Sunday, September 18, 2016

Commands and events with JustSaying and AWS

I've been looking into handing a bit of our messaging infrastructure over to a managed alternative. Managing your own messaging infrastructure that should be highly available is not always an investment you want to make in this day and age. Going through the documentation and relying on experiences from some people I trust, I ended up looking at AWS and SNS/SQS.

Making the Github repository rounds, looking for inspiration, I stumbled on JustSaying: a library by the people from JustEat implementing a message bus on top of AWS.

I wanted to find two messaging patterns in this library:
  1. Command queuing. A common pattern in our components is to react to an event by making an HTTP request to an external partner. To improve reliability and throughput, we generally don't make that HTTP request in the projection itself, but rather drop a command onto a queue which will then be processed in parallel using a bounded amount of retries. When things do go wrong, we either retry the messages by moving them from the error queue back to the input queue or we change the reaction and reset the projection checkpoint, sending the commands again.
  2. Pub-sub. Another pattern used when there is a certain level of familiarity between components, is to have a component publish events. Other components can subscribe to these messages and have them delivered to their own queues.
Both these styles are supported by JustSaying.

In this example, I have two commands: BookFlight and CancelBooking, with two related events: FlightWasBooked and BookingWasCancelled.

Since JustSaying requires messages to inherit from a base class, these message definitions live on the outside, far from the domain. This allows to decouple the domain from the outside contracts and to make sure the events go out to the world in the format I want them to be.

To handle these messages, JustSaying requires you to implement the IHandler interface.

Having this out of the way, we need to configure the bus (publishers and subscribers).

First of all, Amazon needs to know who we are and what we're allowed to do.

We should define which region our infrastructure lives in.

Now we can configure our command queue. Commands should be published using an SQS publisher, directly dropping messages into the "Commands" queue. A point-to-point subscriber will directly pull messages from the "Commands" queue and hand them over to the command handlers.

Events are not directly dropped to an SQS queue, but will be created as an SNS topic. We can use SQS to subscribe to these topics and have them delivered to an "Events" queue.

Once the bus has been created, we can start listening and publishing messages.

JustSaying will create two SNS topics and four SQS queues: two input queues and two error queues.

Those topic and queue names are not that descriptive once you introduce multiple components and might cause names to collide. JustSaying allows you to define a custom naming strategy. I've settled on a strategy that is based on the message type and prefixed with the component name. This has the added advantage that each message type now goes into its own queue.



This whole experiment has had a scary low learning curve (maybe a bit too low). While I'm still in the assess-phase, I'm fairly optimistic that running on top of SNS/SQS might take away some of our operational burden. Going over the JustSaying API and code base, it's quite opinionated and there are things I might have approached differently. Some features I'd like to see, like the library providing a message envelope as a first-class citizen (a base message class is something I've regretted in the past) is being worked on, so I'm keeping my eye on those. Since I'm only using command queuing at the moment, I should be pretty safe from future breaking changes to the message format and such.

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