Re-institute async mode

README.md

@@ -14,8 +14,9 @@ Solid Queue can be used with SQL databases such as MySQL, PostgreSQL, or SQLite,
   - [Dashboard UI Setup](#dashboard-ui-setup)
   - [Incremental adoption](#incremental-adoption)
   - [High performance requirements](#high-performance-requirements)
+- [Workers, dispatchers, and scheduler](#workers-dispatchers-and-scheduler)
+  - [Fork vs. async mode](#fork-vs-async-mode)
 - [Configuration](#configuration)
-  - [Workers, dispatchers, and scheduler](#workers-dispatchers-and-scheduler)
   - [Queue order and priorities](#queue-order-and-priorities)
   - [Queues specification and performance](#queues-specification-and-performance)
   - [Threads, processes, and signals](#threads-processes-and-signals)
@@ -179,9 +180,7 @@ end
 
 Solid Queue was designed for the highest throughput when used with MySQL 8+, MariaDB 10.6+, or PostgreSQL 9.5+, as they support `FOR UPDATE SKIP LOCKED`. You can use it with older versions, but in that case, you might run into lock waits if you run multiple workers for the same queue. You can also use it with SQLite on smaller applications.
 
-## Configuration
-
-### Workers, dispatchers, and scheduler
+## Workers, dispatchers, and scheduler
 
 We have several types of actors in Solid Queue:
 
@@ -190,7 +189,17 @@ We have several types of actors in Solid Queue:
 - The _scheduler_ manages [recurring tasks](#recurring-tasks), enqueuing jobs for them when they're due.
 - The _supervisor_ runs workers and dispatchers according to the configuration, controls their heartbeats, and stops and starts them when needed.
 
-Solid Queue's supervisor will fork a separate process for each supervised worker/dispatcher/scheduler.
+### Fork vs. async mode
+
+By default, Solid Queue runs in `fork` mode. This means the supervisor will fork a separate process for each supervised worker/dispatcher/scheduler. This provides the best isolation and performance, but can have additional memory usage. Alternatively, you can run all workers, dispatchers and schedulers in the same process as the supervisor, in different threads, with an `async` mode. You can choose this mode by running `bin/jobs` as:
+
+```
+bin/jobs --mode async
+```
+
+Or you can also set the environment variable `SOLID_QUEUE_SUPERVISOR_MODE` to `async`. If you use the `async` mode, the `processes` option in the configuration described below will be ignored.
+
+## Configuration
 
 By default, Solid Queue will try to find your configuration under `config/queue.yml`, but you can set a different path using the environment variable `SOLID_QUEUE_CONFIG` or by using the `-c/--config_file` option with `bin/jobs`, like this:
 
@@ -254,7 +263,7 @@ Here's an overview of the different options:
 
 - `threads`: this is the max size of the thread pool that each worker will have to run jobs. Each worker will fetch this number of jobs from their queue(s), at most and will post them to the thread pool to be run. By default, this is `3`. Only workers have this setting.
 It is recommended to set this value less than or equal to the queue database's connection pool size minus 2, as each worker thread uses one connection, and two additional connections are reserved for polling and heartbeat.
-- `processes`: this is the number of worker processes that will be forked by the supervisor with the settings given. By default, this is `1`, just a single process. This setting is useful if you want to dedicate more than one CPU core to a queue or queues with the same configuration. Only workers have this setting.
+- `processes`: this is the number of worker processes that will be forked by the supervisor with the settings given. By default, this is `1`, just a single process. This setting is useful if you want to dedicate more than one CPU core to a queue or queues with the same configuration. Only workers have this setting. **Note**: this option will be ignored if [running in `async` mode](#fork-vs-async-mode).
 - `concurrency_maintenance`: whether the dispatcher will perform the concurrency maintenance work. This is `true` by default, and it's useful if you don't use any [concurrency controls](#concurrency-controls) and want to disable it or if you run multiple dispatchers and want some of them to just dispatch jobs without doing anything else.
 
 
@@ -334,7 +343,7 @@ queues: back*
 
 Workers in Solid Queue use a thread pool to run work in multiple threads, configurable via the `threads` parameter above. Besides this, parallelism can be achieved via multiple processes on one machine (configurable via different workers or the `processes` parameter above) or by horizontal scaling.
 
