More Clojure component patterns

Parking the cart before putting the horse to bed.

Like the proper nerd that I am, I’ve been recently playing around with the National Rail data feeds, with a view to drawing some pretty pictures of quite how differently timely our train services are. So, when you’re developing at the REPL, it’s convenient to be able to cleanly shutdown and restart a n service.

One of the first steps I take when looking at a dataset is to load it into a relational database, namely PostgreSQL; in order to more easily get a feel for the shape of the information, and I’ve used core.async and the component library to do this. Whilst this post won’t be about the specifics of the ETL process itself, there are a few patterns around structuring the processes that I’ve found useful.

Parents produce, children consume.

When you’re shutting down a system when there are messages in flight, then in most cases, you’ll want to avoid dropping messages on the floor, and so at the system level, you want to shut down the producers first, then their children, their children, and so on, until you reach your pipline’s egress point. If you start your shutdown from the egress point and work backwards, then because a given producer may well be still producing information after a consumer has shut down, that information will effectively be lost. It’s also possible that the system may deadlock, depending on how your internal queues are configured.

Now, the component library gives us one main way to achieve ordering, and that’s by specifying inter-component dependencies, with the component/using function, eg:

To summarize the behaviour of the component library, a system-map represents a hash-map that represents the topology of the system, and using associates meta-data with each component (eg: the database, or the web application and server) describing which other services it requires. So in this case, the application queries the database, and the http-server requires an application to service requests.

However, in this case, we can think of the web-server being a producer of events (http requests), that are “consumed” (responded to) by the web server. So, logically, if you shut down the application and/or database before the http server, then you’ll have a service which likely isn’t actually able to service requests, and at best will return 500 Internal server errors. To avoid this, we need to shut-down and drain the incoming requests before shutting down the application.

Fully draining inputs.

When I’m using core.async in a pipeline, I associate one or more core.async processes (or go-routine, if you prefer) with each component to actually do the work.

When you shut down a system in the above manner, however, you need to ensure that each component within the system has actually fully processed it’s input before you shut down any components downstream from it. It’s easy enough to construct a fresh input channel when the component starts, and close it when the component is stopped. However, with the default core.async channels, it’s not generally possible to inspect the buffer to check whether it has been fully drained or not.

However, when you start a go-routine (or thread) in core.async, you are returned a channel which represents the value returned from the body of the go-routine when it completes. So, we can stash a reference to this channel within the component state, and use that to determine when this component is fully done. For example:

As you can see, we do a blocking wait on the channel returned by consumer-loop when we shut down. Granted, this is a proof of concept; in fully production-ready code, you’ll need to allow for the risk of the component failing, and timing-out on shut down.