feature: Add optimistic offline lock (#1306)

optimistic-offline-lock/README.md

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+---
+title: Optimistic Offline Lock
+category: Concurrency
+language: en
+tag:
+- Data access
+---
+
+## Intent
+
+Provide an ability to avoid concurrent changes of one record in relational databases.
+
+## Explanation
+
+Each transaction during object modifying checks equation of object's version before start of transaction
+and before commit itself.
+
+**Real world example**
+> Since people love money, the best (and most common) example is banking system:
+> imagine you have 100$ on your e-wallet and two people are trying to send you 50$ both at a time.
+> Without locking, your system will start **two different thread**, each of whose will read your current balance
+> and just add 50$. The last thread won't re-read balance and will just rewrite it.
+> So, instead 200$ you will have only 150$.
+
+**In plain words**
+> Each transaction during object modifying will save object's last version and check it before saving.
+> If it differs, the transaction will be rolled back.
+
+**Wikipedia says**
+> Optimistic concurrency control (OCC), also known as optimistic locking,
+> is a concurrency control method applied to transactional systems such as
+> relational database management systems and software transactional memory.
+
+**Programmatic Example**  
+Let's simulate the case from *real world example*. Imagine we have next entity:
+
+```java
+import lombok.AllArgsConstructor;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+/**
+ * Bank card entity.
+ */
+@Data
+@NoArgsConstructor
+@AllArgsConstructor
+public class Card {
+
+  /**
+   * Primary key.
+   */
+  private long id;
+
+  /**
+   * Foreign key points to card's owner.
+   */
+  private long personId;
+
+  /**
+   * Sum of money. 
+   */
+  private float sum;
+
+  /**
+   * Current version of object;
+   */
+  private int version;
+}
+```
+
+Then the correct modifying will be like this:
+
+```java
+
+import lombok.RequiredArgsConstructor;
+
+/**
+ * Service to update {@link Card} entity.
+ */
+@RequiredArgsConstructor
+public class CardUpdateService implements UpdateService<Card> {
+
+  private final JpaRepository<Card> cardJpaRepository;
+
+  @Override
+  @Transactional(rollbackFor = ApplicationException.class) //will roll back transaction in case ApplicationException
+  public Card doUpdate(Card card, long cardId) {
+    float additionalSum = card.getSum();
+    Card cardToUpdate = cardJpaRepository.findById(cardId);
+    int initialVersion = cardToUpdate.getVersion();
+    float resultSum = cardToUpdate.getSum() + additionalSum;
+    cardToUpdate.setSum(resultSum);
+    //Maybe more complex business-logic e.g. HTTP-requests and so on
+
+    if (initialVersion != cardJpaRepository.getEntityVersionById(cardId)) {
+      String exMessage = String.format("Entity with id %s were updated in another transaction", cardId);
+      throw new ApplicationException(exMessage);
+    }
+
+    cardJpaRepository.update(cardToUpdate);
+    return cardToUpdate;
+  }
+}
+```
+
+## Applicability
+
+Since optimistic locking can cause degradation of system's efficiency and reliability due to
+many retries/rollbacks, it's important to use it safely. They are useful in case when transactions are not so long
+and does not distributed among many microservices, when you need to reduce network/database overhead.
+
+Important to note that you should not choose this approach in case when modifying one object
+in different threads is common situation.
+
+## Tutorials
+
+- [Offline Concurrency Control](https://www.baeldung.com/cs/offline-concurrency-control)
+- [Optimistic lock in JPA](https://www.baeldung.com/jpa-optimistic-locking)
+
+## Known uses
+
+- [Hibernate ORM](https://docs.jboss.org/hibernate/orm/4.3/devguide/en-US/html/ch05.html)
+
+## Consequences
+
+**Advantages**:
+
+- Reduces network/database overhead
+- Let to avoid database deadlock
+- Improve the performance and scalability of the application
+
+**Disadvantages**:
+
+- Increases complexity of the application
+- Requires mechanism of versioning
+- Requires rollback/retry mechanisms
+
+## Related patterns
+
+- [Pessimistic Offline Lock](https://martinfowler.com/eaaCatalog/pessimisticOfflineLock.html)
+
+## Credits
+
+- [Source (Martin Fowler)](https://martinfowler.com/eaaCatalog/optimisticOfflineLock.html)
+- [Advantages and disadvantages](https://www.linkedin.com/advice/0/what-benefits-drawbacks-using-optimistic)
+- [Comparison of optimistic and pessimistic locks](https://www.linkedin.com/advice/0/what-advantages-disadvantages-using-optimistic)

