1. Added unit test LibrarianControllerTest for LibrarianController
class;
2. Fixed 'toString' method implementation for nullable fields
'credentials', 'birthDay' of empty PersonDto instance;
3. Path variables' values in ValidationControllerTest class represented
as method 'perform' parameters.

1. Added short description for project.

1. Added query method 'getNonAvailableBooks' to ApprovalRepository to
fetch set of approved for transfer but yet not returned by customer
books;
2. Method 'saveApproval' of DefaultApprovalService class checks if
requested for approval book is not available for order by calling method
'getNonAvailableBooks' of ApprovalRepository.

1. Added email sender bean 'mailSender' to LibraryApplication class;
2. Added interface NotificationService and implementation class
DefaultNotificationService for email delivery service;
3. Method 'save' of DefaultCustomerService sends email notification
after new customer registered;
4. Fixed mistake in authorization expression for method 'save' of
CustomerService interface (librarian's login doesn't need to coincide
with registered customer's login);
5. Added phone and email parameters to endpoint 'savePersonalData' of
CustomerController class; added UI controls for primary phone and email
to 'register-new-customer' form.

1. Enabled task scheduling in LibraryApplication class;
2. Added blocking queue for events in DefaultNotificationService class;
method 'notifyNewCustomerRegistered' appends new event to queue; method
'deliverMessages' periodically delivers accumulated messages; enum class
Event renamed to EventType; added record class Event to hold message
information.

1. Method 'getAuthorTitlePublishYear' added to entity class Book;
2. Method 'saveRequest' of class DefaultRequestService sends
notification after request entity stored successfully;
3. Added method 'notifyRequestReceived' declaration and implementation
to interface NotificationService and class DefaultNotificationService.

1. Added methods 'notifyApprovalReceived', 'notifyTransferAccomplished',
'notifyReturnAccomplished' to interface NotificationService;
2. Methods DefaultApprovalService.saveApproval,
DefaultTransferService.saveTransfer, DefaultReturnService.saveReturn
call appropriate methods of NotificationService to send notification by
email;
3. Added implementation for methods 'notifyApprovalReceived',
'notifyTransferAccomplished', 'notifyReturnAccomplished' to
DefaultNotificationService class;
4. Added mock for notification service to ApprovalServiceTest class;
checked cases when all books available and some books are unavailable.

1. Added Kafka configuration class KafkaConfiguration that defines beans
for topic list, Kafka admin, producer factory and template;
2. Added record Message which holds generic information to be sent via
sender classes;
3. MessageType extracted into public enum class;
4. Added interface Sender and implementing classes EmailSender,
KafkaSender to deliver messages to customer's email or Kafka topic;
5. Refactored class DefaultNotificationService:
a) added list of sender beans senderList;
b) non-related code extracted to EmailSender class, Message record,
MessageType enum class.

1. Added CantSendKafkaMessageException class;
2. KafkaSender uses CompletableFuture to handle error while sending
message to broker topic and re-throw wrapped exception
CantSendKafkaMessageException.

1. Added KafkaMessageSerializationException class;
2. Changed type of message from String to Message for producer factory
and template beans in KafkaConfiguration class;
3. Added methods 'getBookIds' to Message and MessageType classes;
4. Added custom Kafka message serializer class MessageSerializer.

1. Added interface CustomerRelated for customer related events, abstract
class Event and derived classes NewCustomerCreatedEvent,
RequestReceivedEvent, ApprovalReceivedEvent, ReturnAccomplishedEvent,
TransferAccomplishedEvent;
2. Added method 'accepts' to interface Sender,
DefaultNotificationService checks if sender accepts given kind of event;
3. Class EmailSender sends Email only for customer related events;
4. Class KafkaSender maps event to Kafka topic according to event class
name and sends message to that topic via MessageSerializer; topic list
moved from KafkaConfiguration to KafkaSender class;
5. Removed classes Message, MessageType;
6. Added UnacceptableOperationException class.

1. List of Kafka topics converted to map with event class as key in
KafkaSender class; method 'mapEventToTopic' fetches topic from the map
by event class;
2. Refactored method 'send' of class EmailSender which accepts only
CustomerRelated events.