-The supervisor is in charge of managing these processes, and it responds to the following signals:
+The supervisor is in charge of managing these processes, and it responds to the following signals when running in its own process via `bin/jobs` or with [the Puma plugin](#puma-plugin) with the default `fork` mode:
 - `TERM`, `INT`: starts graceful termination. The supervisor will send a `TERM` signal to its supervised processes, and it'll wait up to `SolidQueue.shutdown_timeout` time until they're done. If any supervised processes are still around by then, it'll send a `QUIT` signal to them to indicate they must exit.
 - `QUIT`: starts immediate termination. The supervisor will send a `QUIT` signal to its supervised processes, causing them to exit immediately.
 
@@ -603,6 +612,20 @@ that you set in production only. This is what Rails 8's default Puma config look
 
 **Note**: phased restarts are not supported currently because the plugin requires [app preloading](https://github.com/puma/puma?tab=readme-ov-file#cluster-mode) to work.
 
+### Running as a fork or asynchronously
+
+By default, the Puma plugin will fork additional processes for each worker and dispatcher so that they run in different processes. This provides the best isolation and performance, but can have additional memory usage.
+
+Alternatively, workers and dispatchers can be run within the same Puma process(s). To do so just configure the plugin as:
+
+```ruby
+plugin :solid_queue
+solid_queue_mode :async
+```
+
+Note that in this case, the `processes` configuration option will be ignored. See also [Fork vs. async mode](#fork-vs-async-mode).
+
+
 ## Jobs and transactional integrity
 :warning: Having your jobs in the same ACID-compliant database as your application data enables a powerful yet sharp tool: taking advantage of transactional integrity to ensure some action in your app is not committed unless your job is also committed and vice versa, and ensuring that your job won't be enqueued until the transaction within which you're enqueuing it is committed. This can be very powerful and useful, but it can also backfire if you base some of your logic on this behaviour, and in the future, you move to another active job backend, or if you simply move Solid Queue to its own database, and suddenly the behaviour changes under you. Because this can be quite tricky and many people shouldn't need to worry about it, by default Solid Queue is configured in a different database as the main app.
 

lib/puma/plugin/solid_queue.rb

@@ -1,41 +1,87 @@
 require "puma/plugin"
 
+module Puma
+  class DSL
+    def solid_queue_mode(mode = :fork)
+      @options[:solid_queue_mode] = mode.to_sym
+    end
+  end
+end
+
 Puma::Plugin.create do
   attr_reader :puma_pid, :solid_queue_pid, :log_writer, :solid_queue_supervisor
 
   def start(launcher)
     @log_writer = launcher.log_writer
     @puma_pid = $$
 
-    in_background do
-      monitor_solid_queue
+    if launcher.options[:solid_queue_mode] == :async
+      start_async(launcher)
+    else
+      start_forked(launcher)
     end
+  end
 
-    if Gem::Version.new(Puma::Const::VERSION) < Gem::Version.new("7")
-      launcher.events.on_booted do
-        @solid_queue_pid = fork do
-          Thread.new { monitor_puma }
-          SolidQueue::Supervisor.start
+  private
+    def start_forked(launcher)
+      in_background do
+        monitor_solid_queue
+      end
+
+      if Gem::Version.new(Puma::Const::VERSION) < Gem::Version.new("7")
+        launcher.events.on_booted do
+          @solid_queue_pid = fork do
+            Thread.new { monitor_puma }
+            SolidQueue::Supervisor.start(mode: :fork)
+          end
         end
+
+        launcher.events.on_stopped { stop_solid_queue_fork }
+        launcher.events.on_restart { stop_solid_queue_fork }
+      else
+        launcher.events.after_booted do
+          @solid_queue_pid = fork do
+            Thread.new { monitor_puma }
+            start_solid_queue(mode: :fork)
+          end
+        end
+
+        launcher.events.after_stopped { stop_solid_queue_fork }
+        launcher.events.before_restart { stop_solid_queue_fork }
       end
+    end
 
