The Mystery of a Missing Greeting

#elixir #journey #debugging #durableworkflow

"Sure, I can build things quickly with durable workflows, but how do I debug them?"

I hear this question a lot, so let's solve this mystery!

Source Code and Setup

The source code for this exercise is available at https://github.com/markmark206/journey-introspect.

You can clone that repo and follow its README's setup steps to run this sequence on your machine, or you can just follow along!

Some output in fragments below is omitted for brevity.

With logistics out of the way, let's go!

Welcome, Customers!

You are a technical co-founder. You just implemented the "welcome" sequence of your onboarding application, as a durable workflow. Your co-founder is surprised at how quickly you did it and how well it works.

Honestly, you are also surprised at how little code you needed to write:

iex(1)> import Journey.Node
Journey.Node
iex(2)> graph = Journey.new_graph(
  "Customer Onboarding",
  [
    input(:name),
    input(:email_address),
    compute(
      :greeting,
      [:name, :email_address],
      fn values ->
        welcome = "Welcome, #{values.name} at #{values.email_address}"
        IO.puts(welcome)
        {:ok, welcome}
      end
    )
  ]
)

The application spins up an execution of this workflow (Journey.start(graph)) for every customer, as they navigate your website. Customers are getting greeted. The business is booming.

The Mystery of a Missing Greeting

All of a sudden you get a call from your co-founder, Ms. Too-Ticky: "Mr. Hemulen, our newest customer, did not get his 'welcome' greeting! Why?!?"

No worries, Ms. Too-Ticky! Journey.Tools.introspect/1 to the rescue!

Mystery Solved?

Let's introspect Mr. Hemulen's onboarding execution. Did :greeting compute?

iex(5)> Journey.Tools.introspect(eid) |> IO.puts()
Values:
- Set:
  - name: '"Hemulen"' | :input
    set at 2025-12-10 09:02:16Z | rev: 1

- Not set:
  - email_address: <unk> | :input
  - greeting: <unk> | :compute

Computations:
- Outstanding:
  - greeting: ⬜ :not_set (not yet attempted) | :compute
       :and
        ├─ ✅ :name | &provided?/1 | rev 1
        └─ 🛑 :email_address | &provided?/1

Aha! :greeting is blocked – we have Mr. Hemulen's :name, but not his :email_address.

Mystery Solved! Hello, Mr. Hemulen!

As soon as Mr. Hemulen provides his email address, he is greeted:

iex> Journey.set(eid, :email_address, "hemulen@gojourney.dev")
Welcome, Hemulen at hemulen@gojourney.dev

All done!

:greeting should now be computed. Is it?

iex(7)> Journey.Tools.introspect(eid) |> IO.puts()
Values:
- Set:
  - greeting: '"Welcome, Hemulen at hemulen@gojourney.dev"' | :compute
    computed at 2025-12-10 09:03:06Z | rev: 4

  - email_address: '"hemulen@gojourney.dev"' | :input
    set at 2025-12-10 09:03:06Z | rev: 2

  - name: '"Hemulen"' | :input
    set at 2025-12-10 09:02:16Z | rev: 1

Computations:
- Completed:
  - :greeting (CMPR7RL0T7T2VTJAG9Z0748): ✅ :success | :compute | rev 4
    inputs used:
       :name (rev 1)
       :email_address (rev 2)