Adding the sourcecode for tutorial tracked under BAEL-2957

java-es-cqrs/.gitignore

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+HELP.md
+/target/
+!.mvn/wrapper/maven-wrapper.jar
+
+### STS ###
+.apt_generated
+.classpath
+.factorypath
+.project
+.settings
+.springBeans
+.sts4-cache
+
+### IntelliJ IDEA ###
+.idea
+*.iws
+*.iml
+*.ipr
+
+### NetBeans ###
+/nbproject/private/
+/nbbuild/
+/dist/
+/nbdist/
+/.nb-gradle/
+/build/
+
+### VS Code ###
+.vscode/

java-es-cqrs/pom.xml

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+<project xmlns="http://maven.apache.org/POM/4.0.0"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+	<modelVersion>4.0.0</modelVersion>
+	<groupId>com.sapient.learning</groupId>
+	<artifactId>java-events</artifactId>
+	<version>0.0.1-SNAPSHOT</version>
+	<name>java-es-cqrs</name>
+	<properties>
+		<maven.compiler.source>1.8</maven.compiler.source>
+		<maven.compiler.target>1.8</maven.compiler.target>
+	</properties>
+	<dependencies>
+		<dependency>
+			<groupId>org.projectlombok</groupId>
+			<artifactId>lombok</artifactId>
+			<version>1.18.12</version>
+		</dependency>
+	</dependencies>
+</project>

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/Main.java

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+package com.baeldung.patterns.cqrs;
+
+import java.util.UUID;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import com.baeldung.patterns.cqrs.aggregates.UserAggregate;
+import com.baeldung.patterns.cqrs.commands.CreateUserCommand;
+import com.baeldung.patterns.cqrs.commands.UpdateUserCommand;
+import com.baeldung.patterns.cqrs.projections.UserProjection;
+import com.baeldung.patterns.cqrs.projectors.UserProjector;
+import com.baeldung.patterns.cqrs.queries.AddressByRegionQuery;
+import com.baeldung.patterns.cqrs.queries.ContactByTypeQuery;
+import com.baeldung.patterns.cqrs.repository.UserReadRepository;
+import com.baeldung.patterns.cqrs.repository.UserWriteRepository;
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.domain.User;
+
+public class Main {
+
+    public static void main(String[] args) throws Exception {
+        UserWriteRepository writeRepository = new UserWriteRepository();
+        UserReadRepository readRepository = new UserReadRepository();
+        UserProjector projector = new UserProjector(readRepository);
+        UserAggregate userAggregate = new UserAggregate(writeRepository);
+        UserProjection userProjection = new UserProjection(readRepository);
+
+        String userId = UUID.randomUUID()
+            .toString();
+        User user = null;
+        CreateUserCommand createUserCommand = new CreateUserCommand(userId, "Tom", "Sawyer");
+        user = userAggregate.handleCreateUserCommand(createUserCommand);
+        projector.project(user);
+
+        UpdateUserCommand updateUserCommand = new UpdateUserCommand(user.getUserid(), Stream.of(new Address("New York", "NY", "10001"), new Address("Los Angeles", "CA", "90001"))
+            .collect(Collectors.toSet()),
+            Stream.of(new Contact("EMAIL", "tom.sawyer@gmail.com"), new Contact("EMAIL", "tom.sawyer@rediff.com"))
+                .collect(Collectors.toSet()));
+        user = userAggregate.handleUpdateUserCommand(updateUserCommand);
+        projector.project(user);
+
+        updateUserCommand = new UpdateUserCommand(userId, Stream.of(new Address("New York", "NY", "10001"), new Address("Housten", "TX", "77001"))
+            .collect(Collectors.toSet()),
+            Stream.of(new Contact("EMAIL", "tom.sawyer@gmail.com"), new Contact("PHONE", "700-000-0001"))
+                .collect(Collectors.toSet()));
+        user = userAggregate.handleUpdateUserCommand(updateUserCommand);
+        projector.project(user);
+
+        AddressByRegionQuery addressByRegionQuery = new AddressByRegionQuery(user.getUserid(), "NY");
+        System.out.println(userProjection.handle(addressByRegionQuery));
+
+        ContactByTypeQuery contactByTypeQuery = new ContactByTypeQuery(user.getUserid(), "EMAIL");
+        System.out.println(userProjection.handle(contactByTypeQuery));
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/aggregates/UserAggregate.java

