RabbitMQによるSagaパターン
RabbitMQを使って長時間実行される分散トランザクションを管理するSagaパターンを実装します。複数のマイクロサービスにまたがる複雑なワークフローを確実にオーケストレーションします。
「RabbitMQによるSagaパターン」はCoddyKit上の無料RabbitMQ Messaging & Async Systemsレッスンです。 これはレッスン2/4です。 下記で完全なレッスンを無料で読むことができます。その後、ブラウザ内の組み込みコードエディタと24時間対応のAIチューターでハンズオン演習できます。 これはRabbitMQ Messaging & Async Systems学習パスの一部であり、ウェブとCoddyKitアプリ全体で進捗が同期されます。 RabbitMQ Messaging & Async Systemsコースには全4レッスンが含まれています。
このレッスンの一部はまだ翻訳されておらず、英語で表示されています。
Distributed Transactions Unveiled
In microservices, a single business operation often spans multiple services. For example, placing an order might involve an Order Service, Inventory Service, and Payment Service.
A distributed transaction ensures that all these separate operations either succeed together or fail together, maintaining data consistency across your system.
Beyond Two-Phase Commit
Traditional database transactions often use a Two-Phase Commit (2PC) protocol to ensure atomicity. However, 2PC isn't ideal for microservices because:
- It creates tight coupling between services.
- It can lead to long-held locks, impacting availability.
- It's complex to implement and manage across different technologies.
We need a more flexible approach for distributed systems.
Introducing the Saga Pattern
The Saga pattern is a way to manage distributed transactions. Instead of a single atomic transaction, a saga is a sequence of local transactions, each updating its own service's database.
- Each local transaction publishes an event upon completion.
- These events trigger the next step in the saga.
- If a step fails, compensating transactions are used to undo previous successful steps.
The goal is eventual consistency.
Two Saga Flavors
There are two main ways to implement a Saga:
- Orchestration: A central "Saga Orchestrator" service manages and directs the workflow, telling each participant what to do next.
- Choreography: Each service produces and listens to events, deciding its own next action without a central coordinator.
For this lesson, we'll focus on the Orchestration approach, which often pairs well with message brokers like RabbitMQ.
The Brain of the Saga
The Saga Orchestrator is a dedicated service responsible for:
- Receiving the initial command (e.g., "Create Order").
- Sending commands to saga participants (microservices).
- Listening for events from participants.
- Maintaining the saga's state.
- Deciding the next step or initiating compensating transactions if a step fails.
RabbitMQ is perfect for the orchestrator to send commands and receive events.
Participants & Local Transactions
A Saga Participant is a microservice involved in the distributed transaction. When it receives a command from the orchestrator, it:
- Performs its own local ACID transaction.
- Updates its database.
- Publishes an event (e.g., "OrderCreatedEvent", "StockReservedEvent") indicating success or failure.
These events are crucial for the orchestrator to continue the saga.
Undo Actions: Compensation
What happens if a step in the saga fails? This is where compensating transactions come in. They are operations designed to reverse the effects of previously completed local transactions.
For example, if a "Process Payment" step fails, a compensating transaction for "Reserve Stock" might be to release the reserved items back into inventory.
This ensures the system returns to a consistent state, even if not fully rolled back.
Orchestrator Kicks Off
Let's imagine an order creation saga. The orchestrator receives a request and sends a command to the first participant (e.g., "Order Service"). Here's a simplified Java example:
public class OrderSagaOrchestrator {
public void startOrderCreationSaga(String orderId, String userId, double amount) {
System.out.println("Orchestrator: Starting saga for Order " + orderId);
// Simulate sending a message to Order Service
String command = "CreateOrderCommand { orderId: " + orderId + ", userId: " + userId + ", amount: " + amount + " }";
System.out.println("Orchestrator: Sending command to Order Service: " + command);
// In a real app, this would be a RabbitMQ message send
}
public static void main(String[] args) {
OrderSagaOrchestrator orchestrator = new OrderSagaOrchestrator();
orchestrator.startOrderCreationSaga("ORD-001", "user123", 99.99);
}
}Participant Responds
Now, let's look at the "Order Service" (a participant) receiving the command. It processes the order locally and then publishes an event.
