In my recent talk on Evil by Design, I showed how I’ve been visualizing event streams as a means to get a better grip on how aggregates behave in production. The talk’s scope kept me from showing the code that goes together with the examples shown. Consider this post as an addendum to that talk.

First off, we need a few types: a string that identifies a stream, an event containing a timestamp and its name. A stream which is a composition of an identifier and a sequence of events. We also need a function that’s able to read a stream based on its identifier.

type StreamId = string
type Event = { Timestamp : DateTime; Name : string; Payload : string; }
type Stream = { StreamId : StreamId; Events : seq<Event> }
type ReadStream = StreamId -> Stream

let readStream ( streamId : StreamId ) =
  {
    StreamId = streamId
    Events = seq {
      // implementation goes here
    }
  }

Once we’ve implemented that, we want to go ahead and visualize a single stream. Having some experience with Google Charts, I used the XPlot.GoogleCharts package.

I want to visualize my event stream as a timeline. For that, it makes only sense to use the Timeline graph. This means that I’ll have to make sure I transform my data into a format the Timeline chart can work with, which is a sequence of tuples.

// Timeline data
data : seq<string * string * #value * #value>
data : seq<string * #value * #value>

let asTimelineEventStream stream =
    stream.Events
    |> Seq.map (fun e ->
    (
        stream.StreamId,
        e.Name,
        e.Timestamp,
        e.Timestamp.AddMilliseconds(float 1)
    ))

So we write a function which accepts a stream, and returns a sequence of tuples containing the stream identifier, the event name and the timestamp of the event.

With just a few lines of code, we can already compose our way to a timeline.

"withdrawrequest/..."
|> readStream
|> asTimelineEventStream
|> Chart.Timeline
|> Chart.Show

The result tells a small story: a withdrawal to a casino was requested at 9:44PM, approved at 12:15PM the next day, and eventually completed 7 hours later.

From an operational perspective, this visualization can be used as visual assistance for your support team when users have a question or a complaint. From a more technical perspective, it can be used to get a feel of the domain language and business processes without having to look at code or tests. I could even see this being used in the front-end, where you enable users to monitor a process; think package tracking, document verification and so on.

Once you start exploring aggregates, you will notice that some aggregates look healthier than others; lean and short-lived. While other aggregates are fat and long-lived which can introduce a set of problems:

• rebuilding state from a large event stream might kill performance
• there’s often more contention on larger aggregates making optimistic (or pessimistic) concurrency very annoying

Spotting one of these instances is an invitation to review your model, to revise true invariants and to break things apart.

We’ve now looked at an aggregate’s event stream in isolation, but often something happening in one place leads to a reaction somewhere else. A simple example: when a new user registers, a promotion is awarded. We can visualize this by rendering multiple streams on one timeline.

Technically, we need to transform a sequence of streams to a single sequence of tuples which we can feed the chart. It’s as simple as mapping each stream for then to flatten the result into a single sequence.

let asTimelineEventStreams streams =
    streams
    |> Seq.map asTimelineEventStream
    |> Seq.concat

correlatedStreams
|> asTimelineEventStreams
|> Chart.Timeline
|> Chart.Show

This one extra step makes the result even more useful.

There’s more potential though; consider showing the payload when hovering over an event, adding commands in the mix, zooming out, zooming in, filtering…

If this is something you could see being useful to you or your organization, let me know! Maybe I can port some bits and polish the concept in the open.