feat: Implement Write-Ahead Log (WAL) pattern (#3576) (#3582)

* feat: Implement Write-Ahead Log (WAL) pattern (#3576) * test: Add exception handling tests and verify coverage
2026-09-03 04:18:19 +00:00 · 2026-08-25 07:47:35 +03:00
parent aa11f6b6be
commit b0c34db084
12 changed files with 1116 additions and 0 deletions
@@ -253,6 +253,7 @@
    <module>view-helper</module>
    <module>virtual-proxy</module>
    <module>visitor</module>
+    <module>write-ahead-log</module>
    <module>backpressure</module>
    <module>actor-model</module>
    <module>rate-limiting-pattern</module>
@@ -0,0 +1,217 @@
+---
+title: "Write-Ahead Log (WAL) Pattern in Java: Ensuring Data Durability and Crash Recovery"
+shortTitle: Write-Ahead Log
+description: "Learn about the Write-Ahead Log (WAL) design pattern in Java. Discover how append-only logging guarantees data durability and fast crash recovery in database engines and distributed systems."
+category: Data Access
+language: en
+tag:
+  - Data access
+  - Storage
+  - Fault tolerance
+  - Transactions
+  - Performance
+---
+
+## Also known as
+
+* Append-Only Log
+* Redo Log
+* Journaling
+
+## Intent of Write-Ahead Log Pattern
+
+The Write-Ahead Log (WAL) design pattern ensures data durability and system recoverability in database engines, distributed consensus protocols, and transactional systems. It enforces a strict order of operations where any state mutation (e.g., insert, update, delete) must be written sequentially to an append-only log file on stable storage (disk) before it is applied to the main database state or in-memory storage structures.
+
+## Detailed Explanation of Write-Ahead Log Pattern with Real-World Examples
+
+Real-world example
+
+> Imagine an accountant managing a company's ledger. Before modifying the main financial summary balance sheets, the accountant immediately records every incoming transaction line-by-line into a sequential physical logbook. If power cuts out mid-day or the summary balance sheets are damaged, the accountant can re-open the physical logbook, replay every recorded entry from the beginning, and perfectly recalculate the final financial state.
+
+In plain words
+
+> Write-Ahead Log guarantees that no state mutation is lost during sudden system crashes by writing changes to a fast append-only disk log file before updating the in-memory store.
+
+Wikipedia says
+
+> In computer science, write-ahead logging (WAL) is a family of techniques for providing atomicity and durability (two of the ACID properties) in database systems. In a system using WAL, all modifications are written to a log before they are applied. Usually both redo and undo information are stored in the log.
+
+Class Diagram
+
+```mermaid
+classDiagram
+    class OperationType {
+        <<enumeration>>
+        SET
+        DELETE
+        CHECKPOINT
+    }
+
+    class LogEntry {
+        -long sequenceNumber
+        -OperationType type
+        -String key
+        -String value
+        +toLogString() String
+        +fromLogString(String line)$ LogEntry
+    }
+
+    class WriteAheadLog {
+        -File logFile
+        -AtomicLong sequenceNumberCounter
+        +append(OperationType type, String key, String value) LogEntry
+        +readAll() List~LogEntry~
+        +clear() void
+    }
+
+    class DatabaseStore {
+        -WriteAheadLog wal
+        -Map~String, String~ memTable
+        +put(String key, String value) void
+        +delete(String key) void
+        +get(String key) String
+        +checkpoint() void
+        +simulateCrash() void
+        +recover() void
+    }
+
+    DatabaseStore --> WriteAheadLog
+    WriteAheadLog --> LogEntry
+    LogEntry --> OperationType
+```
+
+## Programmatic Example of Write-Ahead Log Pattern in Java
+
+The `WriteAheadLog` class manages append-only sequential writes to disk:
+
+```java
+public class WriteAheadLog {
+  private final File logFile;
+  private final AtomicLong sequenceNumberCounter = new AtomicLong(0);
+
+  public synchronized LogEntry append(OperationType type, String key, String value) throws IOException {
+    long nextSeq = sequenceNumberCounter.incrementAndGet();
+    LogEntry entry = new LogEntry(nextSeq, type, key, value);
+
