feat(worker): scale connection pool with worker threads

This will automatically increase the DB connection pool size if the number of threads needed in a worker is less than the maximum pool size configured.

app/lib/worker/process.rb

@@ -60,6 +60,7 @@ module Worker
     def run
       logger.tagged(component: "worker") do
         setup_traps
+        ensure_connection_pool_size_is_suitable
         start_work_threads
         start_tasks_thread
         wait_for_threads
@@ -96,6 +97,23 @@ module Worker
       @exit_pipe_read.wait_readable(wait_time) ? true : false
     end
 
+    # Ensure that the connection pool is big enough for the number of threads
+    # configured.
+    #
+    # @return [void]
+    def ensure_connection_pool_size_is_suitable
+      current_pool_size = ActiveRecord::Base.connection_pool.size
+      desired_pool_size = @thread_count + 3
+
+      return if current_pool_size >= desired_pool_size
+
+      logger.warn "number of worker threads (#{@thread_count}) is more  " \
+                  "than the db connection pool size (#{current_pool_size}+3), " \
+                  "increasing connection pool size to #{desired_pool_size}"
+
+      Postal.change_database_connection_pool_size(desired_pool_size)
+    end
+
     # Wait for all threads to complete
     #
     # @return [void]

lib/postal/config.rb

@@ -22,6 +22,8 @@ module Postal
 
   class << self
 
+    attr_writer :current_process_type
+
     # Return the path to the config file
     #
     # @return [String]
@@ -136,6 +138,21 @@ module Postal
       end
     end
 
+    # Change the connection pool size to the given size.
+    #
+    # @param new_size [Integer]
+    # @return [void]
+    def change_database_connection_pool_size(new_size)
+      ActiveRecord::Base.connection_pool.disconnect!
+
+      config = ActiveRecord::Base.configurations
+                                 .configs_for(env_name: Rails.env)
+                                 .first
+                                 .configuration_hash
+
+      ActiveRecord::Base.establish_connection(config.merge(pool: new_size))
+    end
+
   end
 
   Config = initialize_config

spec/lib/postal_spec.rb

@@ -9,4 +9,10 @@ RSpec.describe Postal do
       expect(Postal.signer.private_key.to_pem).to eq OpenSSL::PKey::RSA.new(File.read(Postal::Config.postal.signing_key_path)).to_pem
     end
   end
+
+  describe "#change_database_connection_pool_size" do
+    it "changes the connection pool size" do
+      expect { Postal.change_database_connection_pool_size(8) }.to change { ActiveRecord::Base.connection_pool.size }.from(5).to(8)
+    end
+  end
 end