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+package com.baeldung.patterns.cqrs.aggregates;
+
+import com.baeldung.patterns.cqrs.commands.CreateUserCommand;
+import com.baeldung.patterns.cqrs.commands.UpdateUserCommand;
+import com.baeldung.patterns.cqrs.repository.UserWriteRepository;
+import com.baeldung.patterns.domain.User;
+
+public class UserAggregate {
+
+    private UserWriteRepository writeRepository;
+
+    public UserAggregate(UserWriteRepository repository) {
+        this.writeRepository = repository;
+    }
+
+    public User handleCreateUserCommand(CreateUserCommand command) {
+        User user = new User(command.getUserId(), command.getFiratName(), command.getLastName());
+        writeRepository.addUser(user.getUserid(), user);
+        return user;
+    }
+
+    public User handleUpdateUserCommand(UpdateUserCommand command) {
+        User user = writeRepository.getUser(command.getUserId());
+        user.setAddresses(command.getAddresses());
+        user.setContacts(command.getContacts());
+        writeRepository.addUser(user.getUserid(), user);
+        return user;
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/commands/CreateUserCommand.java

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+package com.baeldung.patterns.cqrs.commands;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Data
+@AllArgsConstructor
+public class CreateUserCommand {
+
+    private String userId;
+    private String firatName;
+    private String lastName;
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/commands/UpdateUserCommand.java

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+package com.baeldung.patterns.cqrs.commands;
+
+import java.util.HashSet;
+import java.util.Set;
+
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Data
+@AllArgsConstructor
+public class UpdateUserCommand {
+
+    private String userId;
+    private Set<Address> addresses = new HashSet<>();
+    private Set<Contact> contacts = new HashSet<>();
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/projections/UserProjection.java

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+package com.baeldung.patterns.cqrs.projections;
+
+import java.util.Set;
+
+import com.baeldung.patterns.cqrs.queries.AddressByRegionQuery;
+import com.baeldung.patterns.cqrs.queries.ContactByTypeQuery;
+import com.baeldung.patterns.cqrs.repository.UserReadRepository;
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.domain.UserAddress;
+import com.baeldung.patterns.domain.UserContact;
+
+public class UserProjection {
+
+    private UserReadRepository repository;
+
+    public UserProjection(UserReadRepository repository) {
+        this.repository = repository;
+    }
+
+    public Set<Contact> handle(ContactByTypeQuery query) throws Exception {
+        UserContact userContact = repository.getUserContact(query.getUserId());
+        if (userContact == null)
+            throw new Exception("User does not exist.");
+        return userContact.getContactByType()
+            .get(query.getContactType());
+    }
+
+    public Set<Address> handle(AddressByRegionQuery query) throws Exception {
+        UserAddress userAddress = repository.getUserAddress(query.getUserId());
+        if (userAddress == null)
+            throw new Exception("User does not exist.");
+        return userAddress.getAddressByRegion()
+            .get(query.getState());
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/projectors/UserProjector.java

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+package com.baeldung.patterns.cqrs.projectors;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+import com.baeldung.patterns.cqrs.repository.UserReadRepository;
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.domain.User;
+import com.baeldung.patterns.domain.UserAddress;
+import com.baeldung.patterns.domain.UserContact;
+
+public class UserProjector {
+
+    UserReadRepository readRepository = new UserReadRepository();
+
+    public UserProjector(UserReadRepository readRepository) {
+        this.readRepository = readRepository;
+    }
+
+    public void project(User user) {
+        UserContact userContact = readRepository.getUserContact(user.getUserid());
+        if (userContact == null)
+            userContact = new UserContact();
+        userContact.setContacts(user.getContacts());
+        Map<String, Set<Contact>> contactByType = new HashMap<>();
+        for (Contact contact : user.getContacts()) {
+            Set<Contact> contacts = contactByType.get(contact.getType());
+            if (contacts == null)
+                contacts = new HashSet<>();
+            contacts.add(contact);
+            contactByType.put(contact.getType(), contacts);
+        }
+        userContact.setContactByType(contactByType);
+        readRepository.addUserContact(user.getUserid(), userContact);
+
+        UserAddress userAddress = readRepository.getUserAddress(user.getUserid());
+        if (userAddress == null)
+            userAddress = new UserAddress();
+        userAddress.setAddresses(user.getAddresses());
+        Map<String, Set<Address>> addressByRegion = new HashMap<>();
+        for (Address address : user.getAddresses()) {
+            Set<Address> addresses = addressByRegion.get(address.getState());
+            if (addresses == null)
+                addresses = new HashSet<>();
+            addresses.add(address);
+            addressByRegion.put(address.getState(), addresses);
+        }
+        userAddress.setAddressByRegion(addressByRegion);
+        readRepository.addUserAddress(user.getUserid(), userAddress);
+
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/queries/AddressByRegionQuery.java

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+package com.baeldung.patterns.cqrs.queries;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Data
+@AllArgsConstructor
+public class AddressByRegionQuery {
+
+    private String userId;
+    private String state;
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/queries/ContactByTypeQuery.java

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+package com.baeldung.patterns.cqrs.queries;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Data
+@AllArgsConstructor
+public class ContactByTypeQuery {
+
+    private String userId;
+    private String contactType;
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/repository/UserReadRepository.java

