Tune concurrency via env, fixes #2985

Changes.md

@@ -1,5 +1,23 @@
 # Sidekiq Changes
 
+HEAD
+-----------
+
+- Allow tuning of concurrency with the `RAILS_MAX_THREADS` env var. [#2985]
+  This is the same var used by Puma so you can tune all of your systems
+  the same way:
+```sh
+web: RAILS_MAX_THREADS=5 bundle exec puma ...
+worker: RAILS_MAX_THREADS=10 bundle exec sidekiq ...
+```
+Using `-c` or `config/sidekiq.yml` overrides this setting.  I recommend
+adjusting your `config/database.yml` to use it too so connections are
+auto-scaled:
+```yaml
+  pool: <%= ENV['RAILS_MAX_THREADS'] || 5 %>
+```
+
+
 4.1.4
 -----------
 

lib/sidekiq/cli.rb

@@ -208,6 +208,7 @@ module Sidekiq
       opts = parse_config(cfile).merge(opts) if cfile
 
       opts[:strict] = true if opts[:strict].nil?
+      opts[:concurrency] = Integer(ENV["RAILS_MAX_THREADS"]) if !opts[:concurrency] && ENV["RAILS_MAX_THREADS"]
 
       options.merge!(opts)
     end

test/test_cli.rb

@@ -42,6 +42,18 @@ class TestCli < Sidekiq::Test
       assert_equal 60, Sidekiq.options[:concurrency]
     end
 
+    it 'changes concurrency with ENV' do
+      begin
+        ENV['RAILS_MAX_THREADS'] = '9'
+        @cli.parse(['sidekiq', '-c', '60', '-r', './test/fake_env.rb'])
+        assert_equal 60, Sidekiq.options[:concurrency]
+        @cli.parse(['sidekiq', '-r', './test/fake_env.rb'])
+        assert_equal 9, Sidekiq.options[:concurrency]
+      ensure
+        ENV.delete('RAILS_MAX_THREADS')
+      end
+    end
+
     it 'changes queues' do
       @cli.parse(['sidekiq', '-q', 'foo', '-r', './test/fake_env.rb'])
       assert_equal ['foo'], Sidekiq.options[:queues]