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EVENT DRIVEN MICROSERVICES

EVENT BASED MICROSERVICES - Event Sourcing

In a Microservice Architecture, especially with Database per Microservice, the Microservices need to exchange data. For resilient, highly scalable, and fault-tolerant systems, they should communicate asynchronously by exchanging Events. In such a case, you may want to have Atomic operations, e.g., update the Database and send the message. If you have SQL databases and want to have distributed transactions for a high volume of data, you cannot use the two-phase locking (2PL) as it does not scale. If you use NoSQL Databases and want to have a distributed transaction, you cannot use 2PL as many NoSQL databases do not support two-phase locking. In such scenarios, use Event based Architecture with Event Sourcing. In traditional databases, the Business Entity with the current “state” is directly stored. In Event Sourcing, any state-changing event or other significant events are stored instead of the entities. It means the modifications of a Business Entity is saved as a series of immutable events. The State of a Business entity is deducted by reprocessing all the Events of that Business entity at a given time. Because data is stored as a series of events rather than via direct updates to data stores, various services can replay events from the event store to compute the appropriate state of their respective data stores.

Pros

  • Provide atomicity to highly scalable systems.
  • Automatic history of the entities, including time travel functionality.
  • Loosely coupled and event-driven Microservices.

Cons

  • Reading entities from the Event store becomes challenging and usually need an additional data store (CQRS pattern)
  • The overall complexity of the system increases and usually need Domain-Driven Design.
  • The system needs to handle duplicate events (idempotent) or missing events.
  • Migrating the Schema of events becomes challenging.

When to use Event Sourcing

  • Highly scalable transactional systems with SQL Databases.
  • Transactional systems with NoSQL Databases.
  • Highly scalable and resilient Microservice Architecture.
Typical Message Driven or Event-Driven systems (e-commerce, booking, and reservation systems).

When not to use Event Sourcing

  • Lowly scalable transactional systems with SQL Databases.
  • In simple Microservice Architecture where Microservices can exchange data synchronously (e.g., via API).

Enabling Technology Examples


Event Store: EventStoreDB, Apache Kafka, Confluent Cloud, AWS Kinesis, Azure Event Hub, GCP Pub/Sub, Azure Cosmos DB, MongoDB, Cassandra, Amazon DynamoDB.
Frameworks: Lagom, Akka, Spring, akkatecture, Axon, Eventuate

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