feat: use a sequence for the verification number on postgres

db/migrations/20201023_create_verification_number_sequence.rb

@@ -0,0 +1,20 @@
+require_relative 'migration_helper'
+
+Sequel.migration do
+  up do
+    if PactBroker::MigrationHelper.postgres?
+      row = from(:verification_sequence_number).select(:value).limit(1).first
+      start = row ? row[:value] + 1 : 1
+      run("CREATE SEQUENCE verification_number_sequence START WITH #{start}")
+    end
+  end
+
+  down do
+    if PactBroker::MigrationHelper.postgres?
+      nextval = execute("SELECT nextval('verification_number_sequence') as val") { |v| v.first["val"].to_i }
+      # Add a safety margin!
+      from(:verification_sequence_number).update(value: nextval + 100)
+      run("DROP SEQUENCE verification_number_sequence")
+    end
+  end
+end

lib/db.rb

@@ -60,6 +60,10 @@ def self.mysql?
     !!(PACT_BROKER_DB.adapter_scheme.to_s =~ /mysql/)
   end
 
+  def self.postgres?
+    !!(PACT_BROKER_DB.adapter_scheme.to_s == "postgres")
+  end
+
   PACT_BROKER_DB ||= connection_for_env ENV.fetch('RACK_ENV')
 
   def self.health_check

lib/pact_broker/verifications/sequence.rb

@@ -1,4 +1,6 @@
 require 'sequel'
+require 'pact_broker/repositories/helpers'
+
 
 module PactBroker
   module Verifications
@@ -7,22 +9,26 @@ class Sequence < Sequel::Model(:verification_sequence_number)
         # The easiest way to implement a cross database compatible sequence.
         # Sad, I know.
         def next_val
-          db.transaction do
-            for_update.first
-            select_all.update(value: Sequel[:value]+1)
-            row = first
-            if row
-              row.value
-            else
-              # The first row should have been created in the migration, so this code
-              # should only ever be executed in a test context.
-              # There would be a risk of a race condition creating two rows if this
-              # code executed in prod, as I don't think you can lock an empty table
-              # to prevent another record being inserted.
-              max_verification_number = PactBroker::Domain::Verification.max(:number)
-              value = max_verification_number ? max_verification_number + 100 : 1
-              insert(value: value)
-              value
+          if PactBroker::Repositories::Helpers.postgres?
+            db.execute("SELECT nextval('verification_number_sequence') as val") { |v| v.first["val"].to_i }
+          else
+            db.transaction do
+              for_update.first
+              select_all.update(value: Sequel[:value]+1)
+              row = first
+              if row
+                row.value
+              else
+                # The first row should have been created in the migration, so this code
+                # should only ever be executed in a test context.
+                # There would be a risk of a race condition creating two rows if this
+                # code executed in prod, as I don't think you can lock an empty table
+                # to prevent another record being inserted.
+                max_verification_number = PactBroker::Domain::Verification.max(:number)
+                value = max_verification_number ? max_verification_number + 100 : 1
+                insert(value: value)
+                value
+              end
             end
           end
         end

spec/lib/pact_broker/verifications/sequence_spec.rb

@@ -4,41 +4,67 @@ module PactBroker
   module Verifications
     describe Sequence do
       describe "#next_val", migration: true do
+        context "for proper databases with proper sequences", skip: !::DB.postgres? do
+          it "increments the value each time" do
+            PactBroker::Database.migrate
+            expect(Sequence.next_val).to eq 101
+            expect(Sequence.next_val).to eq 102
+          end
 
-        before do
-          PactBroker::Database.migrate
-        end
-
-        context "when there is a row in the verification_sequence_number table" do
-          before do
-            Sequence.select_all.delete
-            Sequence.insert(value: 1)
+          it "can rollback without duplicating a sequence number" do
+            PactBroker::Database.migrate
+            row = database.from(:verification_sequence_number).select(:value).limit(1).first
+            expect(row[:value]).to eq 100
+            Sequence.next_val
+            PactBroker::Database.migrate(20201006)
+            row = database.from(:verification_sequence_number).select(:value).limit(1).first
+            expect(row[:value]).to eq 202
           end
 
-          it "increments the value and returns it" do
-            expect(Sequence.next_val).to eq 2
+          it "can deal with there not being an existing value in the verification_sequence_number table" do
+            PactBroker::Database.migrate(20201006)
+            database.from(:verification_sequence_number).delete
+            PactBroker::Database.migrate
+            expect(Sequence.next_val).to eq 1
           end
         end
 
-        context "when there is no row in the verification_sequence_number table and no existing verifications" do
+        context "for databases without sequences", skip: ::DB.postgres? do
           before do
-            Sequence.select_all.delete
+            PactBroker::Database.migrate
           end
 
-          it "inserts and returns the value 1" do
-            expect(Sequence.next_val).to eq 1
+          context "when there is a row in the verification_sequence_number table" do
+            before do
+              Sequence.select_all.delete
+              Sequence.insert(value: 1)
+            end
+
+            it "increments the value and returns it" do
+              expect(Sequence.next_val).to eq 2
+            end
           end
-        end
 
-        context "when there is no row in the verification_sequence_number table and there are existing verifications" do
-          before do
-            Sequence.select_all.delete
-            TestDataBuilder.new.create_pact_with_hierarchy("A", "1", "B")
-              .create_verification(provider_version: "2")
+          context "when there is no row in the verification_sequence_number table and no existing verifications" do
+            before do
+              Sequence.select_all.delete
+            end
+
+            it "inserts and returns the value 1" do
+              expect(Sequence.next_val).to eq 1
+            end
           end
 
-          it "inserts a number that is guaranteed to be higher than any of the existing verification numbers from when we tried to do this without a sequence" do
-            expect(Sequence.next_val).to eq 101
+          context "when there is no row in the verification_sequence_number table and there are existing verifications" do
+            before do
+              Sequence.select_all.delete
+              TestDataBuilder.new.create_pact_with_hierarchy("A", "1", "B")
+                .create_verification(provider_version: "2")
+            end
+
+            it "inserts a number that is guaranteed to be higher than any of the existing verification numbers from when we tried to do this without a sequence" do
+              expect(Sequence.next_val).to eq 101
+            end
           end
         end
       end

tasks/database.rb

@@ -35,6 +35,7 @@ def ensure_database_dir_exists
     def drop_objects
       drop_views
       drop_tables
+      drop_sequences
     end
 
     def drop_tables
@@ -56,6 +57,12 @@ def drop_views
       end
     end
 
+    def drop_sequences
+      if psql?
+        database.run('DROP SEQUENCE verification_number_sequence')
+      end
+    end
+
     def create
       if psql?
         system('psql postgres -c "create database pact_broker"')