+    try (BufferedWriter writer = new BufferedWriter(new FileWriter(logFile, true))) {
+      writer.write(entry.toLogString());
+      writer.newLine();
+      writer.flush();
+    }
+    return entry;
+  }
+}
+```
+
+The `DatabaseStore` class coordinates writing to the log before modifying its in-memory `MemTable`:
+
+```java
+public class DatabaseStore {
+  private final WriteAheadLog wal;
+  private final Map<String, String> memTable = new HashMap<>();
+
+  public synchronized void put(String key, String value) throws IOException {
+    wal.append(OperationType.SET, key, value);
+    memTable.put(key, value);
+  }
+
+  public synchronized void delete(String key) throws IOException {
+    wal.append(OperationType.DELETE, key, null);
+    memTable.remove(key);
+  }
+
+  public synchronized void recover() {
+    memTable.clear();
+    List<LogEntry> entries = wal.readAll();
+    for (LogEntry entry : entries) {
+      if (entry.getType() == OperationType.SET) {
+        memTable.put(entry.getKey(), entry.getValue());
+      } else if (entry.getType() == OperationType.DELETE) {
+        memTable.remove(entry.getKey());
+      }
+    }
+  }
+}
+```
+
+The `App` class demonstrates initialization, writes, crash simulation, and WAL recovery:
+
+```java
+@Slf4j
+public class App {
+  public static void main(String[] args) {
+    try {
+      File logFile = File.createTempFile("wal_demo", ".log");
+      WriteAheadLog wal = new WriteAheadLog(logFile);
+      DatabaseStore store = new DatabaseStore(wal);
+
+      store.put("user:101", "Alice");
+      store.put("user:102", "Bob");
+      store.delete("user:103");
+
+      // Simulating system crash where in-memory state is lost
+      store.simulateCrash();
+
+      // System reboot & recovery from WAL log replay
+      store.recover();
+
+      LOGGER.info("MemTable post recovery: {}", store.getMemTableSnapshot());
+    } catch (IOException e) {
+      LOGGER.error("Error running WAL demo", e);
+    }
+  }
+}
+```
+
+Program output:
+
+```text
+15:45:00.100 [main] INFO com.iluwatar.writeaheadlog.App -- === 1. Initializing Storage Engine with WAL ===
+15:45:00.105 [main] INFO com.iluwatar.writeaheadlog.WriteAheadLog -- WAL Entry appended & flushed to disk: LogEntry(sequenceNumber=1, type=SET, key=user:101, value=Alice)
+15:45:00.106 [main] INFO com.iluwatar.writeaheadlog.DatabaseStore -- Applied SET operation to MemTable: user:101 = Alice
+15:45:00.107 [main] INFO com.iluwatar.writeaheadlog.App -- === 3. Simulating Unexpected System Crash ===
+15:45:00.108 [main] INFO com.iluwatar.writeaheadlog.DatabaseStore -- !!! SIMULATED SYSTEM CRASH: In-memory MemTable has been wiped !!!
+15:45:00.109 [main] INFO com.iluwatar.writeaheadlog.App -- === 4. System Restart & Recovery from WAL ===
+15:45:00.110 [main] INFO com.iluwatar.writeaheadlog.DatabaseStore -- Starting recovery process from WAL...
+15:45:00.112 [main] INFO com.iluwatar.writeaheadlog.DatabaseStore -- Recovery completed. Replayed 5 log entries into MemTable.
+15:45:00.113 [main] INFO com.iluwatar.writeaheadlog.App -- MemTable snapshot post recovery: {user:101=Alice, user:102=Bob Smith}
+```
+
+## When to Use the Write-Ahead Log Pattern in Java
+
+* Building storage engines or key-value data stores requiring ACID durability guarantees.
+* Implementing fault-tolerant distributed consensus protocols (e.g., Raft, Paxos).
+* System architectures where random disk I/O is expensive, allowing sequential append-only writes for maximum throughput.
+* Message brokers or event streams requiring replayability after failure.
+
+## Real-World Applications of Write-Ahead Log Pattern in Java
+
+* **PostgreSQL / MySQL (InnoDB):** Uses WAL / Redo Log for crash recovery and replication.
+* **SQLite:** Write-Ahead Logging mode for concurrency and atomic commits.
+* **Apache Cassandra / RocksDB:** Appends mutations to CommitLog / WAL before MemTable updates.
+* **Apache Kafka / Raft:** Log replication across distributed nodes for consensus and state machine replication.
+
+## Benefits and Trade-offs of Write-Ahead Log Pattern
+
+Benefits:
+
+* **High Performance:** Sequential disk writes are significantly faster than random disk updates (e.g., updating B-Trees directly).
