Discussion on: Active Record, Sidekiq, pools and threads

[Janko Marohnić]

Notice that the real problem is that ActiveRecord doesn't automatically release checked out connections back into the connection pool. You can see that by running the following script:

require "active_record"

ActiveRecord::Base.establish_connection(
  adapter:  "sqlite3",
  database: ":memory:"
)

threads = []

10.times do
  threads << Thread.new do
    ActiveRecord::Base.connection.execute "SELECT 1"
  end
end

threads.each(&:join)

ActiveRecord::ConnectionTimeoutError: could not obtain a connection from the pool within 5.000 seconds (waited 5.001 seconds); all pooled connections were in use

Contrast that with Sequel which checks out connections just fine:

require "sequel"

DB = Sequel.sqlite

threads = []

10.times do
  threads << Thread.new do
    DB.run "SELECT 1"
  end
end

threads.each(&:join)

(no errors)