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+package com.baeldung.patterns.cqrs.repository;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import com.baeldung.patterns.domain.UserAddress;
+import com.baeldung.patterns.domain.UserContact;
+
+public class UserReadRepository {
+
+    private Map<String, UserAddress> userAddress = new HashMap<>();
+
+    private Map<String, UserContact> userContact = new HashMap<>();
+
+    public void addUserAddress(String id, UserAddress user) {
+        userAddress.put(id, user);
+    }
+
+    public UserAddress getUserAddress(String id) {
+        return userAddress.get(id);
+    }
+
+    public void addUserContact(String id, UserContact user) {
+        userContact.put(id, user);
+    }
+
+    public UserContact getUserContact(String id) {
+        return userContact.get(id);
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/cqrs/repository/UserWriteRepository.java

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+package com.baeldung.patterns.cqrs.repository;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import com.baeldung.patterns.domain.User;
+
+public class UserWriteRepository {
+
+    private Map<String, User> store = new HashMap<>();
+
+    public void addUser(String id, User user) {
+        store.put(id, user);
+    }
+
+    public User getUser(String id) {
+        return store.get(id);
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/crud/Main.java

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+package com.baeldung.patterns.crud;
+
+import java.util.UUID;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import com.baeldung.patterns.crud.repository.UserRepository;
+import com.baeldung.patterns.crud.service.UserService;
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+
+public class Main {
+
+    public static void main(String[] args) throws Exception {
+
+        UserRepository repository = new UserRepository();
+        UserService service = new UserService(repository);
+        String userId = UUID.randomUUID()
+            .toString();
+
+        service.createUser(userId, "Tom", "Sawyer");
+        service.updateUser(userId, 
+            Stream.of(
+              new Contact("EMAIL", "tom.sawyer@gmail.com"), 
+              new Contact("EMAIL", "tom.sawyer@rediff.com"), 
+              new Contact("PHONE", "700-000-0001"))
+            .collect(Collectors.toSet()),
+            Stream.of(
+              new Address("New York", "NY", "10001"), 
+              new Address("Los Angeles", "CA", "90001"), 
+              new Address("Housten", "TX", "77001"))
+            .collect(Collectors.toSet()));
+        service.updateUser(userId, 
+            Stream.of(
+              new Contact("EMAIL", "tom.sawyer@gmail.com"), 
+              new Contact("PHONE", "700-000-0001"))
+            .collect(Collectors.toSet()),
+            Stream.of(
+              new Address("New York", "NY", "10001"), 
+              new Address("Housten", "TX", "77001"))
+            .collect(Collectors.toSet()));
+
+        System.out.println(service.getContactByType(userId, "EMAIL"));
+        System.out.println(service.getAddressByRegion(userId, "NY"));
+
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/crud/repository/UserRepository.java

@@ -0,0 +1,20 @@
+package com.baeldung.patterns.crud.repository;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import com.baeldung.patterns.domain.User;
+
+public class UserRepository {
+
+    private Map<String, User> store = new HashMap<>();
+
+    public void addUser(String id, User user) {
+        store.put(id, user);
+    }
+
+    public User getUser(String id) {
+        return store.get(id);
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/crud/service/UserService.java

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+package com.baeldung.patterns.crud.service;
+
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import com.baeldung.patterns.crud.repository.UserRepository;
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.domain.User;
+
+public class UserService {
+
+    private UserRepository repository;
+
+    public UserService(UserRepository repository) {
+        this.repository = repository;
+    }
+
+    public void createUser(String userId, String firstName, String lastName) {
+        User user = new User(userId, firstName, lastName);
+        repository.addUser(userId, user);
+    }
+
+    public void updateUser(String userId, Set<Contact> contacts, Set<Address> addresses) throws Exception {
+        User user = repository.getUser(userId);
+        if (user == null)
+            throw new Exception("User does not exist.");
+        user.setContacts(contacts);
+        user.setAddresses(addresses);
+        repository.addUser(userId, user);
+    }
+
+    public Set<Contact> getContactByType(String userId, String contactType) throws Exception {
+        User user = repository.getUser(userId);
+        if (user == null)
+            throw new Exception("User does not exit.");
+        Set<Contact> contacts = user.getContacts();
+        return contacts.stream()
+            .filter(c -> c.getType()
+                .equals(contactType))
+            .collect(Collectors.toSet());
+    }
+
+    public Set<Address> getAddressByRegion(String userId, String state) throws Exception {
+        User user = repository.getUser(userId);
+        if (user == null)
+            throw new Exception("User does not exist.");
+        Set<Address> addresses = user.getAddresses();
+        return addresses.stream()
+            .filter(a -> a.getState()
+                .equals(state))
+            .collect(Collectors.toSet());
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/domain/Address.java