+* **Durability & Fault Tolerance:** Guarantees no committed transaction is lost during sudden system crashes.
+* **Simplicity of Recovery:** Replaying ordered log records deterministically restores the exact last-known state.
+
+Trade-offs:
+
+* **Storage Overhead:** Log files grow over time, requiring periodic checkpointing and log truncation.
+* **Recovery Time:** Large log files without checkpoints can lead to slow startup/recovery times.
+
+## Related Java Design Patterns
+
+* [Event Sourcing](https://java-design-patterns.com/patterns/event-sourcing/): Captures state mutations as a sequence of events, similar to log replay.
+* [Command](https://java-design-patterns.com/patterns/command/): Encapsulates requests as objects, which can be serialized into WAL entries.
+* [Memento](https://java-design-patterns.com/patterns/memento/): Stores state snapshots (checkpoints) to truncate logs.
+
+## References and Credits
+
+* [Designing Data-Intensive Applications (Martin Kleppmann)](https://www.oreilly.com/library/view/designing-data-intensive-applications/9781491903063/)
+* [PostgreSQL Documentation: Write-Ahead Logging (WAL)](https://www.postgresql.org/docs/current/wal-intro.html)
+* [Raft Consensus Algorithm Paper](https://raft.github.io/)
@@ -0,0 +1,87 @@
+<?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>write-ahead-log</artifactId>
+  <version>1.26.0-SNAPSHOT</version>
+  <name>write-ahead-log</name>
+  <url>http://maven.apache.org</url>
+  <properties>
+    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+  </properties>
+  <dependencies>
+    <dependency>
+      <groupId>org.slf4j</groupId>
+      <artifactId>slf4j-api</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>ch.qos.logback</groupId>
+      <artifactId>logback-classic</artifactId>
+    </dependency>
+    <dependency>
+      <groupId>org.projectlombok</groupId>
+      <artifactId>lombok</artifactId>
+      <version>${lombok.version}</version>
+      <scope>provided</scope>
+    </dependency>
+    <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>
+  <build>
+    <plugins>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-assembly-plugin</artifactId>
+        <executions>
+          <execution>
+            <configuration>
+              <archive>
+                <manifest>
+                  <mainClass>com.iluwatar.writeaheadlog.App</mainClass>
+                </manifest>
+              </archive>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
+    </plugins>
+  </build>
+</project>
@@ -0,0 +1,82 @@
+/*
+ * 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.
+ */
+
+package com.iluwatar.writeaheadlog;
+
+import java.io.File;
+import java.io.IOException;
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * Main application class demonstrating the Write-Ahead Log (WAL) design pattern.
+ *
+ * <p>The WAL pattern guarantees durability by ensuring every mutation (SET, DELETE) is written to a
+ * persistent append-only log file on disk BEFORE updating in-memory state. If the system crashes
+ * unexpectedly, replaying log entries from the WAL file restores state.
+ */
+@Slf4j
+public class App {
+
+  /**
+   * Application entry point.
+   *
+   * @param args command line arguments
+   */
+  public static void main(String[] args) {
+    try {
+      File logFile = File.createTempFile("wal_demo", ".log");
+      logFile.deleteOnExit();
+
+      LOGGER.info(
+          "=== 1. Initializing Storage Engine with WAL at {} ===", logFile.getAbsolutePath());
+      WriteAheadLog wal = new WriteAheadLog(logFile);
+      DatabaseStore store = new DatabaseStore(wal);
+
+      LOGGER.info("=== 2. Performing Data Operations (Write-Ahead Logging) ===");
+      store.put("user:101", "Alice");
+      store.put("user:102", "Bob");
+      store.put("user:103", "Charlie");
+      store.put("user:102", "Bob Smith");
+      store.delete("user:103");
+      store.checkpoint();
+
+      LOGGER.info("MemTable snapshot before crash: {}", store.getMemTableSnapshot());
+
+      LOGGER.info("=== 3. Simulating Unexpected System Crash ===");
+      store.simulateCrash();
+      LOGGER.info("MemTable snapshot after crash: {}", store.getMemTableSnapshot());
+
+      LOGGER.info("=== 4. System Restart & Recovery from WAL ===");
+      store.recover();
+      LOGGER.info("MemTable snapshot post recovery: {}", store.getMemTableSnapshot());
+
+      if (logFile.exists()) {
+        logFile.delete();
+      }
+    } catch (IOException e) {
+      LOGGER.error("An error occurred during WAL demonstration: {}", e.getMessage(), e);
+    }
+  }
+}
@@ -0,0 +1,131 @@
+/*
+ * 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.
