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In my understanding when database transactions span across microservices ,we can solve this problem with using message-broker(kafka,RabbitMQ etc) by publishing events so that Subscriber Microservices can update their database by listening to these events.
In case of exception we can send event for failure ,so that Subscriber services can update their state.
Is this not sufficient? What is the problem with this approach?
why and when we need event sourcing?
Do we need really event sourcing ?
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CQRS is implemented by a separation of responsibilities between commands and queries, and event sourcing is implemented by using the sequence of events to track changes in data.
CQRS is one of the important pattern when querying between microservices. We can use CQRS design pattern in order to avoid complex queries to get rid of inefficient joins. CQRS stands for Command and Query Responsibility Segregation.
Main reason for this failure was wrong implementation and most of them are not related to CQRS. What I learned from this is that we must understand key concepts of both Microservice and CQRS, only then we can merge these.
Event sourcing has several benefits: It solves one of the key problems in implementing an event-driven architecture and makes it possible to reliably publish events whenever state changes. Because it persists events rather than domain objects, it mostly avoids the object‑relational impedance mismatch problem.
I think you're confused about the microservices stuff :) They're a solution to design a scalable application, nothing to do with db transactions. Even more, a db transaction (ideally) shouldn't span across processes, nevermind microservices.
The pub-sub approach is how different micro services (works for in process models as well) communicate. Nothing to do with db transaction. Event sourcing is all about looking at the domain state as a collection of relevant changes. Very different objective compared to microservices.
We're using ES because we like to store domain events as being the 'single source of truth', microservices or not. It's all about domain model design.
Learn more about ES, DDD, CQRS and leave microservices for the future. They're too buzzwordy for now, few apps needs them, few developers can actually use them properly.
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Not at all. You can have well a very well defined microservices-styled architecture without CQRS and Event Sourcing. CQRS and Event Sourcing is a solution for intra-microservice design. You can choose to implement all or some of your microservices using CQRS and Event Sourcing.
Let's see how Event Sourcing may help you. Event Sourcing is an alternative to restoring your current state of an entity using events instead of an ORM like Entity Framework or Hibernate and a SQL database. Suppose you have a microservice to store data about Books. If you use SQL, you would have controllers and end-points to Create, Update, and Delete a book and store those books in a SQL table. If you want to update that book, then in order to get the current state you would go back to SQL table and query for that book (by its id) and then your ORM would convert that table representation into a book object (object‑relational impedance mismatch problem) and then you would apply the changes and save the changed book object back into SQL table. As an alternative, you can store events for the books objects in a NoSQL database like MongoDB or maybe an event store. Now in order to update the book, first you would want to restore the current state and you can do that by getting back all the events related to this book and replaying these events to restore the current state. Your events became a source of truth and you completely avoided the bottleneck of ORM mapping and SQL joins. Events are stored as JSON documents and are usually ultra-fast.
Now, coming to CQRS - CQRS is purely a pattern for separation of concerns. You would use CQRS to bifurcate your read-side from the write-side. End-points related to write-side like create, update, and delete live in one service and end-point for read-side live in another service. The advantage you get here is independent scaling, deploying, maintenance and many more. If your application is read-intensive, then have multiple numbers of instances deployed for read-side service.
If you want to learn more, feel free to PM me. Good Luck!
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You are describing a compensation pattern as a replacement of a distributed transaction. In a microservice-oriented architecture, this is a good approach to focus on availability by utilizing eventual consistency: Instead of having one centrally coordinated, distributed transaction across services, each service will execute its sub-task without a transactional context. If something goes wrong, each service will be informed about the failure and execute some kind of (semantic) compensation of the previous action. Thus, the transactional operation is eventually undone.
As you have already stated, communication can be done via a message bus system and there is no need for Event Sourcing or CQRS, the compensation pattern does not depend on these principles.
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