-      launcher.events.on_stopped { stop_solid_queue }
-      launcher.events.on_restart { stop_solid_queue }
-    else
-      launcher.events.after_booted do
-        @solid_queue_pid = fork do
-          Thread.new { monitor_puma }
-          SolidQueue::Supervisor.start
+    def start_async(launcher)
+      if Gem::Version.new(Puma::Const::VERSION) < Gem::Version.new("7")
+        launcher.events.on_booted do
+          start_solid_queue(mode: :async, standalone: false)
+        end
+
+        launcher.events.on_stopped { solid_queue_supervisor&.stop }
+
+        launcher.events.on_restart do
+          solid_queue_supervisor&.stop
+          start_solid_queue(mode: :async, standalone: false)
+        end
+      else
+        launcher.events.after_booted do
+          start_solid_queue(mode: :async, standalone: false)
+        end
+
+        launcher.events.after_stopped { solid_queue_supervisor&.stop }
+
+        launcher.events.before_restart do
+          solid_queue_supervisor&.stop
+          start_solid_queue(mode: :async, standalone: false)
         end
       end
+    end
 
-      launcher.events.after_stopped { stop_solid_queue }
-      launcher.events.before_restart { stop_solid_queue }
+    def start_solid_queue(**options)
+      @solid_queue_supervisor = SolidQueue::Supervisor.start(**options)
     end
-  end
 
-  private
-    def stop_solid_queue
+    def stop_solid_queue_fork
       Process.waitpid(solid_queue_pid, Process::WNOHANG)
       log "Stopping Solid Queue..."
       Process.kill(:INT, solid_queue_pid) if solid_queue_pid
@@ -48,7 +94,7 @@ def monitor_puma
     end
 
     def monitor_solid_queue
-      monitor(:solid_queue_dead?, "Detected Solid Queue has gone away, stopping Puma...")
+      monitor(:solid_queue_fork_dead?, "Detected Solid Queue has gone away, stopping Puma...")
     end
 
     def monitor(process_dead, message)
@@ -62,7 +108,7 @@ def monitor(process_dead, message)
       end
     end
 
-    def solid_queue_dead?
+    def solid_queue_fork_dead?
       if solid_queue_started?
         Process.waitpid(solid_queue_pid, Process::WNOHANG)
       end

lib/solid_queue/app_executor.rb

@@ -17,5 +17,15 @@ def handle_thread_error(error)
         SolidQueue.on_thread_error.call(error)
       end
     end
+
+    def create_thread(&block)
+      Thread.new do
+        Thread.current.name = name
+        block.call
+      rescue Exception => exception
+        handle_thread_error(exception)
+        raise
+      end
+    end
   end
 end

lib/solid_queue/async_supervisor.rb

@@ -0,0 +1,50 @@
+# frozen_string_literal: true
+
+module SolidQueue
+  class AsyncSupervisor < Supervisor
+    after_shutdown :terminate_gracefully, unless: :standalone?
+
+    def stop
+      super
+      @thread&.join
+    end
+
+    private
+      def supervise
+        if standalone? then super
+        else
+          @thread = create_thread { super }
+        end
+      end
+
+      def check_and_replace_terminated_processes
+        terminated_threads = process_instances.select { |thread_id, instance| !instance.alive? }
+        terminated_threads.each { |thread_id, instance| replace_thread(thread_id, instance) }
+      end
+
+      def replace_thread(thread_id, instance)
+        SolidQueue.instrument(:replace_thread, supervisor_pid: ::Process.pid) do |payload|
+          payload[:thread] = instance
+
+          error = Processes::ThreadTerminatedError.new(terminated_instance.name)
+          release_claimed_jobs_by(terminated_instance, with_error: error)
+
+          start_process(configured_processes.delete(thread_id))
+        end
+      end
+
+      def perform_graceful_termination
+        process_instances.values.each(&:stop)
+
+        Timer.wait_until(SolidQueue.shutdown_timeout, -> { all_processes_terminated? })
+      end
+
+      def perform_immediate_termination
+        exit!
+      end
+
+      def all_processes_terminated?
+        process_instances.values.none?(&:alive?)
+      end
+  end
+end

lib/solid_queue/cli.rb

@@ -8,6 +8,10 @@ class Cli < Thor
       desc: "Path to config file (default: #{Configuration::DEFAULT_CONFIG_FILE_PATH}).",
       banner: "SOLID_QUEUE_CONFIG"
 
