Organizing (Commands, Events & Handlers) in Microservices

In a message-driven architecture such as SOA or Microservices, organizing commands and events, along with their respected handlers, can have a big impact on have you navigate your codebase. You might be surprised by the progression of how I organized them initially to what I do now! Also a tip on how to name commands and Events instead of after CRUD.

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Building Blocks

When moving into a message-driven system, you’re likely going to fall down the path of creating very explicit actions for your system rather than CRUD. When you do this, you’ll find yourself following CQRS and likely also moving to an event-driven architecture (where it makes sense).

The building blocks are messages and message handlers. Messages can be in the form of Commands, Queries, and Events. Commands and Queries will have a single handler that consumes the message. Events can have zero or many handlers that consume them.

Organizing commands and events (and their handlers) can be a bit of a message when you first start. I get asked often on my channel how I organize these building blocks of messages and handlers. Here’s how my progression over time.

Organize by Feature Folder

Put all relevant messages and handlers in the same folder. This is likely the most common or intuitive approach you might start with when moving into a message-driven system. Putting all the relevant types into their own files in a single folder.

I often put them nested inside a folder called Features. This is because I view a single or collection of messages and handlers as an individual feature. For example, setting an order as ReadyToShip is a command, but also occurs when the ShippingCreatedEvent occurs.

Single File with Suffix

The next step in the journey of organization for me was putting all relevant types into a single file. Especially for commands and queries, where there is only a single handler. To me, this simplifies not having to jump around files as the command/query and handler are directly related to each other. Why bother with two files?

The following example contains a Controller, Command, Handler, and Saga. Everything related to the feature of PlaceOrder is located in this file.

Events

In the above example, there is an OrderPlaced event that is published. However, it is not defined in this file. This is because this event is consumed and shared with other boundaries/services. For that reason, I put shared messages into a Contracts project which is shared with other services.

Static Wrapper Class

My next progression in organizing was I did not like having a redundant name for Command/Handler/Event. In the above examples for the PlaceOrder feature, there was a PlaceOrderCommand and PlaceOrderHandler. I found the naming to be a bit redundant and tiresome for typing.

To combat this, I simply place the types inside a static class. The static class is the name of the feature.

In the following example, the feature CancelOrder contains a static class named that, and nested within are the Command and Handler. When creating a new instance of a command: new CancelOrder.Command()

Suffix & Consistency

I tend not to suffix events with “Event”. Hence why you see OrderPlaced and OrderCancelled and not OrderPlacedEvent or OrderCancelledEvent. This is a matter of preference because events are always named in the past tense.

At the end of the day, it’s your preference. My recommendation is ultimately to be consistent in your naming. Regardless of how you organize or name, just be consistent as it will make navigating your codebase that much easier.

Source Code

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REST APIs for Microservices? Beware!

Are you using REST APIs for a Microservices architecture? If you’re using REST, HTTP APIs, gRPC, or any other Request/Response model as the primary way to communicate between microservices, you’re going to need to deal with possibly hard to debug latency issues and address availability concerns.

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In-Process Single Transaction

First, let’s take a step back and talk about a monolith. When in a monolith, you’d typically have a single process communicating with your database. If you’re using an ACID-compliant database, then you’d be using transactions to make all database statements atomic.

REST APIs for Microservices? Beware!

In the example above, our monolith has some clear boundaries of Sales, Catalog, Billing, and Warehouse. If an order is placed, which is started in sales, all three of these boundaries might be involved in that business process. Billing will have to create an Invoice and the Warehouse will have to allocate the products to ship. That entire business process would be encapsulated in one transaction. If anything goes wrong through that process, the transaction will rollback. We don’t have to worry about data consistency because we have the safety net of the atomic transaction.

Distributed Transaction

When you move out of a single process and into a distributed system, you’re going to lose that single database connection and transaction.

If you’re developing a microservices architecture with REST APIs, then what happens to your single transaction?

In the same scenario as above, when placing an order, there are multiple services that are required as apart of this long-running process.

If all the interactions from Sales to Billing and Warehouse are done over a REST HTTP API (or any synchronous request/response), then all the dependent services must be available in order to place an order.

If an order was placed, then Sales called Billing to create an Invoice and it succeeded. Then it calls the Warehouse to allocate products for shipping. If that request to the Warehouse fails, for whatever reason, then we have to manually perform some “undo” or compensating action to try and revert out changes along the way. This means you must make a call back to Billing to cancel the invoice.

