A queue that keeps order per key. ≈ SQS FIFO message groups, Kafka partitions, Pub/Sub ordering keys.

The events of one order run one after another, in the order they reached Postgres; events of other orders run in parallel, on any node.

# where the event arrives, inside a Repo.transact/1
Ordered.insert(MyWorker.new(%{...}), "order:42")

# first thing in MyWorker.perform/1
with :ok <- Ordered.turn(job), do: ...

• Order is arrival at Postgres, not the nodes' clocks. insert/2 takes a lock on the key until the transaction commits, so a job can't become visible, and run, before an earlier one of the same key.
• A failing job blocks its key while it retries, like a Kafka partition. Once it is discarded (out of attempts) it stops blocking.
• A node that dies mid-job leaves it executing, which blocks the key until Oban's lifeline gives the job back — set rescue_after short enough for you.
• Needs the index from Dandelion.Migration, and the default Oban instance (the one started as {Oban, …}).
• Use an ordered queue (queue: :ordered) for the worker. One fixed key makes a global queue: one job at a time for the whole cluster.