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+package com.baeldung.patterns.domain;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Data
+@AllArgsConstructor
+public class Address {
+
+    private String city;
+    private String state;
+    private String postcode;
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/domain/Contact.java

@@ -0,0 +1,13 @@
+package com.baeldung.patterns.domain;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+
+@Data
+@AllArgsConstructor
+public class Contact {
+
+    private String type;
+    private String detail;
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/domain/User.java

@@ -0,0 +1,22 @@
+package com.baeldung.patterns.domain;
+
+import java.util.HashSet;
+import java.util.Set;
+
+import lombok.Data;
+import lombok.NonNull;
+import lombok.RequiredArgsConstructor;
+
+@Data
+@RequiredArgsConstructor
+public class User {
+    @NonNull
+    private String userid;
+    @NonNull
+    private String firstname;
+    @NonNull
+    private String lastname;
+    private Set<Contact> contacts = new HashSet<>();
+    private Set<Address> addresses = new HashSet<>();
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/domain/UserAddress.java

@@ -0,0 +1,16 @@
+package com.baeldung.patterns.domain;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+import lombok.Data;
+
+@Data
+public class UserAddress {
+
+    private Set<Address> addresses = new HashSet<>();
+    private Map<String, Set<Address>> addressByRegion = new HashMap<>();
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/domain/UserContact.java

@@ -0,0 +1,16 @@
+package com.baeldung.patterns.domain;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Set;
+
+import lombok.Data;
+
+@Data
+public class UserContact {
+
+    private Set<Contact> contacts = new HashSet<>();
+    private Map<String, Set<Contact>> contactByType = new HashMap<>();
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/Main.java

@@ -0,0 +1,48 @@
+package com.baeldung.patterns.es;
+
+import java.util.UUID;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.es.repository.EventStore;
+import com.baeldung.patterns.es.service.UserService;
+
+public class Main {
+
+    public static void main(String[] args) throws Exception {
+
+        EventStore repository = new EventStore();
+        UserService service = new UserService(repository);
+        String userId = UUID.randomUUID()
+            .toString();
+
+        service.createUser(userId, "Tom", "Sawyer");
+        service.updateUser(userId, 
+            Stream.of(
+              new Contact("EMAIL", "tom.sawyer@gmail.com"), 
+              new Contact("EMAIL", "tom.sawyer@rediff.com"), 
+              new Contact("PHONE", "700-000-0001"))
+            .collect(Collectors.toSet()),
+            Stream.of(
+              new Address("New York", "NY", "10001"), 
+              new Address("Los Angeles", "CA", "90001"), 
+              new Address("Housten", "TX", "77001"))
+            .collect(Collectors.toSet()));
+        service.updateUser(userId, 
+            Stream.of(
+              new Contact("EMAIL", "tom.sawyer@gmail.com"), 
+              new Contact("PHONE", "700-000-0001"))
+            .collect(Collectors.toSet()),
+            Stream.of(
+              new Address("New York", "NY", "10001"), 
+              new Address("Housten", "TX", "77001"))
+            .collect(Collectors.toSet()));
+
+        System.out.println(service.getContactByType(userId, "EMAIL"));
+        System.out.println(service.getAddressByRegion(userId, "NY"));
+
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/events/Event.java

@@ -0,0 +1,15 @@
+package com.baeldung.patterns.es.events;
+
+import java.util.Date;
+import java.util.UUID;
+
+import lombok.ToString;
+
+@ToString
+public abstract class Event {
+
+    public final UUID id = UUID.randomUUID();
+
+    public final Date created = new Date();
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/events/UserAddressAddedEvent.java

@@ -0,0 +1,16 @@
+package com.baeldung.patterns.es.events;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+
+@Data
+@AllArgsConstructor
+@EqualsAndHashCode(callSuper = false)
+public class UserAddressAddedEvent extends Event {
+
+    private String city;
+    private String state;
+    private String postCode;
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/events/UserAddressRemovedEvent.java

@@ -0,0 +1,16 @@
+package com.baeldung.patterns.es.events;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+
+@Data
+@AllArgsConstructor
+@EqualsAndHashCode(callSuper = false)
+public class UserAddressRemovedEvent extends Event {
+
+    private String city;
+    private String state;
+    private String postCode;
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/events/UserContactAddedEvent.java

@@ -0,0 +1,15 @@
+package com.baeldung.patterns.es.events;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+
+@Data
+@AllArgsConstructor
+@EqualsAndHashCode(callSuper = false)
+public class UserContactAddedEvent extends Event {
+
+    private String contactType;
+    private String contactDetails;
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/events/UserContactRemovedEvent.java

