Merge pull request #24376 from jeremy/cable/downsize-worker-pool-default-size

Cable: reconcile default worker pool size with low db conn pool size

2 files changed, 12 insertions, 2 deletions

diff --git a/actioncable/lib/action_cable/server/base.rb b/actioncable/lib/action_cable/server/base.rb
index 778f5ffeed..b1a0e11631 100644
--- a/actioncable/lib/action_cable/server/base.rb
+++ b/actioncable/lib/action_cable/server/base.rb
@@ -52,7 +52,17 @@ module ActionCable
         @event_loop || @mutex.synchronize { @event_loop ||= config.event_loop_class.new }
       end
 
-      # The thread worker pool for handling all the connection work on this server. Default size is set by config.worker_pool_size.
+      # The worker pool is where we run connection callbacks and channel actions. We do as little as possible on the server's main thread.
+      # The worker pool is an executor service that's backed by a pool of threads working from a task queue. The thread pool size maxes out
+      # at 4 worker threads by default. Tune the size yourself with config.action_cable.worker_pool_size.
+      #
+      # Using Active Record, Redis, etc within your channel actions means you'll get a separate connection from each thread in the worker pool.
+      # Plan your deployment accordingly: 5 servers each running 5 Puma workers each running an 8-thread worker pool means at least 200 database
+      # connections.
+      #
+      # Also, ensure that your database connection pool size is as least as large as your worker pool size. Otherwise, workers may oversubscribe
+      # the db connection pool and block while they wait for other workers to release their connections. Use a smaller worker pool or a larger
+      # db connection pool instead.
       def worker_pool
         @worker_pool || @mutex.synchronize { @worker_pool ||= ActionCable::Server::Worker.new(max_size: config.worker_pool_size) }
       end
diff --git a/actioncable/lib/action_cable/server/configuration.rb b/actioncable/lib/action_cable/server/configuration.rb
index 5fe71caed2..0bb378cf03 100644
--- a/actioncable/lib/action_cable/server/configuration.rb
+++ b/actioncable/lib/action_cable/server/configuration.rb
@@ -14,7 +14,7 @@ module ActionCable
         @log_tags = []
 
         @connection_class = ActionCable::Connection::Base
-        @worker_pool_size = 100
+        @worker_pool_size = 4
 
         @disable_request_forgery_protection = false
       end