Prepare for zero downtime migrationszero-downtime-migrations

Starting with GitLab 9.1.0 we will no longer allow downtime migrations
unless absolutely necessary. This commit updates the various developer
guides and adds code that is necessary to make zero downtime migrations
less painful.

1 files changed, 52 insertions, 0 deletions

diff --git a/lib/gitlab/database/multi_threaded_migration.rb b/lib/gitlab/database/multi_threaded_migration.rb
new file mode 100644
index 00000000000..7ae5a4c17c8
--- /dev/null
+++ b/lib/gitlab/database/multi_threaded_migration.rb
@@ -0,0 +1,52 @@
+module Gitlab
+  module Database
+    module MultiThreadedMigration
+      MULTI_THREAD_AR_CONNECTION = :thread_local_ar_connection
+
+      # This overwrites the default connection method so that every thread can
+      # use a thread-local connection, while still supporting all of Rails'
+      # migration methods.
+      def connection
+        Thread.current[MULTI_THREAD_AR_CONNECTION] ||
+          ActiveRecord::Base.connection
+      end
+
+      # Starts a thread-pool for N threads, along with N threads each using a
+      # single connection. The provided block is yielded from inside each
+      # thread.
+      #
+      # Example:
+      #
+      #     with_multiple_threads(4) do
+      #       execute('SELECT ...')
+      #     end
+      #
+      # thread_count - The number of threads to start.
+      #
+      # join - When set to true this method will join the threads, blocking the
+      #        caller until all threads have finished running.
+      #
+      # Returns an Array containing the started threads.
+      def with_multiple_threads(thread_count, join: true)
+        pool = Gitlab::Database.create_connection_pool(thread_count)
+
+        threads = Array.new(thread_count) do
+          Thread.new do
+            pool.with_connection do |connection|
+              begin
+                Thread.current[MULTI_THREAD_AR_CONNECTION] = connection
+                yield
+              ensure
+                Thread.current[MULTI_THREAD_AR_CONNECTION] = nil
+              end
+            end
+          end
+        end
+
+        threads.each(&:join) if join
+
+        threads
+      end
+    end
+  end
+end