diff --git a/.test.env b/.test.env
new file mode 100644
index 0000000..dcae954
--- /dev/null
+++ b/.test.env
@@ -0,0 +1,2 @@
+FIRESTORE_EMULATOR_HOST=localhost:8787
+MYSQL_ADDR=localhost
diff --git a/Makefile b/Makefile
index 6ae5a71..e3a7f8e 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,6 @@
 include .env
+include .test.env
+export
 
 .PHONY: openapi
 openapi: openapi_http openapi_js
@@ -44,4 +46,16 @@ lint:
 
 .PHONY: mycli
 mycli:
-	mycli -u ${MYSQL_USER} -p ${MYSQL_PASSWORD} ${MYSQL_DATABASE}
\ No newline at end of file
+	mycli -u ${MYSQL_USER} -p ${MYSQL_PASSWORD} ${MYSQL_DATABASE}
+
+INERNAL_PACKAGES := $(wildcard internal/*)
+
+ifeq (test,$(firstword $(MAKECMDGOALS)))
+  TEST_ARGS := $(subst $$,$$$$,$(wordlist 2,$(words $(MAKECMDGOALS)),$(MAKECMDGOALS)))
+  $(eval $(TEST_ARGS):;@:)
+endif
+
+.PHONY: test $(INERNAL_PACKAGES)
+test: $(INERNAL_PACKAGES)
+$(INERNAL_PACKAGES):
+	@(cd $@ && go test -count=1 -race ./... $(subst $$$$,$$,$(TEST_ARGS)))
diff --git a/README.md b/README.md
index 7d082e6..5f34ab8 100644
--- a/README.md
+++ b/README.md
@@ -15,7 +15,8 @@ No application is perfect from the beginning. With over a dozen coming articles,
 5. [**Business Applications in Go: Things to know about DRY**](https://threedots.tech/post/things-to-know-about-dry/?utm_source=github.com)
 6. [**When microservices in Go are not enough: introduction to DDD Lite**](https://threedots.tech/post/ddd-lite-in-go-introduction/?utm_source=github.com)
 7. [**Repository pattern: painless way to simplify your Go service logic**](https://threedots.tech/post/repository-pattern-in-go/?utm_source=github.com)
-8. *More articles are on the way!*
+8. [**4 practical principles of high-quality database integration tests in Go**](https://threedots.tech/post/database-integration-testing/?utm_source=github.com)
+9. *More articles are on the way!*
 
 ### Directories
 
diff --git a/internal/common/go.mod b/internal/common/go.mod
index 0cc3b33..7addfa1 100644
--- a/internal/common/go.mod
+++ b/internal/common/go.mod
@@ -3,6 +3,7 @@ module github.com/ThreeDotsLabs/wild-workouts-go-ddd-example/internal/common
 go 1.14
 
 require (
+	cloud.google.com/go v0.38.0
 	firebase.google.com/go v3.12.0+incompatible
 	github.com/dgrijalva/jwt-go v3.2.0+incompatible
 	github.com/go-chi/chi v4.1.0+incompatible
diff --git a/internal/trainer/hour_repository_test.go b/internal/trainer/hour_repository_test.go
index 2bf0bb9..847e3e5 100644
--- a/internal/trainer/hour_repository_test.go
+++ b/internal/trainer/hour_repository_test.go
@@ -145,11 +145,14 @@ func testUpdateHour_parallel(t *testing.T, repository hour.Repository) {
 	})
 	require.NoError(t, err)
 
-	workersCount := 10
+	workersCount := 20
 	workersDone := sync.WaitGroup{}
 	workersDone.Add(workersCount)
 
+	// closing startWorkers will unblock all workers at once,
+	// thanks to that it will be more likely to have race condition
 	startWorkers := make(chan struct{})
+	// if training was successfully scheduled, number of the worker is sent to this channel
 	trainingsScheduled := make(chan int, workersCount)
 
 	// we are trying to do race condition, in practice only one worker should be able to finish transaction
@@ -163,9 +166,11 @@ func testUpdateHour_parallel(t *testing.T, repository hour.Repository) {
 			schedulingTraining := false
 
 			err := repository.UpdateHour(ctx, hourTime, func(h *hour.Hour) (*hour.Hour, error) {
+				// training is already scheduled, nothing to do there
 				if h.HasTrainingScheduled() {
 					return h, nil
 				}
+				// training is not scheduled yet, so let's try to do that
 				if err := h.ScheduleTraining(); err != nil {
 					return nil, err
 				}
@@ -176,12 +181,15 @@ func testUpdateHour_parallel(t *testing.T, repository hour.Repository) {
 			})
 
 			if schedulingTraining && err == nil {
+				// training is only scheduled if UpdateHour didn't return an error
 				trainingsScheduled <- workerNum
 			}
 		}()
 	}
 
 	close(startWorkers)
+
+	// we are waiting, when all workers did the job
 	workersDone.Wait()
 	close(trainingsScheduled)