optimistic-offline-lock/pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+    This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+
+    The MIT License
+    Copyright © 2014-2022 Ilkka Seppälä
+
+    Permission is hereby granted, free of charge, to any person obtaining a copy
+    of this software and associated documentation files (the "Software"), to deal
+    in the Software without restriction, including without limitation the rights
+    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+    copies of the Software, and to permit persons to whom the Software is
+    furnished to do so, subject to the following conditions:
+
+    The above copyright notice and this permission notice shall be included in
+    all copies or substantial portions of the Software.
+
+    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+    THE SOFTWARE.
+
+-->
+<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>
+
+    <parent>
+        <groupId>com.iluwatar</groupId>
+        <artifactId>java-design-patterns</artifactId>
+        <version>1.26.0-SNAPSHOT</version>
+    </parent>
+
+    <artifactId>optimistic-offline-lock</artifactId>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.mockito</groupId>
+            <artifactId>mockito-core</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+</project>

optimistic-offline-lock/src/main/java/com/iluwatar/api/UpdateService.java

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+package com.iluwatar.api;
+
+/**
+ * Service for entity update.
+ *
+ * @param <T> target entity
+ */
+public interface UpdateService<T> {
+
+  /**
+   * Update entity.
+   *
+   * @param obj entity to update
+   * @param id  primary key
+   * @return modified entity
+   */
+  T doUpdate(T obj, long id);
+}

optimistic-offline-lock/src/main/java/com/iluwatar/exception/ApplicationException.java

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+package com.iluwatar.exception;
+
+/**
+ * Exception happens in application during business-logic execution.
+ */
+public class ApplicationException extends RuntimeException {
+
+  /**
+   * Inherited constructor with exception message.
+   *
+   * @param message exception message
+   */
+  public ApplicationException(String message) {
+    super(message);
+  }
+}

optimistic-offline-lock/src/main/java/com/iluwatar/model/Card.java

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+package com.iluwatar.model;
+
+
+import lombok.AllArgsConstructor;
+import lombok.Builder;
+import lombok.Data;
+import lombok.NoArgsConstructor;
+
+/**
+ * Bank card entity.
+ */
+@Data
+@Builder
+@NoArgsConstructor
+@AllArgsConstructor
+public class Card {
+
+  /**
+   * Primary key.
+   */
+  private long id;
+
+  /**
+   * Foreign key points to card's owner.
+   */
+  private long personId;
+
+  /**
+   * Sum of money.
+   */
+  private float sum;
+
+  /**
+   * Current version of object.
+   */
+  private int version;
+}

optimistic-offline-lock/src/main/java/com/iluwatar/repository/JpaRepository.java

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+package com.iluwatar.repository;
+
+/**
+ * Imitation of Spring's JpaRepository.
+ *
+ * @param <T> target database entity
+ */
+public interface JpaRepository<T> {
+
+  /**
+   * Get object by it's PK.
+   *
+   * @param id primary key
+   * @return {@link T}
+   */
+  T findById(long id);
+
+  /**
+   * Get current object version.
+   *
+   * @param id primary key
+   * @return object's version
+   */
+  int getEntityVersionById(long id);
+
+  /**
+   * Update object.
+   *
+   * @param obj entity to update
+   * @return number of modified records
+   */
+  int update(T obj);
+}

optimistic-offline-lock/src/main/java/com/iluwatar/service/CardUpdateService.java

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+package com.iluwatar.service;
+
+import com.iluwatar.api.UpdateService;
+import com.iluwatar.exception.ApplicationException;
+import com.iluwatar.model.Card;
+import com.iluwatar.repository.JpaRepository;
+import lombok.RequiredArgsConstructor;
+
+/**
+ * Service to update {@link Card} entity.
+ */
+@RequiredArgsConstructor
+public class CardUpdateService implements UpdateService<Card> {
+
+  private final JpaRepository<Card> cardJpaRepository;
+
+  @Override
+  public Card doUpdate(Card obj, long id) {
+    float additionalSum = obj.getSum();
+    Card cardToUpdate = cardJpaRepository.findById(id);
+    int initialVersion = cardToUpdate.getVersion();
+    float resultSum = cardToUpdate.getSum() + additionalSum;
+    cardToUpdate.setSum(resultSum);
+    //Maybe more complex business-logic e.g. HTTP-requests and so on
+
+    if (initialVersion != cardJpaRepository.getEntityVersionById(id)) {
+      String exMessage =
+          String.format("Entity with id %s were updated in another transaction", id);
+      throw new ApplicationException(exMessage);
+    }
+
+    cardJpaRepository.update(cardToUpdate);
+    return cardToUpdate;
+  }
+}