diff --git a/database/client.go b/database/client.go
index 3188d56..9501b90 100644
--- a/database/client.go
+++ b/database/client.go
@@ -26,6 +26,14 @@ type Client struct {
 	userService UserService
 }
 
+// Service represents a service for interacting with the database.
+type Service interface {
+	Read() error
+	Write() error
+	Records() []user.Item
+	AddRecord(user.Item)
+}
+
 // NewClient returns a new database client.
 func NewClient(path string) *Client {
 	c := &Client{
@@ -88,5 +96,15 @@ func (c *Client) Write() error {
 	return err
 }
 
+// AddRecord adds a record to the database.
+func (c *Client) AddRecord(rec user.Item) {
+	c.data.Records = append(c.data.Records, rec)
+}
+
+// Records retrieves all records from the database.
+func (c *Client) Records() []user.Item {
+	return c.data.Records
+}
+
 // UserService returns the user service associated with the client.
 func (c *Client) UserService() user.Service { return &c.userService }
diff --git a/database/client_mock.go b/database/client_mock.go
new file mode 100644
index 0000000..a971636
--- /dev/null
+++ b/database/client_mock.go
@@ -0,0 +1,38 @@
+package database
+
+import (
+	"sync"
+
+	"github.com/josephspurrier/gocleanarchitecture/domain/user"
+)
+
+// MockService represents a service for managing users.
+type MockService struct {
+	records []user.Item
+	mutex   sync.RWMutex
+}
+
+// Reads reads database.
+func (c *MockService) Read() error {
+	return nil
+}
+
+// Write saves the database.
+func (c *MockService) Write() error {
+	return nil
+}
+
+// AddRecord adds a record to the database.
+func (c *MockService) AddRecord(rec user.Item) {
+	c.mutex.Lock()
+	c.records = append(c.records, rec)
+	c.mutex.Unlock()
+}
+
+// Records retrieves all records from the database.
+func (c *MockService) Records() []user.Item {
+	c.mutex.RLock()
+	r := c.records
+	c.mutex.RUnlock()
+	return r
+}
diff --git a/database/client_mock_test.go b/database/client_mock_test.go
new file mode 100644
index 0000000..123afd8
--- /dev/null
+++ b/database/client_mock_test.go
@@ -0,0 +1,23 @@
+package database_test
+
+import (
+	"testing"
+
+	"github.com/josephspurrier/gocleanarchitecture/database"
+	"github.com/josephspurrier/gocleanarchitecture/domain/user"
+)
+
+// TestMockService ensures the mock service works correctly.
+func TestMockService(t *testing.T) {
+	// Test the reading and writing.
+	s := new(database.MockService)
+	AssertEqual(t, s.Read(), nil)
+	AssertEqual(t, s.Write(), nil)
+
+	// Test adding a record and reading it.
+	u := new(user.Item)
+	u.Email = "jdoe@example.com"
+	u.Password = "Pa$$w0rd"
+	s.AddRecord(*u)
+	AssertEqual(t, len(s.Records()), 1)
+}
diff --git a/database/client_test.go b/database/client_test.go
index 0be8269..576a744 100644
--- a/database/client_test.go
+++ b/database/client_test.go
@@ -7,20 +7,6 @@ import (
 	"github.com/josephspurrier/gocleanarchitecture/database"
 )
 
-// AssertEqual throws an error if the two values are not equal.
-func AssertEqual(t *testing.T, actualValue interface{}, expectedValue interface{}) {
-	if actualValue != expectedValue {
-		t.Errorf("\n got: %v\nwant: %v", actualValue, expectedValue)
-	}
-}
-
-// AssertNotNil throws an error if the value is nil.
-func AssertNotNil(t *testing.T, actualValue interface{}) {
-	if actualValue == nil {
-		t.Errorf("\n got: %v\ndidn't want: %v", actualValue, nil)
-	}
-}
-
 // TestNew ensures the NewClient function works properly.
 func TestNewClient(t *testing.T) {
 	c := database.NewClient("db.json")
diff --git a/database/user_service.go b/database/user_service.go
index ec3e954..a27e516 100644
--- a/database/user_service.go
+++ b/database/user_service.go
@@ -8,7 +8,7 @@ import (
 
 // UserService represents a service for managing users.
 type UserService struct {
-	client *Client
+	client Service
 }
 
 // Authenticate returns an error if the email and password don't match.
@@ -20,7 +20,7 @@ func (s *UserService) Authenticate(d *user.Item) error {
 	}
 
 	// Determine if the record exists.
-	for _, v := range s.client.data.Records {
+	for _, v := range s.client.Records() {
 		if v.Email == d.Email {
 			if v.Password == d.Password {
 				return nil
@@ -43,7 +43,7 @@ func (s *UserService) User(email string) (*user.Item, error) {
 	}
 
 	// Determine if the record exists.
-	for _, v := range s.client.data.Records {
+	for _, v := range s.client.Records() {
 		if v.Email == email {
 			// Return the record.
 			return &v, nil
@@ -61,7 +61,7 @@ func (s *UserService) CreateUser(d *user.Item) error {
 	}
 
 	// Check if the user already exists
-	for _, v := range s.client.data.Records {
+	for _, v := range s.client.Records() {
 		if strings.ToLower(v.Email) == strings.ToLower(d.Email) {
 			// Return an error since the record exists.
 			return user.ErrAlreadyExist
@@ -69,7 +69,7 @@ func (s *UserService) CreateUser(d *user.Item) error {
 	}
 
 	// Add the record.
-	s.client.data.Records = append(s.client.data.Records, *d)
+	s.client.AddRecord(*d)
 
 	// Save the record to the database.
 	return s.client.Write()
diff --git a/database/util_test.go b/database/util_test.go
new file mode 100644
index 0000000..4313e76
--- /dev/null
+++ b/database/util_test.go
@@ -0,0 +1,17 @@
+package database_test
+
+import "testing"
+
+// AssertEqual throws an error if the two values are not equal.
+func AssertEqual(t *testing.T, actualValue interface{}, expectedValue interface{}) {
+	if actualValue != expectedValue {
+		t.Errorf("\n got: %v\nwant: %v", actualValue, expectedValue)
+	}
+}
+
+// AssertNotNil throws an error if the value is nil.
+func AssertNotNil(t *testing.T, actualValue interface{}) {
+	if actualValue == nil {
+		t.Errorf("\n got: %v\ndidn't want: %v", actualValue, nil)
+	}
+}