+ */
+
+package com.iluwatar.writeaheadlog;
+
+import java.io.IOException;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import lombok.Getter;
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * Storage engine demonstrating Write-Ahead Log (WAL) pattern. All mutations are logged and flushed
+ * to persistent WAL storage before modifying the in-memory MemTable.
+ */
+@Slf4j
+public class DatabaseStore {
+
+  @Getter private final WriteAheadLog wal;
+  private final Map<String, String> memTable = new HashMap<>();
+
+  /**
+   * Constructs DatabaseStore with the specified WriteAheadLog.
+   *
+   * @param wal persistent write-ahead log manager
+   */
+  public DatabaseStore(WriteAheadLog wal) {
+    this.wal = wal;
+  }
+
+  /**
+   * Stores a key-value pair. First writes to WAL on disk, then updates the in-memory MemTable.
+   *
+   * @param key target entry key
+   * @param value target entry value
+   * @throws IOException if writing to WAL fails
+   */
+  public synchronized void put(String key, String value) throws IOException {
+    wal.append(OperationType.SET, key, value);
+    memTable.put(key, value);
+    LOGGER.info("Applied SET operation to MemTable: {} = {}", key, value);
+  }
+
+  /**
+   * Removes a key-value pair. First writes DELETE operation to WAL on disk, then updates the
+   * in-memory MemTable.
+   *
+   * @param key target entry key to delete
+   * @throws IOException if writing to WAL fails
+   */
+  public synchronized void delete(String key) throws IOException {
+    wal.append(OperationType.DELETE, key, null);
+    memTable.remove(key);
+    LOGGER.info("Applied DELETE operation to MemTable: {}", key);
+  }
+
+  /**
+   * Retrieves value associated with key from the in-memory MemTable.
+   *
+   * @param key key to lookup
+   * @return value or null if non-existent
+   */
+  public synchronized String get(String key) {
+    return memTable.get(key);
+  }
+
+  /**
+   * Returns an unmodifiable view of current in-memory MemTable state.
+   *
+   * @return unmodifiable map of stored data
+   */
+  public synchronized Map<String, String> getMemTableSnapshot() {
+    return Collections.unmodifiableMap(new HashMap<>(memTable));
+  }
+
+  /**
+   * Writes a CHECKPOINT log record.
+   *
+   * @throws IOException if writing to WAL fails
+   */
+  public synchronized void checkpoint() throws IOException {
+    wal.append(OperationType.CHECKPOINT, null, null);
+    LOGGER.info("Checkpoint written to WAL.");
+  }
+
+  /** Simulates a system crash or power outage where in-memory state is wiped. */
+  public synchronized void simulateCrash() {
+    memTable.clear();
+    LOGGER.info("!!! SIMULATED SYSTEM CRASH: In-memory MemTable has been wiped !!!");
+  }
+
+  /** Replays log entries from persistent Write-Ahead Log to fully recover in-memory state. */
+  public synchronized void recover() {
+    LOGGER.info("Starting recovery process from WAL...");
+    memTable.clear();
+    List<LogEntry> entries = wal.readAll();
+
+    for (LogEntry entry : entries) {
+      if (entry.getType() == OperationType.SET) {
+        memTable.put(entry.getKey(), entry.getValue());
+      } else if (entry.getType() == OperationType.DELETE) {
+        memTable.remove(entry.getKey());
+      }
+    }
+    LOGGER.info("Recovery completed. Replayed {} log entries into MemTable.", entries.size());
+  }
+}
@@ -0,0 +1,83 @@
+/*
+ * 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.
+ */
+
+package com.iluwatar.writeaheadlog;
+
+import lombok.AllArgsConstructor;
+import lombok.EqualsAndHashCode;
+import lombok.Getter;
+import lombok.ToString;
+
+/** Represents a single record entry in the Write-Ahead Log. */
+@Getter
+@AllArgsConstructor
+@EqualsAndHashCode
+@ToString
+public class LogEntry {
+
+  private static final String DELIMITER = "|";
+
+  private final long sequenceNumber;
+  private final OperationType type;
+  private final String key;
+  private final String value;
+
+  /**
+   * Serializes the LogEntry to a delimited string format suitable for append-only logging.
