In a microservices architecture, services are divided into smaller ones to work together to process business transactions. This gives us the advantage of being able to develop services quicker and scale them independently.
However, designing a distributed workflow is challenging and communication between the services is complex.
One approach we could take is to implement a centralized orchestrator that acknowledges all incoming requests and delegates them to the relevant services. The orchestrator manages the entire workflow of the transaction.
The orchestrator helps to reduce point-to-point communication between services, but it does result in tight coupling between the orchestrator and other services. If you want to add or remove services, this will result in you having to rewrite parts of your application.
If you want to learn more about orchestration in a Saga pattern, check out this blog post that I wrote on the subject.
Instead of relying on a central point of control, we can implement the Choreography pattern, where each component in the system participates in the workflow of a business transaction.
In this article, I’ll talk about how we can use the Choreography pattern to perform tasks in a workflow transaction, when we should use the Choreography pattern, and some things we need to keep in mind when using the Choreography pattern.
What is the Choreography pattern?
Instead of relying on a centralized service, let each service within your architecture decide when and how a business operation is processed. We could implement asynchronous messaging to coordinate business operations between our different services:
Using asynchronous messaging, clients will publish messages to a message queue. These messages are then pushed to subscribers who are interested in that particular message.
Each subscriber performs the operation that the message prescribes. The subscriber then replies to the message queue with the outcome of that operation. If it’s successful, the service can send a message to initiate the next step in the workflow. If it fails, the message broker can retry the operation.
Using this approach, services choreograph the transaction workflow among themselves without needing an orchestrator, or needing direct communication between them. There’s also no bottleneck caused by the orchestrator, since it doesn’t exist!
I previously worked as a Software Engineer for a bank where we implemented the Choreography pattern as part of a payment workflow. I’ve simplified the process in the diagram below.
Every week we would pick up a file from Blob Storage, which would kick off the transaction. We had an Azure Function that would perform some validation work against a reference table to ensure that the File was in a valid format. If it was valid, we would send the payload in a message to an Adaptor function, where we would transform the file into a format that another transaction would pick up. This payload would be sent to a FileCreator Function, which would produce the file in a format that could be used by other banking systems.
Should there be any failures in the transaction, we would send compensating events to undo successful operations in the transaction, as well as send the original payload to the dead-letter queue, so we can investigate what went wrong.
What should we keep in mind when using the Choreography pattern?
While removing the orchestrator removes a centralized performance bottleneck, decentralizing our workflow can create some issues. Rather than a single service being the single point of failure, the failure is distributed between all services. Now, you may think that this is better, but each service becomes responsible for the entire workflow, which needs to be considered within the logic of each service. Handling retries, terminating requests gracefully, communicating results etc. all adds complexity to each of your applications.
If an operation fails, it can be difficult to recover from that failure. We can solve this by implementing compensating transactions to undo steps in the workflow that were successful. We could also fire events to retry operations, or time them out.
If your workflow needs to occur in a sequence, you’ll need to consider implementing multiple message brokers to send messages in the required order.
As the number of services grow within your architecture, the complexity of your choreography will increase. Tracing all of this is also something you should keep in mind!
When should I use the Choreography pattern?
If you’re using a centralized orchestrator within your architecture and your experiencing performance bottlenecks with it, you may want to consider implementing the choreography pattern instead.
This also makes it easier if you’re adding or removing services from your architecture, as this will cause minimal disruption to your communication paths.
In this article, we discussed the Choreography pattern, how we can implement it, what we need to keep in mind when implementing it, and when we should use this pattern.
If you want to read more about this pattern, check out the following resources:
If you have any questions, feel free to reach out to me on X/Twitter @willvelida
Until next time, Happy coding! 🤓🖥️