public class OrderServiceParticipant {
public void handleCreateOrderCommand(String commandMessage) {
System.out.println("OrderService: Received command: " + commandMessage);
// Simulate local transaction (e.g., save order to DB)
String orderId = "ORD-001"; // Extract from commandMessage in real app
System.out.println("OrderService: Successfully created local order " + orderId);
// Simulate publishing an event back to the orchestrator
String event = "OrderCreatedEvent { orderId: " + orderId + ", status: 'PENDING_PAYMENT' }";
System.out.println("OrderService: Publishing event: " + event);
// In a real app, this would be a RabbitMQ message publish
}
public static void main(String[] args) {
OrderServiceParticipant participant = new OrderServiceParticipant();
participant.handleCreateOrderCommand("CreateOrderCommand { orderId: ORD-001, userId: user123, amount: 99.99 }");
}
}Orchestrator Continues Flow
The orchestrator listens for events like OrderCreatedEvent. Upon receiving it, it updates the saga's state and sends the next command, perhaps to an "Inventory Service" to reserve stock.
public class OrderSagaOrchestratorContinued {
public void handleOrderCreatedEvent(String eventMessage) {
System.out.println("Orchestrator: Received event: " + eventMessage);
String orderId = "ORD-001"; // Extract from eventMessage
// Update saga state (e.g., mark order as 'OrderCreated')
// Decide next step: send command to Inventory Service
String command = "ReserveStockCommand { orderId: " + orderId + ", productId: 'PROD-A', quantity: 2 }";
System.out.println("Orchestrator: Sending command to Inventory Service: " + command);
}
public static void main(String[] args) {
OrderSagaOrchestratorContinued orchestrator = new OrderSagaOrchestratorContinued();
orchestrator.handleOrderCreatedEvent("OrderCreatedEvent { orderId: ORD-001, status: 'PENDING_PAYMENT' }");
}
}Saga Essentials Check
Which of the following are key components or characteristics of the Saga Orchestration pattern?
Saga for Reliability
The Saga pattern is a powerful way to manage complex, long-running distributed transactions in microservice architectures.
- It enables eventual consistency without tight coupling.
- It uses local transactions and compensating transactions for resilience.
- RabbitMQ provides the perfect backbone for the orchestrator and participants to communicate reliably through commands and events.
While adding complexity, Sagas are essential for building robust distributed systems.
よくある質問
「RabbitMQによるSagaパターン」レッスンは無料ですか？
はい。「RabbitMQによるSagaパターン」の完全なテキストはこのウェブで無料で読めます。インタラクティブに演習し（組み込みコードエディタと24時間対応のAIチューター）、RabbitMQ Messaging & Async Systemsコースの残りをアンロックするには、CoddyKit PROにアップグレードしてください。 RabbitMQ Messaging & Async Systemsコースには全4レッスンが含まれています。
「RabbitMQによるSagaパターン」で何を学びますか？
RabbitMQを使って長時間実行される分散トランザクションを管理するSagaパターンを実装します。複数のマイクロサービスにまたがる複雑なワークフローを確実にオーケストレーションします。 ブラウザで直接実行するハンズオンコードでRabbitMQ Messaging & Async Systemsを演習し、24時間対応のAIチューターがレッスンを進める中での質問に答えます。
RabbitMQ Messaging & Async Systemsを始めるのに経験は必要ですか？
事前経験は必要ありません。CoddyKitのRabbitMQ Messaging & Async Systemsは初級者から上級者向けに構成されているため、ここから始めるか最初から始めて、自分のペースで進むことができます。 これはレッスン2/4です。
「RabbitMQによるSagaパターン」レッスンにはどのくらい時間がかかりますか？
ほとんどのCoddyKitレッスンは約5～10分かかります。各レッスンはコンパクトでインタラクティブなので、着実に進歩し、ウェブとアプリ全体で正確に前回の場所から再開できます。
このRabbitMQ Messaging & Async Systemsレッスンでコードを書いて実行できますか？
はい。すべてのRabbitMQ Messaging & Async Systemsレッスンに組み込みコードエディタが含まれているため、ブラウザでリアルコードを書いて実行し、即座のAIフィードバックを取得できます。ローカル設定は不要です。
このコースのすべてのレッスン
- メッセージ処理における冪等性
- RabbitMQによるSagaパターン
- Command-Query Responsibility Segregation（CQRS）
- 信頼性の高い公開を実現するOutboxパターン