+   *
+   * @return delimited log line representation
+   */
+  public String toLogString() {
+    return sequenceNumber
+        + DELIMITER
+        + type
+        + DELIMITER
+        + (key != null ? key : "")
+        + DELIMITER
+        + (value != null ? value : "");
+  }
+
+  /**
+   * Deserializes a delimited string line into a LogEntry instance.
+   *
+   * @param line serialized log line
+   * @return parsed LogEntry
+   * @throws IllegalArgumentException if the log line format is invalid
+   */
+  public static LogEntry fromLogString(String line) {
+    if (line == null || line.isBlank()) {
+      throw new IllegalArgumentException("Log line cannot be null or blank");
+    }
+    String[] parts = line.split("\\|", -1);
+    if (parts.length < 4) {
+      throw new IllegalArgumentException("Invalid log line format: " + line);
+    }
+    long sequenceNumber = Long.parseLong(parts[0]);
+    OperationType type = OperationType.valueOf(parts[1]);
+    String key = parts[2].isEmpty() ? null : parts[2];
+    String value = parts[3].isEmpty() ? null : parts[3];
+    return new LogEntry(sequenceNumber, type, key, value);
+  }
+}
@@ -0,0 +1,33 @@
+/*
+ * 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.
+ */
+
+package com.iluwatar.writeaheadlog;
+
+/** Enumeration representing the type of operations logged in the Write-Ahead Log. */
+public enum OperationType {
+  SET,
+  DELETE,
+  CHECKPOINT
+}
@@ -0,0 +1,134 @@
+/*
+ * 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.
+ */
+
+package com.iluwatar.writeaheadlog;
+
+import java.io.BufferedReader;
+import java.io.BufferedWriter;
+import java.io.File;
+import java.io.FileReader;
+import java.io.FileWriter;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.util.ArrayList;
+import java.util.List;
+import java.util.concurrent.atomic.AtomicLong;
+import lombok.Getter;
+import lombok.extern.slf4j.Slf4j;
+
+/**
+ * Manages sequential append-only logging to persistent disk storage. Ensures state changes are
+ * flushed to stable storage before in-memory updates.
+ */
+@Slf4j
+public class WriteAheadLog {
+
+  @Getter private final File logFile;
+  private final AtomicLong sequenceNumberCounter = new AtomicLong(0);
+
+  /**
+   * Initializes WriteAheadLog with target log file. If log file exists, calculates the initial
+   * sequence number from existing entries.
+   *
+   * @param logFile file to use for append-only log entries
+   */
+  public WriteAheadLog(File logFile) {
+    this.logFile = logFile;
+    initSequenceNumber();
+  }
+
+  private void initSequenceNumber() {
+    if (logFile.exists()) {
+      List<LogEntry> existingEntries = readAll();
+      if (!existingEntries.isEmpty()) {
+        long maxSeq = existingEntries.get(existingEntries.size() - 1).getSequenceNumber();
+        sequenceNumberCounter.set(maxSeq);
+      }
+    }
+  }
+
+  /**
+   * Appends an entry to the log file and flushes to ensure persistence.
+   *
+   * @param type operation type (SET, DELETE, CHECKPOINT)
+   * @param key operation target key
+   * @param value operation target value
+   * @return recorded LogEntry
+   * @throws IOException if writing to persistent storage fails
+   */
+  public synchronized LogEntry append(OperationType type, String key, String value)
+      throws IOException {
+    long nextSeq = sequenceNumberCounter.incrementAndGet();
+    LogEntry entry = new LogEntry(nextSeq, type, key, value);
+
+    try (BufferedWriter writer = new BufferedWriter(new FileWriter(logFile, true))) {
+      writer.write(entry.toLogString());
+      writer.newLine();
+      writer.flush();
+    }
+    LOGGER.info("WAL Entry appended & flushed to disk: {}", entry);
+    return entry;
+  }
+
+  /**
+   * Reads all log entries sequentially from the persistent log file.
