Event Sourcing Taught Me to Delete Code

July 9, 2025 · event-sourcing · design · elixir

I came to event sourcing expecting an audit log and got something more useful: a reason to delete about a third of the inventory service.

The code that went away

The old service had a table of stock levels and a great deal of code protecting it. Compensating updates when a shipment was cancelled. A nightly reconciliation job. A stock_adjustments table nobody could explain, holding rows that existed only to correct earlier rows.

All of that was there because the current level was the only thing we stored. When it went wrong, we had no way to ask how, so we wrote code to guess.

Storing decisions instead of state removed the guessing:

defmodule Inventory.Events do
  defstruct StockReceived, [:sku, :quantity, :received_at, :purchase_order]
  defstruct StockPicked, [:sku, :quantity, :picked_at, :order_id]
  defstruct StockWrittenOff, [:sku, :quantity, :reason, :approved_by]
end

The level is now a fold:

def level(sku) do
  sku
  |> Store.stream()
  |> Enum.reduce(0, fn
    %StockReceived{quantity: n}, level -> level + n
    %StockPicked{quantity: n}, level -> level - n
    %StockWrittenOff{quantity: n}, level -> level - n
  end)
end

The reconciliation job had nothing left to reconcile. The adjustments table became a write-off event with a reason and an approved_by, which is what everyone had wanted from it in the first place.

What it did not fix

It is worth being clear about what event sourcing cost us, because the posts that skip this part are the ones that get people into trouble.

Versioning is forever. The moment an event is persisted, its shape is a public API. We now write upcasters, and we treat an event struct with the same care as a database migration.

Projections drift. Every read model is eventually consistent with the stream, and every part of the UI that pretended otherwise had to learn to say "pending".

Streams grow. A fold over five years of movements for a fast-moving SKU is not free. Snapshots solve it, and snapshots are cache invalidation wearing a hat.

Those are real costs. I would pay them again for the deletions alone. The service went from a thing we were afraid to touch on a Friday to one where the question "how did this SKU get to negative four" has a literal answer.