@@ -0,0 +1,15 @@
+package com.baeldung.patterns.es.events;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+
+@Data
+@AllArgsConstructor
+@EqualsAndHashCode(callSuper = false)
+public class UserContactRemovedEvent extends Event {
+
+    private String contactType;
+    private String contactDetails;
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/events/UserCreatedEvent.java

@@ -0,0 +1,16 @@
+package com.baeldung.patterns.es.events;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+
+@Data
+@AllArgsConstructor
+@EqualsAndHashCode(callSuper = false)
+public class UserCreatedEvent extends Event {
+
+    private String userId;
+    private String firstName;
+    private String lastName;
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/events/UserRemovedEvent.java

@@ -0,0 +1,12 @@
+package com.baeldung.patterns.es.events;
+
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.EqualsAndHashCode;
+
+@Data
+@AllArgsConstructor
+@EqualsAndHashCode(callSuper = false)
+public class UserRemovedEvent extends Event {
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/repository/EventStore.java

@@ -0,0 +1,29 @@
+package com.baeldung.patterns.es.repository;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+import com.baeldung.patterns.es.events.Event;
+
+public class EventStore {
+
+    private Map<String, List<Event>> store = new HashMap<>();
+
+    public void addEvent(String id, Event event) {
+        List<Event> events = store.get(id);
+        if (events == null) {
+            events = new ArrayList<Event>();
+            events.add(event);
+            store.put(id, events);
+        } else {
+            events.add(event);
+        }
+    }
+
+    public List<Event> getEvents(String id) {
+        return store.get(id);
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/service/UserService.java

@@ -0,0 +1,94 @@
+package com.baeldung.patterns.es.service;
+
+import java.util.List;
+import java.util.Set;
+import java.util.stream.Collectors;
+
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.domain.User;
+import com.baeldung.patterns.es.events.UserAddressAddedEvent;
+import com.baeldung.patterns.es.events.UserAddressRemovedEvent;
+import com.baeldung.patterns.es.events.UserContactAddedEvent;
+import com.baeldung.patterns.es.events.UserContactRemovedEvent;
+import com.baeldung.patterns.es.events.UserCreatedEvent;
+import com.baeldung.patterns.es.repository.EventStore;
+
+public class UserService {
+
+    private EventStore repository;
+
+    public UserService(EventStore repository) {
+        this.repository = repository;
+    }
+
+    public void createUser(String userId, String firstName, String lastName) {
+        UserCreatedEvent event = new UserCreatedEvent(userId, firstName, lastName);
+        repository.addEvent(userId, event);
+    }
+
+    public void updateUser(String userId, Set<Contact> contacts, Set<Address> addresses) throws Exception {
+        User user = UserUtility.recreateUserState(repository, userId);
+        if (user == null)
+            throw new Exception("User does not exist.");
+
+        List<Contact> contactsToRemove = user.getContacts()
+            .stream()
+            .filter(c -> !contacts.contains(c))
+            .collect(Collectors.toList());
+        for (Contact contact : contactsToRemove) {
+            UserContactRemovedEvent contactRemovedEvent = new UserContactRemovedEvent(contact.getType(), contact.getDetail());
+            repository.addEvent(userId, contactRemovedEvent);
+        }
+
+        List<Contact> contactsToAdd = contacts.stream()
+            .filter(c -> !user.getContacts()
+                .contains(c))
+            .collect(Collectors.toList());
+        for (Contact contact : contactsToAdd) {
+            UserContactAddedEvent contactAddedEvent = new UserContactAddedEvent(contact.getType(), contact.getDetail());
+            repository.addEvent(userId, contactAddedEvent);
+        }
+
+        List<Address> addressesToRemove = user.getAddresses()
+            .stream()
+            .filter(a -> !addresses.contains(a))
+            .collect(Collectors.toList());
+        for (Address address : addressesToRemove) {
+            UserAddressRemovedEvent addressRemovedEvent = new UserAddressRemovedEvent(address.getCity(), address.getState(), address.getPostcode());
+            repository.addEvent(userId, addressRemovedEvent);
+        }
+
+        List<Address> addressesToAdd = addresses.stream()
+            .filter(a -> !user.getAddresses()
+                .contains(a))
+            .collect(Collectors.toList());
+        for (Address address : addressesToAdd) {
+            UserAddressAddedEvent addressAddedEvent = new UserAddressAddedEvent(address.getCity(), address.getState(), address.getPostcode());
+            repository.addEvent(userId, addressAddedEvent);
+        }
+    }
+
+    public Set<Contact> getContactByType(String userId, String contactType) throws Exception {
+        User user = UserUtility.recreateUserState(repository, userId);
+        if (user == null)
+            throw new Exception("User does not exist.");
+        Set<Contact> contacts = user.getContacts();
+        return contacts.stream()
+            .filter(c -> c.getType()
+                .equals(contactType))
+            .collect(Collectors.toSet());
+    }
+
+    public Set<Address> getAddressByRegion(String userId, String state) throws Exception {
+        User user = UserUtility.recreateUserState(repository, userId);
+        if (user == null)
+            throw new Exception("User does not exist.");
+        Set<Address> addresses = user.getAddresses();
+        return addresses.stream()
+            .filter(a -> a.getState()
+                .equals(state))
+            .collect(Collectors.toSet());
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/es/service/UserUtility.java