+   *
+   * @return list of parsed LogEntries in sequential order
+   */
+  public synchronized List<LogEntry> readAll() {
+    List<LogEntry> entries = new ArrayList<>();
+    if (!logFile.exists()) {
+      return entries;
+    }
+
+    try (BufferedReader reader = new BufferedReader(new FileReader(logFile))) {
+      String line;
+      while ((line = reader.readLine()) != null) {
+        if (!line.isBlank()) {
+          entries.add(LogEntry.fromLogString(line));
+        }
+      }
+    } catch (IOException e) {
+      LOGGER.error("Failed to read log entries from WAL file: {}", logFile.getAbsolutePath(), e);
+    }
+    return entries;
+  }
+
+  /**
+   * Clears the log file and resets the sequence number counter. Typically invoked after a
+   * successful checkpoint.
+   */
+  public synchronized void clear() {
+    if (logFile.exists()) {
+      try {
+        Files.delete(logFile.toPath());
+        sequenceNumberCounter.set(0);
+        LOGGER.info("WAL log cleared successfully.");
+      } catch (IOException e) {
+        LOGGER.error("Failed to clear WAL file: {}", logFile.getAbsolutePath(), e);
+      }
+    }
+  }
+}
@@ -0,0 +1,38 @@
+/*
+ * 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.
+ */
+
+package com.iluwatar.writeaheadlog;
+
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
+
+import org.junit.jupiter.api.Test;
+
+class AppTest {
+
+  @Test
+  void testAppMainExecutesWithoutExceptions() {
+    assertDoesNotThrow(() -> App.main(new String[0]));
+  }
+}
@@ -0,0 +1,96 @@
+/*
+ * 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.
+ */
+
+package com.iluwatar.writeaheadlog;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.Map;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class DatabaseStoreTest {
+
+  private File tempFile;
+  private WriteAheadLog wal;
+  private DatabaseStore store;
+
+  @BeforeEach
+  void setUp() throws IOException {
+    tempFile = File.createTempFile("db_store_test", ".log");
+    wal = new WriteAheadLog(tempFile);
+    store = new DatabaseStore(wal);
+  }
+
+  @AfterEach
+  void tearDown() {
+    if (tempFile != null && tempFile.exists()) {
+      tempFile.delete();
+    }
+  }
+
+  @Test
+  void testPutAndGet() throws IOException {
+    store.put("key1", "val1");
+    assertEquals("val1", store.get("key1"));
+  }
+
+  @Test
+  void testDelete() throws IOException {
+    store.put("key1", "val1");
+    assertEquals("val1", store.get("key1"));
+
+    store.delete("key1");
+    assertNull(store.get("key1"));
+  }
+
+  @Test
+  void testSimulateCrashAndRecovery() throws IOException {
+    store.put("key1", "val1");
+    store.put("key2", "val2");
+    store.put("key1", "val1_updated");
+    store.delete("key2");
+    store.checkpoint();
+
+    Map<String, String> beforeCrashSnapshot = store.getMemTableSnapshot();
+    assertEquals("val1_updated", beforeCrashSnapshot.get("key1"));
+    assertNull(beforeCrashSnapshot.get("key2"));
+
+    store.simulateCrash();
+    assertNull(store.get("key1"));
+    assertNull(store.get("key2"));
+    assertTrue(store.getMemTableSnapshot().isEmpty());
+
+    store.recover();
+    Map<String, String> recoveredSnapshot = store.getMemTableSnapshot();
+    assertEquals("val1_updated", recoveredSnapshot.get("key1"));
+    assertNull(recoveredSnapshot.get("key2"));
+  }
+}
@@ -0,0 +1,80 @@
+/*
+ * 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.