But what happens if the request to cancel the Invoice in Billing fails? Now you’ve created an order, that has an invoice, has no allocated products.

REST APIs for Microservices? Beware!

Since there is no single transaction, you need a distributed transaction (or a two-phase commit). One way to solve this is what a transaction coordinator. But this means that all services must be using compatible technologies for databases. The likely hood of this is not high if you haven’t planned this initially.

The solution to this is through Event orchestration or choreography. But hang on, before that, there are more issues to discuss.

Latency

When making a network hop, you’re going to add latency. The issue is, if you’re inside one service, making an HTTP call to another service, that’s where the visibility ends. You are unaware of what the internals are of the other service is that you’re calling. What happens when the service you’re calling, has to call another service? And so on!?

In the example above, there is a network call chain from service to service. If each service takes (on a happy path) 200ms to respond to the other service, in my example above, it would take a total of 800ms roundtrip to the client.

REST APIs for Microservices? Beware!

The above is a best-case scenario. But because there’s so much coupling, that can’t directly be seen, latency can skyrocket if availability issues arise anywhere in the call chain.

What happens if the catalog is having performance issues and isn’t responding at 200ms, but at 700ms?

From our client, this is now going to take 1.3s. All because of downstream issues that are going to be very difficult to identify. From the perspective of Sales, it’s getting a slow response from the Warehouse. But the warehouse is getting a slow response from Billing. Who’s fault is it? Clearly, in my diagram, it’s the Catalog service, but without clear metrics, this is going to be very difficult to address. Beyond that, every network hop added in any downstream service is going to add latency.

Distributed Ball of Mud

It gets worst. The reality is if a service must make HTTP calls to other services, either for getting data or performing an action, then all the dependent services must be available for it to perform correctly.

For example, let’s say that Sales, Warehouse, and Billing all require the Catalog Service. If the Catalog service is unavailable or is having performance issues, then not only is it an issue, all the services are going to be failing.

This is a distributed monolith. Services require each other to be available and cannot function without other services.

Boundaries & Asynchronous Messaging

One solution to this distributed monolith is using asynchronous messaging between well defined boundaries.

Services should be autonomous. They shouldn’t depend directly on other services through synchronous interactions. If communication is done asynchronously through messaging, this means a service does not require other services to be available.

In order to make services autonomous, you need to have boundaries defined correctly. This means focusing on the behavior of the services and then the data required for those behaviors.

Once you have well-defined boundaries around capabilities, then you can use asynchronous messaging to define the workflow. Check out my post on Event Choreography and Orchestration to develop long-running business processes between services.

Source Code

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Messaging Commands & Events Explained!

In service oriented architecture (including microservices) that leverage messaging, you’re going to run into both types of messages: Commands & Events. What’s the difference between Commands & Events? When should you use a command and when should use an event?

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What’s a Command?

Commands are messages with the intent to change the state of the system.

They are actions to be performed that have side effects.

They may be created by an end-user or another part of the system that could be automated.

They are away to execute behavior.

What’s an Event

Events are statements of fact.

They are named in the past tense because they represent something that has already occured.

Events are things that have happened within a system.

They are the result (not in the method/return sense) of an executed command.

They are notifications.

They are used to tell other parts of the system that something has occurred.

Consumers

One important distinction between Commands & Events are consumers.

A command will have exactly one consumer. There will only be one consumer that will handle an instance of a command. When your producer sends a command to a message broker, it will send that command to only one consumer.

Commands & Events Explained!

An event will have zero to many consumers. When a producer publishes an event to a message broker, it will deliver it to as many consumers that want it. Each consumer will receive a copy of the event. There may not be any consumers that want to receive an event. The publisher does not know who the consumers are, or how many of them exist for an event.

Commands & Events Explained!

When to use a Command?

When you want to tell the system to perform some type of behavior. For example, sending a PlaceOrder command. You’re telling the system to perform an action that could have a side effect.

Name your commands like actions or tasks. They should be contain a verb.

When to use an Event?

When something has occurred as the result of a command and you want to tell other parts of the system. For example, OrderPlaced event can let other parts of the system know that an order has been placed. This likely would have resulted from a PlaceOrder command.

Because events are things that have occurred, they will be named in the past tense.

Commands & Events: Orchestration

You can leverage both commands and events together to orchestrate a business process or workflow. An orchestrator will send commands and consume the resulting events to kick off the next command.

Check out my blog post and video on Event Choreography & Orchestration (Sagas) for more details.

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