+    class_option :mode, type: :string, default: "fork", enum: %w[ fork async ],
+      desc: "Whether to fork processes for workers and dispatchers (fork) or to run these in the same process as the supervisor (async) (default: fork).",
+      banner: "SOLID_QUEUE_SUPERVISOR_MODE"
+
     class_option :recurring_schedule_file, type: :string,
       desc: "Path to recurring schedule definition (default: #{Configuration::DEFAULT_RECURRING_SCHEDULE_FILE_PATH}).",
       banner: "SOLID_QUEUE_RECURRING_SCHEDULE"

lib/solid_queue/configuration.rb

@@ -56,6 +56,14 @@ def error_messages
       end
     end
 
+    def mode
+      @options[:mode].to_s.inquiry
+    end
+
+    def standalone?
+      mode.fork? || @options[:standalone]
+    end
+
     private
       attr_reader :options
 
@@ -84,6 +92,8 @@ def ensure_correctly_sized_thread_pool
 
       def default_options
         {
+          mode: ENV["SOLID_QUEUE_SUPERVISOR_MODE"] || :fork,
+          standalone: true,
           config_file: Rails.root.join(ENV["SOLID_QUEUE_CONFIG"] || DEFAULT_CONFIG_FILE_PATH),
           recurring_schedule_file: Rails.root.join(ENV["SOLID_QUEUE_RECURRING_SCHEDULE"] || DEFAULT_RECURRING_SCHEDULE_FILE_PATH),
           only_work: false,
@@ -110,7 +120,12 @@ def skip_recurring_tasks?
 
       def workers
         workers_options.flat_map do |worker_options|
-          processes = worker_options.fetch(:processes, WORKER_DEFAULTS[:processes])
+          processes = if mode.fork?
+            worker_options.fetch(:processes, WORKER_DEFAULTS[:processes])
+          else
+            1
+          end
+
           processes.times.map { Process.new(:worker, worker_options.with_defaults(WORKER_DEFAULTS)) }
         end
       end

lib/solid_queue/dispatcher.rb

@@ -3,6 +3,7 @@
 module SolidQueue
   class Dispatcher < Processes::Poller
     include LifecycleHooks
+
     attr_reader :batch_size
 
     after_boot :run_start_hooks

lib/solid_queue/fork_supervisor.rb

@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+module SolidQueue
+  class ForkSupervisor < Supervisor
+    private
+
+    def perform_graceful_termination
+      term_forks
+
+      Timer.wait_until(SolidQueue.shutdown_timeout, -> { all_processes_terminated? }) do
+        reap_terminated_forks
+      end
+    end
+
+    def perform_immediate_termination
+      quit_forks
+    end
+
+    def term_forks
+      signal_processes(process_instances.keys, :TERM)
+    end
+
+    def quit_forks
+      signal_processes(process_instances.keys, :QUIT)
+    end
+
+    def check_and_replace_terminated_processes
+      loop do
+        pid, status = ::Process.waitpid2(-1, ::Process::WNOHANG)
+        break unless pid
+
+        replace_fork(pid, status)
+      end
+    end
+
+    def reap_terminated_forks
+      loop do
+        pid, status = ::Process.waitpid2(-1, ::Process::WNOHANG)
+        break unless pid
+
+        if (terminated_fork = process_instances.delete(pid)) && !status.exited? || status.exitstatus > 0
+          error = Processes::ProcessExitError.new(status)
+          release_claimed_jobs_by(terminated_fork, with_error: error)
+        end
+
+        configured_processes.delete(pid)
+      end
+    rescue SystemCallError
+      # All children already reaped
+    end
+
+    def replace_fork(pid, status)
+      SolidQueue.instrument(:replace_fork, supervisor_pid: ::Process.pid, pid: pid, status: status) do |payload|
+        if terminated_fork = process_instances.delete(pid)
+          payload[:fork] = terminated_fork
+          error = Processes::ProcessExitError.new(status)
+          release_claimed_jobs_by(terminated_fork, with_error: error)
+
+          start_process(configured_processes.delete(pid))
+        end
+      end
+    end
+
+    def all_processes_terminated?
+      process_instances.empty?
+    end
+  end
+end