@@ -0,0 +1,66 @@
+package com.baeldung.patterns.es.service;
+
+import java.util.List;
+import java.util.UUID;
+
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.domain.User;
+import com.baeldung.patterns.es.events.Event;
+import com.baeldung.patterns.es.events.UserAddressAddedEvent;
+import com.baeldung.patterns.es.events.UserAddressRemovedEvent;
+import com.baeldung.patterns.es.events.UserContactAddedEvent;
+import com.baeldung.patterns.es.events.UserContactRemovedEvent;
+import com.baeldung.patterns.es.events.UserCreatedEvent;
+import com.baeldung.patterns.es.events.UserRemovedEvent;
+import com.baeldung.patterns.es.repository.EventStore;
+
+public class UserUtility {
+
+    public static User recreateUserState(EventStore store, String userId) {
+        User user = null;
+
+        List<Event> events = store.getEvents(userId);
+        for (Event event : events) {
+            if (event instanceof UserCreatedEvent) {
+                UserCreatedEvent e = (UserCreatedEvent) event;
+                user = new User(UUID.randomUUID()
+                    .toString(), e.getFirstName(), e.getLastName());
+            }
+            if (event instanceof UserRemovedEvent) {
+                user = null;
+            }
+            if (event instanceof UserAddressAddedEvent) {
+                UserAddressAddedEvent e = (UserAddressAddedEvent) event;
+                Address address = new Address(e.getCity(), e.getState(), e.getPostCode());
+                if (user != null)
+                    user.getAddresses()
+                        .add(address);
+            }
+            if (event instanceof UserAddressRemovedEvent) {
+                UserAddressRemovedEvent e = (UserAddressRemovedEvent) event;
+                Address address = new Address(e.getCity(), e.getState(), e.getPostCode());
+                if (user != null)
+                    user.getAddresses()
+                        .remove(address);
+            }
+            if (event instanceof UserContactAddedEvent) {
+                UserContactAddedEvent e = (UserContactAddedEvent) event;
+                Contact contact = new Contact(e.getContactType(), e.getContactDetails());
+                if (user != null)
+                    user.getContacts()
+                        .add(contact);
+            }
+            if (event instanceof UserContactRemovedEvent) {
+                UserContactRemovedEvent e = (UserContactRemovedEvent) event;
+                Contact contact = new Contact(e.getContactType(), e.getContactDetails());
+                if (user != null)
+                    user.getContacts()
+                        .remove(contact);
+            }
+        }
+
+        return user;
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/escqrs/Main.java

@@ -0,0 +1,61 @@
+package com.baeldung.patterns.escqrs;
+
+import java.util.List;
+import java.util.UUID;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import com.baeldung.patterns.cqrs.commands.CreateUserCommand;
+import com.baeldung.patterns.cqrs.commands.UpdateUserCommand;
+import com.baeldung.patterns.cqrs.projections.UserProjection;
+import com.baeldung.patterns.cqrs.queries.AddressByRegionQuery;
+import com.baeldung.patterns.cqrs.queries.ContactByTypeQuery;
+import com.baeldung.patterns.cqrs.repository.UserReadRepository;
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.es.events.Event;
+import com.baeldung.patterns.es.repository.EventStore;
+import com.baeldung.patterns.escqrs.aggregates.UserAggregate;
+import com.baeldung.patterns.escqrs.projectors.UserProjector;
+
+public class Main {
+
+    public static void main(String[] args) throws Exception {
+
+        EventStore writeRepository = new EventStore();
+        UserReadRepository readRepository = new UserReadRepository();
+        UserProjector projector = new UserProjector(readRepository);
+        UserAggregate userAggregate = new UserAggregate(writeRepository);
+        UserProjection userProjection = new UserProjection(readRepository);
+
+        String userId = UUID.randomUUID()
+            .toString();
+        List<Event> events = null;
+        CreateUserCommand createUserCommand = new CreateUserCommand(userId, "Kumar", "Chandrakant");
+        events = userAggregate.handleCreateUserCommand(createUserCommand);
+
+        projector.project(userId, events);
+
+        UpdateUserCommand updateUserCommand = new UpdateUserCommand(userId, Stream.of(new Address("New York", "NY", "10001"), new Address("Los Angeles", "CA", "90001"))
+            .collect(Collectors.toSet()),
+            Stream.of(new Contact("EMAIL", "tom.sawyer@gmail.com"), new Contact("EMAIL", "tom.sawyer@rediff.com"))
+                .collect(Collectors.toSet()));
+        events = userAggregate.handleUpdateUserCommand(updateUserCommand);
+        projector.project(userId, events);
+
+        updateUserCommand = new UpdateUserCommand(userId, Stream.of(new Address("New York", "NY", "10001"), new Address("Housten", "TX", "77001"))
+            .collect(Collectors.toSet()),
+            Stream.of(new Contact("EMAIL", "tom.sawyer@gmail.com"), new Contact("PHONE", "700-000-0001"))
+                .collect(Collectors.toSet()));
+        events = userAggregate.handleUpdateUserCommand(updateUserCommand);
+        projector.project(userId, events);
+
+        AddressByRegionQuery addressByRegionQuery = new AddressByRegionQuery(userId, "NY");
+        System.out.println(userProjection.handle(addressByRegionQuery));
+
+        ContactByTypeQuery contactByTypeQuery = new ContactByTypeQuery(userId, "EMAIL");
+        System.out.println(userProjection.handle(contactByTypeQuery));
+
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/escqrs/aggregates/UserAggregate.java

