Idempotency is an annoying word that makes people think you are an asshole when you use it, at least if they aren't exactly sure what it means.

But it's such a useful concept that I use it anyway and will just suffer the consequences.

For the uninitiated, it simply means that I can run the same code over and over and over but the result will always be the same.

Consider these two versions of a function trying to add tax to a bill:

defmodule Bill do
  defstruct total: 0.0, subtotal: 0.0
end

defmodule Tax do
  def calc_tax(bill) do
    %Bill{bill | total: bill.total * 1.07}
  end

  def calc_tax_w_subtotal(bill) do
    tax = bill.subtotal * 0.07
    %Bill{bill | total: bill.subtotal + tax}
  end
end

Both would work fine, if they are only called one time. In both cases, the current total is increased by 7%. The second version is more complex because we have the concept of both a subtotal and a total.

But the first version is not idempotent.

A race condition, bad calling code, or any number of other reasons could result in something like the following:

bill1 = %Bill{total: 10.0}
bill1 = Tax.calc_tax(bill1)
bill1 = Tax.calc_tax(bill1)
bill1.total # 11.449

vs:

bill2 = %Bill{subtotal: 10.0}
bill2 = Tax.calc_tax_w_subtotal(bill2)
bill2 = Tax.calc_tax_w_subtotal(bill2)
bill2.total # 10.7

In the case of the first, the tax compounds because we're only working with a single total and we can't tell if that total already includes tax. In the second case, we can calculate tax as much as we want but it doesn't effect the result after the first time.

Where it matters - Seeds

I see the need for idempotency in a lot of places but a very likely place you'll run into and need to care about it is with your seeds code.

If you run your seeds multiple times you probably don't want your default users or system categories or whatever created more than once.

I tried out a new form today to achieve idempotency in my seeds.exs and I found it cleaner than a simple if based approach so I thought I'd share. I'd love to hear if others have found better forms.

Approach one - ecto.reset

I suppose should start with the brute force method. If you always run your seeds as part of mix ecto.reset then you are achieving idempotency by always starting with a blank slate.

Similarly, you could create another approach in which you try to delete specific entities if they exist, before creating them in the seeds. (We can call that 1B)

Approach two - try

We could rely on database unique constraints to prevent multiple similar entries. We just try to create every time and ignore any errors:

try do
  Repo.insert!(%Category{
    name: "Books",
  })
rescue
  _ ->
    IO.puts("Already seeded")
end

Approach three - if

I've done this one a fair amount. It's not too bad, and rather readable, especially if all you want to do is create the entry (but not use it later in the seeds).

For example, this doesn't look horrible:

if Repo.get_by(Category, name: "Books") == nil do
  Repo.insert!(%Category{
    name: "Books",
  })
end

But it looks worse if you want to use that entity later:

book_category = 
  if Repo.get_by(Category, name: "Books") == nil do
    Repo.insert!(%Category{
      name: "Books",
    })
  else
    Repo.get_by(Category, name: "Books")
  end

(Yes, you could keep the result of the first get_by but the form doesn't really get much cleaner.)

lookup = Repo.get_by(Category, name: "Books")
book_category = 
  if lookup == nil do
    Repo.insert!(%Category{
      name: "Books",
    })
  else
    lookup
  end

Approach four - case

This is my current favorite way of achieving idempotency and capturing the value for later use:

book_category =
  case Repo.get_by(Category, name: "Books") do
    nil -> Repo.insert!(%Category{
      name: "Books",
    })
    category -> category
  end

Is there something even better that you use?

Please share!

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