Rheo

v0.4.0 — Durable consumer-group semantics over searchable databases. Backends today: MongoDB (durable) and ETS (ephemeral, zero-infra). Rheo is an Elixir/OTP library you embed in your supervision tree, not a standalone messaging server.

Delivery guarantee: at-least-once. Duplicates are possible after failures — use stable event IDs for idempotency.

When to use

Use Rheo when you want:

• An immutable, queryable event log in a database you already run
• Consumer groups with leases, ACK, retry, and dead-lettering
• Competing consumers and independent groups on the same stream
• OTP-native demand, concurrency, and lease renewal — without standing up Kafka, RabbitMQ, or a separate broker cluster

Skip Rheo when you need a dedicated broker (massive fan-out, cross-language clients, exactly-once claims) or when a simple job queue is enough.

Compared with other approaches

Databases already store and search historical events well. Message brokers already coordinate consumers well. Rheo combines those strengths: immutable, queryable events in MongoDB, with leases, acknowledgement, retry, and competing consumers in OTP.

Installation

Add Rheo to your mix.exs dependencies:

def deps do
  [
    {:rheo, "~> 0.4.0"}
  ]
end

Then fetch deps:

mix deps.get

Choose a backend:

# Zero-infra (tests, Livebook, ephemeral apps)
{Rheo, name: MyRheo, backend: Rheo.Backend.ETS}

# Durable MongoDB
{Rheo, name: MyRheo, backend: {Rheo.Backend.Mongo, url: "mongodb://localhost:27017/rheo"}}

Quick start

ETS (no Docker)

children = [
  {Rheo, name: MyRheo, backend: Rheo.Backend.ETS},
  {MyApp.RiskConsumer, rheo: MyRheo, concurrency: 8, max_demand: 100}
]

Supervisor.start_link(children, strategy: :one_for_one)

MongoDB

children = [
  {Rheo, name: MyRheo, backend: {Rheo.Backend.Mongo, url: "mongodb://localhost:27017/rheo"}},
  {MyApp.RiskConsumer, rheo: MyRheo, concurrency: 8, max_demand: 100}
]

Supervisor.start_link(children, strategy: :one_for_one)

Define a consumer — handlers only implement handle_event/2; a local Rheo.Group owns fetch, concurrency, lease renewal, and settle:

defmodule MyApp.RiskConsumer do
  use Rheo.Consumer,
    stream: "market-events",
    group: "risk",
    concurrency: 8,
    max_demand: 100

  @impl true
  def handle_event(event, state) do
    case Risk.process(event) do
      :ok ->
        {:ack, state}

      {:temporary_error, reason} ->
        {:retry, reason, state}

      {:permanent_error, reason} ->
        {:reject, reason, state}
    end
  end
end

Create a stream, append events, and query history:

Rheo.create_stream("market-events")
Rheo.create_group("market-events", "risk")

Rheo.append("market-events", %{
  type: "curve_update",
  currency: "EUR",
  price: 2.913
})

Rheo.query("market-events", type: "curve_update", currency: "EUR")

Interactive walkthrough: open notebooks/rheo_demo.livemd in Livebook. The notebook defaults to ETS (no Docker). CLI demo: mix rheo.demo (ETS) or RHEO_BACKEND=mongo mix rheo.demo.

Upgrading:

• 0.3 → 0.4 (additive — search, replay, lineage)
• 0.1 → 0.2 (breaking Group / Query changes)

Documentation

• HexDocs — API reference
• Architecture
• Tutorials
• ADRs
• Livebook demo
• Article 10: Prove it with ETS
• Article 11: Search and Replay
• 0.3 → 0.4 migration
• 0.1 → 0.2 migration
• Changelog
• Roadmap

More examples

Low-level fetch / ACK

{:ok, leases} = Rheo.fetch("market-events", "risk", limit: 10, rheo: MyRheo)

Enum.each(leases, fn lease ->
  # process lease.event
  Rheo.ack(lease, rheo: MyRheo)
end)

Portable queries

Rheo.query("market-events",
  type: "curve_update",
  after_sequence: 100,
  order_by: [sequence: :desc],
  limit: 50
)

{:ok, page} = Rheo.query_page("market-events", type: "curve_update", limit: 100)
Rheo.stream_query("market-events", type: "curve_update", limit: 100) |> Enum.take(250)

Replay without copying events

# Safest: new group from a cursor
Rheo.create_group("market-events", "risk-replay", start_after: 1_000)

# Or reopen an existing group (duplicates expected)
Rheo.replay("market-events", "risk", from_sequence: 1_000)
Rheo.reset_group("market-events", "risk", confirm: true)

Event lineage metadata

meta =
  Rheo.Event.Lineage.put(%{},
    correlation_id: "trade-42",
    causation_id: "cmd-9",
    producer: "pricing-v3",
    schema: "curve_update",
    schema_version: "1"
  )

Rheo.append("market-events", %{type: "curve_update", currency: "EUR", metadata: meta})
Rheo.query("market-events", correlation_id: "trade-42")

Competing consumers and independent groups

Multiple processes can compete for the same durable group. Separate groups on the same stream process every event independently (e.g. "risk" and "surveillance").

Rheo.create_group("market-events", "risk")
Rheo.create_group("market-events", "surveillance")

Lease renewal and concurrency

Rheo.Group renews inflight leases (~half of lease_ms) and runs up to :concurrency handler tasks while bounding outstanding leases with :max_demand.

{MyApp.RiskConsumer,
 rheo: MyRheo,
 concurrency: 8,
 max_demand: 100,
 lease_ms: 30_000,
 poll_ms: 200}

Named instances

Run more than one Rheo instance in the same BEAM:

children = [
  {Rheo, name: MyRheo, backend: {Rheo.Backend.Mongo, url: primary_url}},
  {Rheo, name: MyRheoAudit, backend: {Rheo.Backend.Mongo, url: audit_url}}
]

Pass rheo: MyRheo (or rheo: MyRheoAudit) on APIs and consumers.

Roadmap

Still out of scope through 1.0 unless demand forces it: standalone Rheo server, exactly-once claims, K8s operator, auth frameworks, multi-tenancy. Details in docs/roadmap.md.

License

MIT — see LICENSE.

Building and developing the library

For contributors working on Rheo itself (not application consumers):

mix deps.get
mix test
mix rheo.demo
# Mongo demo:
docker compose up -d && RHEO_BACKEND=mongo mix rheo.demo

Quality gates:

mix format --check-formatted
mix compile --warnings-as-errors
mix credo --strict
mix dialyzer
mix coveralls

Mongo-backed tests run by default when Mongo is available. Heavier end-to-end scenarios are tagged :integration and excluded unless enabled:

RHEO_INTEGRATION=1 mix test
# or
mix test.integration

Unit-only (excludes Mongo):

mix test.unit

Livebook from a clone (ETS by default — no Docker):

livebook server notebooks/rheo_demo.livemd

CI tests Erlang/OTP 27–29 × Elixir 1.17–1.20 (excluding unsupported pairs). Format, Credo, Dialyzer, and Coveralls run on Elixir 1.20.2 / OTP 29.

Coverage publishes to Coveralls from CI via mix coveralls.github. Hex releases publish from annotated tags (v*); set repository secrets COVERALLS_REPO_TOKEN and HEX_API_KEY.