@@ -0,0 +1,87 @@
+package com.baeldung.patterns.escqrs.aggregates;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.stream.Collectors;
+
+import com.baeldung.patterns.cqrs.commands.CreateUserCommand;
+import com.baeldung.patterns.cqrs.commands.UpdateUserCommand;
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.domain.User;
+import com.baeldung.patterns.es.events.Event;
+import com.baeldung.patterns.es.events.UserAddressAddedEvent;
+import com.baeldung.patterns.es.events.UserAddressRemovedEvent;
+import com.baeldung.patterns.es.events.UserContactAddedEvent;
+import com.baeldung.patterns.es.events.UserContactRemovedEvent;
+import com.baeldung.patterns.es.events.UserCreatedEvent;
+import com.baeldung.patterns.es.repository.EventStore;
+import com.baeldung.patterns.es.service.UserUtility;
+
+public class UserAggregate {
+
+    private EventStore writeRepository;
+
+    public UserAggregate(EventStore repository) {
+        this.writeRepository = repository;
+    }
+
+    public List<Event> handleCreateUserCommand(CreateUserCommand command) {
+        UserCreatedEvent event = new UserCreatedEvent(command.getUserId(), command.getFiratName(), command.getLastName());
+        writeRepository.addEvent(command.getUserId(), event);
+        return Arrays.asList(event);
+    }
+
+    public List<Event> handleUpdateUserCommand(UpdateUserCommand command) {
+        User user = UserUtility.recreateUserState(writeRepository, command.getUserId());
+        List<Event> events = new ArrayList<>();
+
+        List<Contact> contactsToRemove = user.getContacts()
+            .stream()
+            .filter(c -> !command.getContacts()
+                .contains(c))
+            .collect(Collectors.toList());
+        for (Contact contact : contactsToRemove) {
+            UserContactRemovedEvent contactRemovedEvent = new UserContactRemovedEvent(contact.getType(), contact.getDetail());
+            events.add(contactRemovedEvent);
+            writeRepository.addEvent(command.getUserId(), contactRemovedEvent);
+        }
+
+        List<Contact> contactsToAdd = command.getContacts()
+            .stream()
+            .filter(c -> !user.getContacts()
+                .contains(c))
+            .collect(Collectors.toList());
+        for (Contact contact : contactsToAdd) {
+            UserContactAddedEvent contactAddedEvent = new UserContactAddedEvent(contact.getType(), contact.getDetail());
+            events.add(contactAddedEvent);
+            writeRepository.addEvent(command.getUserId(), contactAddedEvent);
+        }
+
+        List<Address> addressesToRemove = user.getAddresses()
+            .stream()
+            .filter(a -> !command.getAddresses()
+                .contains(a))
+            .collect(Collectors.toList());
+        for (Address address : addressesToRemove) {
+            UserAddressRemovedEvent addressRemovedEvent = new UserAddressRemovedEvent(address.getCity(), address.getState(), address.getPostcode());
+            events.add(addressRemovedEvent);
+            writeRepository.addEvent(command.getUserId(), addressRemovedEvent);
+        }
+
+        List<Address> addressesToAdd = command.getAddresses()
+            .stream()
+            .filter(a -> !user.getAddresses()
+                .contains(a))
+            .collect(Collectors.toList());
+        for (Address address : addressesToAdd) {
+            UserAddressAddedEvent addressAddedEvent = new UserAddressAddedEvent(address.getCity(), address.getState(), address.getPostcode());
+            events.add(addressAddedEvent);
+            writeRepository.addEvent(command.getUserId(), addressAddedEvent);
+        }
+
+        return events;
+    }
+
+}

java-es-cqrs/src/main/java/com/baeldung/patterns/escqrs/projectors/UserProjector.java