+ */
+
+package com.iluwatar.writeaheadlog;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+class LogEntryTest {
+
+  @Test
+  void testToLogStringAndFromLogString() {
+    LogEntry entry = new LogEntry(1, OperationType.SET, "key1", "val1");
+    String logString = entry.toLogString();
+    assertEquals("1|SET|key1|val1", logString);
+
+    LogEntry parsed = LogEntry.fromLogString(logString);
+    assertEquals(entry, parsed);
+    assertEquals(1, parsed.getSequenceNumber());
+    assertEquals(OperationType.SET, parsed.getType());
+    assertEquals("key1", parsed.getKey());
+    assertEquals("val1", parsed.getValue());
+  }
+
+  @Test
+  void testDeleteLogEntrySerialization() {
+    LogEntry entry = new LogEntry(2, OperationType.DELETE, "key2", null);
+    String logString = entry.toLogString();
+    assertEquals("2|DELETE|key2|", logString);
+
+    LogEntry parsed = LogEntry.fromLogString(logString);
+    assertEquals(entry, parsed);
+    assertEquals("key2", parsed.getKey());
+    assertNull(parsed.getValue());
+  }
+
+  @Test
+  void testInvalidLogStringThrowsException() {
+    assertThrows(IllegalArgumentException.class, () -> LogEntry.fromLogString(null));
+    assertThrows(IllegalArgumentException.class, () -> LogEntry.fromLogString("   "));
+    assertThrows(IllegalArgumentException.class, () -> LogEntry.fromLogString("1|SET|key1"));
+  }
+
+  @Test
+  void testEqualsAndHashCode() {
+    LogEntry entry1 = new LogEntry(1, OperationType.SET, "k", "v");
+    LogEntry entry2 = new LogEntry(1, OperationType.SET, "k", "v");
+    LogEntry entry3 = new LogEntry(2, OperationType.SET, "k", "v");
+
+    assertEquals(entry1, entry2);
+    assertEquals(entry1.hashCode(), entry2.hashCode());
+    assertNotEquals(entry1, entry3);
+  }
+}
@@ -0,0 +1,134 @@
+/*
+ * 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.
+ */
+
+package com.iluwatar.writeaheadlog;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import java.io.File;
+import java.io.IOException;
+import java.nio.file.Files;
+import java.util.List;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class WriteAheadLogTest {
+
+  private File tempFile;
+  private WriteAheadLog wal;
+
+  @BeforeEach
+  void setUp() throws IOException {
+    tempFile = File.createTempFile("wal_test", ".log");
+    wal = new WriteAheadLog(tempFile);
+  }
+
+  @AfterEach
+  void tearDown() {
+    if (tempFile != null && tempFile.exists()) {
+      if (tempFile.isDirectory()) {
+        File[] files = tempFile.listFiles();
+        if (files != null) {
+          for (File f : files) {
+            f.delete();
+          }
+        }
+      }
+      tempFile.delete();
+    }
+  }
+
+  @Test
+  void testAppendAndReadAll() throws IOException {
+    LogEntry e1 = wal.append(OperationType.SET, "k1", "v1");
+    LogEntry e2 = wal.append(OperationType.SET, "k2", "v2");
+    LogEntry e3 = wal.append(OperationType.DELETE, "k1", null);
+
+    assertEquals(1, e1.getSequenceNumber());
+    assertEquals(2, e2.getSequenceNumber());
+    assertEquals(3, e3.getSequenceNumber());
+
+    List<LogEntry> entries = wal.readAll();
+    assertEquals(3, entries.size());
+    assertEquals(e1, entries.get(0));
+    assertEquals(e2, entries.get(1));
+    assertEquals(e3, entries.get(2));
+  }
+
+  @Test
+  void testSequenceNumberResumptionOnReopen() throws IOException {
+    wal.append(OperationType.SET, "k1", "v1");
+    wal.append(OperationType.SET, "k2", "v2");
+
+    WriteAheadLog reopenedWal = new WriteAheadLog(tempFile);
+    LogEntry newEntry = reopenedWal.append(OperationType.SET, "k3", "v3");
+
+    assertEquals(3, newEntry.getSequenceNumber());
+  }
+
+  @Test
+  void testClear() throws IOException {
+    wal.append(OperationType.SET, "k1", "v1");
+    assertTrue(tempFile.exists());
+
+    wal.clear();
+    assertFalse(tempFile.exists());
+
+    List<LogEntry> entries = wal.readAll();
+    assertNotNull(entries);
+    assertTrue(entries.isEmpty());
+  }
+
+  @Test
+  void testReadAllIOExceptionHandling() throws IOException {
+    File dir = Files.createTempDirectory("wal_dir_test").toFile();
+    WriteAheadLog dirWal = new WriteAheadLog(dir);
+
+    List<LogEntry> entries = dirWal.readAll();
+    assertNotNull(entries);
+    assertTrue(entries.isEmpty());
+
+    dir.delete();
+  }
+
+  @Test
+  void testClearIOExceptionHandling() throws IOException {
+    File dir = Files.createTempDirectory("wal_nonempty_dir_test").toFile();
+    File child = new File(dir, "child.txt");
+    child.createNewFile();
+
+    WriteAheadLog dirWal = new WriteAheadLog(dir);
+    dirWal.clear();
+
+    assertTrue(dir.exists());
+
+    child.delete();
+    dir.delete();
+  }
+}