feat: reduce contention when updating the contract_data_updated_at fi…

…eld for integrations (#671)

lib/pact_broker/integrations/repository.rb

@@ -50,18 +50,43 @@ def delete(consumer_id, provider_id)
       # @param [PactBroker::Domain::Pacticipant, nil] consumer the consumer for the integration, or nil if for a provider-only event (eg. Pactflow provider contract published)
       # @param [PactBroker::Domain::Pacticipant] provider the provider for the integration
       def set_contract_data_updated_at(consumer, provider)
-        Integration
-          .where({ consumer_id: consumer&.id, provider_id: provider.id }.compact )
-          .update(contract_data_updated_at: Sequel.datetime_class.now)
+        set_contract_data_updated_at_for_multiple_integrations([OpenStruct.new(consumer: consumer, provider: provider)])
       end
 
 
-      # Sets the contract_data_updated_at for the integrations as specified by an array of objects which each have a consumer and provider
-      # @param [Array<Object>] where each object has a consumer and a provider
+      # Sets the contract_data_updated_at for the integrations as specified by an array of objects which each have a consumer and provider.
+      #
+      # The contract_data_updated_at attribute is only ever used for ordering the list of integrations on the index page of the *Pact Broker* UI,
+      # so that the most recently updated integrations (the ones you're likely working on) are showed at the top of the first page.
+      # There is often contention around updating it however, which can cause deadlocks, and slow down API responses.
+      # Because it's not a critical field (eg. it won't change any can-i-deploy results), the easiest way to reduce this contention
+      # is to just not update it if the row is locked, because if it is locked, the value of contract_data_updated_at is already
+      # going to be a date from a few seconds ago, which is perfectly fine for the purposes for which we are using the value.
+      #
+      # Notes on SKIP LOCKED:
+      # SKIP LOCKED is only supported by Postgres.
+      # When executing SELECT ... FOR UPDATE SKIP LOCKED, the SELECT will run immediately, not waiting for any other transactions,
+      # and only return rows that are not already locked by another transaction.
+      # The FOR UPDATE is required to make it work this way - SKIP LOCKED on its own does not work.
+      #
+      # @param [Array<Object>] where each object MAY have a consumer and does have a provider (for Pactflow provider contract published there is no consumer)
       def set_contract_data_updated_at_for_multiple_integrations(objects_with_consumer_and_provider)
-        consumer_and_provider_ids = objects_with_consumer_and_provider.collect{ | object | [object.consumer.id, object.provider.id] }.uniq
+        consumer_and_provider_ids = objects_with_consumer_and_provider.collect{ | object | { consumer_id: object.consumer&.id, provider_id: object.provider.id }.compact }.uniq
+
+        # MySQL doesn't support an UPDATE with a subquery. FFS. Really need to do a major version release and delete the support code.
+        criteria =  if Integration.dataset.supports_skip_locked?
+                      integration_ids_to_update = Integration
+                                                  .select(:id)
+                                                  .where(Sequel.|(*consumer_and_provider_ids))
+                                                  .for_update
+                                                  .skip_locked
+                      { id: integration_ids_to_update }
+                    else
+                      Sequel.|(*consumer_and_provider_ids)
+                    end
+
         Integration
-          .where([:consumer_id, :provider_id] => consumer_and_provider_ids)
+          .where(criteria)
           .update(contract_data_updated_at: Sequel.datetime_class.now)
       end
     end