You have questions, I have answers… well mainly opinions! Here are the top questions I was asked for this Software Architecture Q&A. As you can expect they are around Microservices, Messaging, CQRS & Event Sourcing.
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What is the difference between SOA and Microservices?
I get this question often and I was expecting it for this Software Architecture Q&A. I believe it’s because of how I present Event Driven Architecture in relation to Microservices. The answer to this question really depends on your definition of both SOA and Microservices. The definition of Microservices that I generally use is from Adrian Cockcroft:
Loosely coupled service oriented architecture with bounded contexts.
If I dissect that definition, Service Oriented Architecture that’s loosely coupled. Loosely coupled implying Event Driven Architecture.
In my mind, SOA was always about being loosely coupled, however that may not have been how the general developer community understood it at the time.
The Bounded Context portion of the definition is from Domain Driven Design. Bounded Contexts are about defining boundaries. Services should own a set of business capabilities and the data behind those capabilities. You aren’t sharing behaviors or data between services. Services are independent. And because they are independent communication is done in a loosely coupled fashion via an Event or Message Driven Architecture.
So my answer to this question is nothing is different between the two.
Is the saga an anti-pattern since you have a transaction spanning multiple bounded contexts?
No, a saga is not an anti-pattern. It’s responsible for coordinating a long-running business process between multiple services where there are no distributed transactions. Since there can be failures, the saga is responsible for handling failures by sending compensating actions to the appropriate services. The saga is a centralized place to handle the workflow of a long-running business process. And when I say long-running business process, that could mean milliseconds to weeks.
An alternative to a Saga is using Event Choreography, which removes the centralized logic of orchestration. Event Choreography is having all services consume and produce events that ultimately fulfill the long-running business process.
Check out my post on Event Choregrophay and Orchestration for more.
When should I choose CQRS over CRUD based “RESTful” endpoints?
Choose CQRS over CRUD when you want to be explicit.
With CRUD, when you’re making state changes via Create, Update, Delete, you aren’t capturing explicitly why it’s occurring. For example, if you’re performing an Update to a customer why is that happening? Did their address change? Did their discount rate change? With CRUD based approach, you don’t know exactly, you’d have to imply it based on the change being made.
With CQRS, all your commands are making those state changes explicit. You’re not “updating a customer” but rather you’re using explicit commands such as ChangeCustomerAddress or IncreaseDiscountRate. Because commands are explicit, you can then infer various events to be published from those commands.
If you wanted to publish an event based on CRUD, you would have very generic events such as CustomerUpdated. If you’re using CQRS, you’d have events such as CustomerAdresssChanged and CustomerDiscountRateIncreased.
How to handle versioning when Event Sourcing?
No Software Architecture Q&A on this Blog/YouTube could happen without a few questions about Event Sourcing.
The definitive answer to versioning is Greg Young’s Versioning in an Event Sourced System eBook.
I often like to compare this to a relational database. If you were adding a new column to a table, you would either need to make the column nullable or give it a default value and then possibly backfill existing data.
With event sourcing, making a new property on an existing event nullable is exactly the same. You’d need to deal with the value being null. If it’s not null and has a default value the situation is still the same as a relational database. The difference is that you cannot backfill and update old events in an existing stream. What you can do in this situation is make sure any old event can be upconverted at runtime to the new event.
Best Practices and Pitfalls when Event Sourcing?
The biggest pitfall is how different the concept can be to developers who have only ever done CRUD and only ever recorded the current state.
While I don’t think the concept of Event Sourcing is difficult, how you deal with an immutable log (event store) is very different than just recording the current state.
An example of this is when using a relational database if you needed to “fix” data, you’d simply go and write an UPDATE statement to update records in your database. With Event Sourcing, since the events are in an immutable log (event stream), you can’t just “update” an existing event. Nor would that actually even make much sense. You generally have to create compensating events to “undo” something.
In the stereotypical example, if you deposited $10 into a bank account, however, that deposit was recorded as $100 by mistake. The bank would actually create a withdrawal of $100 and then a deposit of $10. Meaning they would do a full reversal of your original incorrect deposit.
It’s just different than I believe most developers are used to. As mentioned in the previous question, versioning is another topic that’s just something very different from what most people are used to.
How to handle eventual consistency from eventually consistent projections (read models)
In most event-sourced systems, projections are used to create different read models used oftentimes for specific use cases. If you’re unfamiliar with Projections, check out my post on Projections in Event Sourcing: Build ANY model you want!
However this question doesn’t even need to be specific to event sourcing, but any data source that you’re querying that is eventually consistent. For example, a read replica that has lagged behind the primary.
I think the pitfall here is how you set users’ expectations. If a user performs an action that they know is changing the state of the system, however after they perform the action, you don’t provide them with the data that shows the updated state, they will not have a good experience. They have an expectation of consistency.
From a technical perspective, using versions (numbers). When you perform a query, return a version number in the response to the caller. When the caller needs to re-query, they will know if the update has occurred because the version will have changed. If it hasn’t you can wait and retry to get the data again.
How and when did you start using DDD & CQRS?
I can’t remember the exact year, but I bought the Domain-Driven Design book sometime around 2009. I can’t remember exactly how I was introduced to it but I do know at the time I stumbled upon some blog posts and conference talks by Greg Young and Udi Dahan.
Both Greg and Udi were/are a large influence on how I think about designing and developing software.
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- Event Choreography & Orchestration (Sagas)
- Event Based Architecture: What do you mean by EVENT?
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