot", hosts: lb_host
+        end
       end
     end
   end
@@ -143,10 +169,10 @@ def upgrade
           execute *KAMAL.auditor.record("Rebooted proxy"), verbosity: :debug
           execute *KAMAL.registry.login
 
-          "Stopping and removing Traefik on #{host}, if running..."
+          info "Stopping and removing Traefik on #{host}, if running..."
           execute *KAMAL.proxy.cleanup_traefik
 
-          "Stopping and removing kamal-proxy on #{host}, if running..."
+          info "Stopping and removing kamal-proxy on #{host}, if running..."
           execute *KAMAL.proxy.stop, raise_on_non_zero_exit: false
           execute *KAMAL.proxy.remove_container
           execute *KAMAL.proxy.remove_image
@@ -198,6 +224,12 @@ def restart
   desc "details", "Show details about proxy container from servers"
   def details
     on(KAMAL.proxy_hosts) { |host| puts_by_host host, capture_with_info(*KAMAL.proxy.info), type: "Proxy" }
+
+    if KAMAL.config.proxy.load_balancing?
+      on(KAMAL.config.proxy.effective_loadbalancer) do |host|
+        puts_by_host host, capture_with_info(*KAMAL.proxy.loadbalancer.info), type: "Loadbalancer"
+      end
+    end
   end
 
   desc "logs", "Show log lines from proxy on servers"
@@ -239,13 +271,80 @@ def remove
     end
   end
 
+  desc "loadbalancer STATUS", "Manage the load balancer"
+  def loadbalancer(status)
+    case status
+    when "info"
+      if KAMAL.config.proxy.load_balancing?
+        on(KAMAL.config.proxy.effective_loadbalancer) do |host|
+          puts "Loadbalancer status on #{host}:"
+          puts capture_with_info(*KAMAL.loadbalancer.info)
+        end
+      else
+        puts "Load balancing is not configured"
+      end
+    when "start"
+      if KAMAL.config.proxy.load_balancing?
+        on(KAMAL.config.proxy.effective_loadbalancer) do |host|
+          execute *KAMAL.registry.login
+          execute *KAMAL.loadbalancer.start_or_run
+        end
+      else
+        puts "Load balancing is not configured"
+      end
+    when "stop"
+      if KAMAL.config.proxy.load_balancing?
+        on(KAMAL.config.proxy.effective_loadbalancer) do |host|
+          execute *KAMAL.loadbalancer.stop, raise_on_non_zero_exit: false
+        end
+      else
+        puts "Load balancing is not configured"
+      end
+    when "logs"
+      if KAMAL.config.proxy.load_balancing?
+        on(KAMAL.config.proxy.effective_loadbalancer) do |host|
+          puts_by_host host, capture(*KAMAL.loadbalancer.logs(timestamps: true)), type: "Loadbalancer"
+        end
+      else
+        puts "Load balancing is not configured"
+      end
+    when "deploy"
+      if KAMAL.config.proxy.load_balancing?
+        targets = []
+        KAMAL.config.roles.each do |role|
+          next unless role.running_proxy?
+
+          role.hosts.each do |host|
+            targets << host
+          end
+        end
+
+        on(KAMAL.config.proxy.effective_loadbalancer) do |host|
+          info "Deploying to loadbalancer on #{host} with targets: #{targets.join(', ')}"
+          execute *KAMAL.loadbalancer.deploy(targets: targets)
+        end
+      else
+        puts "Load balancing is not configured"
+      end
+    else
+      puts "Unknown loadbalancer subcommand: #{status}. Available: info, start, stop, logs, deploy"
+    end
+  end
+
   desc "remove_container", "Remove proxy container from servers", hide: true
   def remove_container
     with_lock do
       on(KAMAL.proxy_hosts) do
         execute *KAMAL.auditor.record("Removed proxy container"), verbosity: :debug
         execute *KAMAL.proxy.remove_container
       end
+
+      if KAMAL.config.proxy.load_balancing?
+        on(KAMAL.config.proxy.effective_loadbalancer) do
+          execute *KAMAL.auditor.record("Removed loadbalancer container"), verbosity: :debug
+          execute *KAMAL.loadbalancer.remove_container
+        end
+      end
     end
   end
 
@@ -256,6 +355,13 @@ def remove_image
         execute *KAMAL.auditor.record("Removed proxy image"), verbosity: :debug
         execute *KAMAL.proxy.remove_image
       end
+
+      if KAMAL.config.proxy.load_balancing?
+        on(KAMAL.config.proxy.effective_loadbalancer) do
+          execute *KAMAL.auditor.record("Removed loadbalancer image"), verbosity: :debug
+          execute *KAMAL.loadbalancer.remove_image
+        end
+      end
     end
   end
 

lib/kamal/commander.rb

@@ -113,6 +113,14 @@ def proxy
     @commands[:proxy] ||= Kamal::Commands::Proxy.new(config)
   end
 