@@ -0,0 +1,123 @@
+package com.baeldung.patterns.escqrs.projectors;
+
+import java.util.HashSet;
+import java.util.List;
+import java.util.Set;
+
+import com.baeldung.patterns.cqrs.repository.UserReadRepository;
+import com.baeldung.patterns.domain.Address;
+import com.baeldung.patterns.domain.Contact;
+import com.baeldung.patterns.domain.UserAddress;
+import com.baeldung.patterns.domain.UserContact;
+import com.baeldung.patterns.es.events.Event;
+import com.baeldung.patterns.es.events.UserAddressAddedEvent;
+import com.baeldung.patterns.es.events.UserAddressRemovedEvent;
+import com.baeldung.patterns.es.events.UserContactAddedEvent;
+import com.baeldung.patterns.es.events.UserContactRemovedEvent;
+import com.baeldung.patterns.es.events.UserCreatedEvent;
+import com.baeldung.patterns.es.events.UserRemovedEvent;
+
+public class UserProjector {
+
+    UserReadRepository readRepository = new UserReadRepository();
+
+    public UserProjector(UserReadRepository readRepository) {
+        this.readRepository = readRepository;
+    }
+
+    public void project(String userId, List<Event> events) {
+
+        for (Event event : events) {
+            if (event instanceof UserCreatedEvent)
+                apply(userId, (UserCreatedEvent) event);
+            if (event instanceof UserRemovedEvent)
+                apply(userId, (UserRemovedEvent) event);
+            if (event instanceof UserAddressAddedEvent)
+                apply(userId, (UserAddressAddedEvent) event);
+            if (event instanceof UserAddressRemovedEvent)
+                apply(userId, (UserAddressRemovedEvent) event);
+            if (event instanceof UserContactAddedEvent)
+                apply(userId, (UserContactAddedEvent) event);
+            if (event instanceof UserContactRemovedEvent)
+                apply(userId, (UserContactRemovedEvent) event);
+        }
+
+    }
+
+    public void apply(String userId, UserCreatedEvent event) {
+
+    }
+
+    public void apply(String userId, UserRemovedEvent event) {
+
+    }
+
+    public void apply(String userId, UserAddressAddedEvent event) {
+        Address address = new Address(event.getCity(), event.getState(), event.getPostCode());
+        UserAddress userAddress = readRepository.getUserAddress(userId);
+        if (userAddress == null)
+            userAddress = new UserAddress();
+        userAddress.getAddresses()
+            .add(address);
+        Set<Address> addresses = userAddress.getAddressByRegion()
+            .get(address.getState());
+        if (addresses == null)
+            addresses = new HashSet<>();
+        addresses.add(address);
+        userAddress.getAddressByRegion()
+            .put(address.getState(), addresses);
+        readRepository.addUserAddress(userId, userAddress);
+    }
+
+    public void apply(String userId, UserAddressRemovedEvent event) {
+        Address address = new Address(event.getCity(), event.getState(), event.getPostCode());
+        UserAddress userAddress = readRepository.getUserAddress(userId);
+        if (userAddress != null) {
+            userAddress.getAddresses()
+                .remove(address);
+            Set<Address> addresses = userAddress.getAddressByRegion()
+                .get(address.getState());
+            if (addresses != null) {
+                addresses.remove(address);
+                userAddress.getAddressByRegion()
+                    .put(address.getState(), addresses);
+            }
+            readRepository.addUserAddress(userId, userAddress);
+        }
+    }
+
+    public void apply(String userId, UserContactAddedEvent event) {
+        Contact contact = new Contact(event.getContactType(), event.getContactDetails());
+        UserContact userContact = readRepository.getUserContact(userId);
+        if (userContact == null)
+            userContact = new UserContact();
+        userContact.getContacts()
+            .add(contact);
+        Set<Contact> contacts = userContact.getContactByType()
+            .get(contact.getType());
+        if (contacts == null)
+            contacts = new HashSet<>();
+        contacts.add(contact);
+        userContact.getContactByType()
+            .put(contact.getType(), contacts);
+        readRepository.addUserContact(userId, userContact);
+    }
+
+    public void apply(String userId, UserContactRemovedEvent event) {
+        Contact contact = new Contact(event.getContactType(), event.getContactDetails());
+        UserContact userContact = readRepository.getUserContact(userId);
+        if (userContact != null) {
+            userContact.getContacts()
+                .remove(contact);
+            Set<Contact> contacts = userContact.getContactByType()
+                .get(contact.getType());
+            if (contacts != null) {
+                contacts.remove(contact);
+                userContact.getContactByType()
+                    .put(contact.getType(), contacts);
+            }
+            readRepository.addUserContact(userId, userContact);
+        }
+    }
+
+}