Fact is an Elixir library that provides a file system based event store database.

• Traditional event sourcing capabilities:
  • Append events to event streams
  • Read events from specific event streams or all events in the event store
  • Subscribe to specific event streams or all events in the event store
  • Optimistic concurrency control
• Compliant with the Dynamic Consistency Boundary specification
• Read events from built-in indexes and custom queries.
• Subscribe to built-in indexes and custom queries.
• Just-in-time indexing for event data queries.
• "Pseudo-WORM" storage¹
• Supports multiple instances for siloed isolation in multi-tenancy setups
• Configurable Content-Addressable Storage (CAS)
• Configurable event schemas
• Supported on Elixir 1.13+ and OTP 25+

• User guides
• Backup task
• Restore task
• Data tampering verification task (for CAS)
• Custom Indexers

• Telemetry
• Merkle Mountain Range
  • inclusion proofs, this event exists in the ledger at position N.
  • doesn't prevent tampering, but proves it did or didn't happen
  • Needs one of these 🤔:
    • signed checkpoints
    • cross-system anchoring
    • client-held receipts
• Proof of scale
  • Target: 1M events per day, <= 86.4 ms per write, 11.5 events per second
  • Honestly not trying to build for global scale, if that's what you need use Axon, Kurrent, or UmaDB
• Full stack example application
• A network protocol to enable non-BEAM based languages to interop.
• A gossip protocol to coordinate multiple BEAM nodes

• Graphical user interface to manage and operate the database (like a pgAdmin or Sql Server Management Studio)

The package can be installed by adding fact to your list of dependencies in mix.exs:

def deps do
  [
    {:fact, "~> 0.2.0"}
  ]
end

Then create a database instance.

$ mix fact.create -p data/turtles

# Start a database instance
iex> {:ok, db} = Fact.open("data/turtles")

# Create an event
iex> event = %{
...>   type: "egg_hatched",
...>   data: %{
...>     name: "Turts"
...>   }
...> }

# Append the event to a stream
iex> {:ok, pos} = Fact.append_stream(db, event, "turtle-1")

# Read the event stream
iex> Fact.read(db, {:stream, "turtle-1"}) |> Enum.to_list()
[
  %{
    "event_data" => %{"name" => "Turts"},
    "event_id" => "3bb4808303c847fd9ceb0a1251ef95da",
    "event_tags" => []
    "event_type" => "egg_hatched",
    "event_metadata" => %{},
    "store_position" => 1,
    "store_timestamp" => 1765039106962264,
    "stream_id" => "turtle-1",
    "stream_position" => 1
  }
]

1 - Its "pseudo-WORM" because immutability is enforced at the filesystem level by marking events as read-only. This prevents modification during normal operation, but does not provide hardware-level or regulatory WORM enforcement.