Failed partitions

Chronicle.FailedPartitions provides an idiomatic Elixir API for inspecting Chronicle’s failed partitions.

A partition (an event source, within an observer’s subscription) is marked failed when an observer can’t handle one of its events. In the Elixir client that happens when:

• a reactor’s handle/2 returns {:error, reason}, raises, or returns a side effect that can’t be appended;
• a reducer’s reduce/3 raises. A reducer must return the new model; returning {:error, reason} doesn’t mark a failure, so raise instead.

A failed partition stops advancing while the rest of the observer continues. Mirrors the C# and Kotlin clients’ IFailedPartitions.

Starting point

Start Chronicle.Client first:

children = [
  {Chronicle.Client,
    connection_string: "chronicle://chronicle-dev-client:chronicle-dev-secret@localhost:35000",
    event_store: "banking",
    otp_app: :my_app}
]

Getting all failed partitions

Get every failed partition for any observer (reactor, reducer, and so on) on the current event store:

{:ok, failed_partitions} = Chronicle.FailedPartitions.get_all()

Getting failed partitions for a specific observer

Narrow the lookup to a single observer by its id:

{:ok, failed_partitions} = Chronicle.FailedPartitions.get_for("MyApp.Reactors.OrderNotifier")

The FailedPartition and Attempt structs

Each failed partition is returned as %Chronicle.FailedPartitions.FailedPartition{} with:

• :id
• :observer_id
• :partition — the event source id that failed
• :attempts — the list of failed attempts, oldest first

Each attempt is a %Chronicle.FailedPartitions.Attempt{} with:

• :occurred
• :sequence_number
• :messages
• :stack_trace

Enum.each(failed_partitions, fn failed_partition ->
  IO.puts("#{failed_partition.observer_id}: #{failed_partition.partition}")

  Enum.each(failed_partition.attempts, fn attempt ->
    IO.puts("  ##{attempt.sequence_number}: #{Enum.join(attempt.messages, "; ")}")
  end)
end)

Using a named client

{:ok, failed_partitions} = Chronicle.FailedPartitions.get_all(client: :bank_chronicle)

See Observers for the observer’s own state — including removing one whose declaring code is gone, which also clears any failed partitions kept for it.