+  def loadbalancer_config
+    @loadbalancer_config ||= Kamal::Configuration::Loadbalancer.new(config: config, proxy_config: config.proxy.proxy_config)
+  end
+
+  def loadbalancer
+    @commands[:loadbalancer] ||= Kamal::Commands::Loadbalancer.new(config, loadbalancer_config: loadbalancer_config)
+  end
+
   def prune
     @commands[:prune] ||= Kamal::Commands::Prune.new(config)
   end

lib/kamal/commands/loadbalancer.rb

@@ -0,0 +1,100 @@
+class Kamal::Commands::Loadbalancer < Kamal::Commands::Base
+  delegate :argumentize, :optionize, to: Kamal::Utils
+
+  attr_reader :loadbalancer_config
+
+  def initialize(config, loadbalancer_config: nil)
+    super(config)
+    @loadbalancer_config = loadbalancer_config
+  end
+
+  def run
+    pipe \
+      [ :echo, proxy_image ],
+      xargs(docker(:run,
+        "--name", container_name,
+        "--network", "kamal",
+        "--detach",
+        "--restart", "unless-stopped",
+        "--publish", "80:80",
+        "--publish", "443:443",
+        "--label", "org.opencontainers.image.title=kamal-loadbalancer",
+        "--volume", "kamal-loadbalancer-config:/home/kamal-loadbalancer/.config/kamal-loadbalancer"))
+  end
+
+  def start
+    docker :container, :start, container_name
+  end
+
+  def stop(name: container_name)
+    docker :container, :stop, name
+  end
+
+  def start_or_run
+    combine start, run, by: "||"
+  end
+
+  def deploy(targets: [])
+    target_args = targets.map { |t| "#{t}:80" }
+
+    hosts = loadbalancer_config.hosts
+
+    options = []
+    options << "--target=#{target_args.join(',')}"
+    options << "--host=#{hosts.join(',')}"
+    options << "--tls" if loadbalancer_config.ssl?
+
+    docker :exec, container_name, "kamal-proxy", "deploy", loadbalancer_config.config.service, *options
+  end
+
+  def info
+    docker :ps, "--filter", "name=^#{container_name}$"
+  end
+
+  def version
+    pipe \
+      docker(:inspect, container_name, "--format '{{.Config.Image}}'"),
+      [ :cut, "-d:", "-f2" ]
+  end
+
+  def logs(timestamps: true, since: nil, lines: nil, grep: nil, grep_options: nil)
+    pipe \
+      docker(:logs, container_name, ("--since #{since}" if since), ("--tail #{lines}" if lines), ("--timestamps" if timestamps), "2>&1"),
+      ("grep '#{grep}'#{" #{grep_options}" if grep_options}" if grep)
+  end
+
+  def follow_logs(host:, timestamps: true, grep: nil, grep_options: nil)
+    run_over_ssh pipe(
+      docker(:logs, container_name, ("--timestamps" if timestamps), "--tail", "10", "--follow", "2>&1"),
+      (%(grep "#{grep}"#{" #{grep_options}" if grep_options}) if grep)
+    ).join(" "), host: host
+  end
+
+  def remove_container
+    docker :container, :prune, "--force", "--filter", "label=org.opencontainers.image.title=kamal-loadbalancer"
+  end
+
+  def remove_image
+    docker :image, :prune, "--all", "--force", "--filter", "label=org.opencontainers.image.title=kamal-loadbalancer"
+  end
+
+  def ensure_directory
+    make_directory loadbalancer_config.directory
+  end
+
+  def remove_directory
+    super(loadbalancer_config.directory)
+  end
+
+  private
+    def proxy_image
+      [
+        loadbalancer_config.config.proxy_boot.image_default,
+        Kamal::Configuration::Proxy::Boot::MINIMUM_VERSION
+      ].join(":")
+    end
+
+    def container_name
+      loadbalancer_config.container_name
+    end
+end

lib/kamal/commands/proxy.rb

@@ -105,6 +105,10 @@ def reset_run_command
     remove_file config.proxy_boot.run_command_file
   end
 
+  def loadbalancer
+    @loadbalancer ||= Kamal::Commands::Loadbalancer.new(config, loadbalancer_config: KAMAL.loadbalancer_config)
+  end
+
   private
     def container_name
       config.proxy_boot.container_name

lib/kamal/configuration.rb

@@ -145,6 +145,10 @@ def proxy_roles
     roles.select(&:running_proxy?)
   end
 
+  def load_balancing?
+    proxy&.load_balancing?
+  end
+
   def proxy_role_names
     proxy_roles.flat_map(&:name)
   end

lib/kamal/configuration/docs/proxy.yml

@@ -25,6 +25,13 @@ proxy:
   hosts:
     - foo.example.com
     - bar.example.com
+
+  # Loadbalancer
+  #
+  # Specify a host to run the loadbalancer on. The loadbalancer will distribute requests
+  # to all web hosts. If not specified but multiple web hosts are configured, the first
+  # web host will be used as the loadbalancer host.
+  loadbalancer: lb.example.com
 
   # App port
   #

lib/kamal/configuration/loadbalancer.rb

@@ -0,0 +1,28 @@
+class Kamal::Configuration::Loadbalancer < Kamal::Configuration::Proxy
+  CONTAINER_NAME = "load-balancer".freeze
+
+  def self.validation_config_key
+    "proxy"
+  end
+
+  def initialize(config:, proxy_config:)
+    super(config: config, proxy_config: proxy_config)
+  end
+
+  def deploy_options
+    opts = super
+
+    opts[:host] = hosts if hosts.present?
+    opts[:tls] = proxy_config["ssl"].presence
+
+    opts
+  end
+
+  def directory
+    File.join config.run_directory, "loadbalancer"
+  end
+
+  def container_name
+    CONTAINER_NAME
+  end
+end