Chronicle Elixir Client

Event sourcing for Elixir — the idiomatic client for Cratis Chronicle, the open-source (MIT) event-sourcing database and processing runtime.

cratis_chronicle brings event sourcing and CQRS to Elixir applications: append events to an event store, project them into read models, and react to them — all backed by the Chronicle Kernel. It builds on Chronicle's language-agnostic gRPC API and exposes OTP-native constructs including:

• use Chronicle.Events.EventType — annotate structs as event types with stable IDs
• use Chronicle.ReadModels.ReadModel — declare model-bound projections executed server-side
• use Chronicle.Reactors.Reactor — react to events with side effects
• use Chronicle.Reducers.Reducer — fold events into read models in your own process
• use Chronicle.Seeding.Seeder — seed event stores with baseline events at startup
• Model-bound constraints — unique and unique-event-type constraints on event types
• Context-aware appends — process-scoped identity, correlation, and causation metadata
• Optimistic concurrency — guard appends with scoped tail-sequence checks
• Transactions — buffer and commit multi-event units of work
• Jobs and webhooks — inspect Chronicle jobs and manage webhook registrations
• Resilient connection — automatic reconnection with exponential backoff

We believe event sourcing is worth it for almost any system dealing with information and business flows — and that in Elixir it should feel like Elixir: modules, structs, and use macros rather than a foreign paradigm. The client is designed to keep friction and boilerplate low, so it reads as familiar code even if you have never event-sourced before. It is part of one deliberately simple Cratis ecosystem, built with productivity, quality, and reliability in mind — AI-friendly by design, with free AI skills for building with the stack.

Add cratis_chronicle to your mix.exs dependencies:

defp deps do
  [
    {:cratis_chronicle, "~> 3.5"}
  ]
end

The client needs Elixir 1.18 or later (googleapis requires 1.18). It uses grpc ~> 1.0, Mint 1.11 or later, and contracts 19.19 or later. CI builds and tests with Elixir 1.19.5 on Erlang/OTP 28.5. If your application also connects to gRPC independently, explicitly pass adapter: GRPC.Client.Adapters.Mint to GRPC.Stub.connect/2 (or add Gun as a direct dependency); grpc 1.x no longer requires you to supervise GRPC.Client.Supervisor. Chronicle disables Mint server push; if your application sets config :grpc, GRPC.Client.Adapters.Mint, client_settings: [...], it must include enable_push: false or the client rejects connection startup.

You need a Chronicle kernel before running samples or application code. For local development, pull and run the development image, which bundles MongoDB:

docker pull cratis/chronicle:latest-development
docker run -d --name chronicle \
  -p 127.0.0.1:35000:35000 \
  -p 127.0.0.1:27017:27017 \
  cratis/chronicle:latest-development

The 127.0.0.1: prefixes keep both ports on your machine: the development kernel accepts well-known credentials and its MongoDB has no authentication. Pull before you run, because the kernel must understand the cratis_chronicle_contracts version that mix deps.get resolves.

Get started with the Elixir client walks through installation, connecting, appending an event and reading a read model, including the failure results to expect along the way. The published documentation is at cratis.io, and the API reference is on HexDocs.

defmodule MyApp.Events.AccountOpened do
  use Chronicle.Events.EventType, id: "account-opened"

  # Typed defaults: the client derives the event's JSON schema from them.
  defstruct owner: "", balance: 0
end

defmodule MyApp.ReadModels.Account do
  use Chronicle.ReadModels.ReadModel

  defstruct id: "", owner: "", balance: 0

  # owner and balance are mapped by name; id comes from the event source id.
  from MyApp.Events.AccountOpened, set: [id: :event_source_id]
end

defmodule MyApp.Application do
  use Application

  @impl true
  def start(_type, _args) do
    children = [
      {Chronicle.Client,
       # Development-only credentials for the local development kernel.
       connection_string: "chronicle://chronicle-dev-client:chronicle-dev-secret@localhost:35000",
       event_store: "my-app",
       otp_app: :my_app}
    ]

    Supervisor.start_link(children, strategy: :one_for_one, name: MyApp.Supervisor)
  end
end

Then, with the application running (for example in iex -S mix):

alias Chronicle.Connections.Lifecycle

# The client connects and registers in the background; until then, calls return
# {:error, :not_connected}.
:ok = Lifecycle.wait_until(Lifecycle.name_for(Chronicle.Client), :registered)

:ok = Chronicle.append("account-42", %MyApp.Events.AccountOpened{owner: "Alice", balance: 1000})

# Projections run asynchronously: this can be {:ok, nil} until the projection catches up.
{:ok, account} = Chronicle.read_model(MyApp.ReadModels.Account, "account-42")

• The client skips TLS certificate validation unless the connection string sets skipTlsValidation=false.
• Version 3.5.0 had defects in read model mappings, reducer registration, constraint registration, seeding, read model paging, and sequence number lookups; use 3.5.1 or later.

The Elixir client documentation explains each one.

Source/
  chronicle/       ← cratis_chronicle Hex package
Documentation/     ← Elixir client documentation and client-owned snippets
Samples/
  console/         ← Runnable interactive console sample

cd Source/chronicle
mix deps.get
mix compile
mix test

A working example is in the Samples/console directory. It uses the client from Source/chronicle and starts its own kernel with Docker Compose; see its README for controls and details.

cd Samples/console
docker compose up -d
mix deps.get
mix run --no-halt

Set CHRONICLE_CONNECTION_STRING to connect to another kernel:

CHRONICLE_CONNECTION_STRING="chronicle://client-id:client-secret@myserver:35000?skipTlsValidation=false" mix run --no-halt

This project is part of Cratis — free, MIT-licensed tools for building event-sourced and CQRS applications.

• Chronicle — event-sourcing database and runtime. Orleans-based kernel, pluggable storage (MongoDB default; PostgreSQL, SQL Server, SQLite, in-memory), language-agnostic gRPC contracts. Docs
• Chronicle clients — first-class .NET SDK, plus TypeScript, Kotlin/Java, and this Elixir client; Python coming soon (pre-alpha). AI agents connect through the Chronicle MCP server.
• Arc — opinionated CQRS framework for ASP.NET Core with commands, queries, validation, authorization, and TypeScript proxy generation. Works without event sourcing. Docs
• Components — React components aligned with Arc patterns. Docs
• CLI + Workbench — inspect and diagnose Chronicle from the terminal or the browser. Docs
• Model-first layer (experimental) — Studio, Screenplay, Stage, Scene, Prologue
• Supporting — Fundamentals, Specifications, Synopsis, Lens, Narrator, and free AI tooling (preview); Ensemble coming soon (pre-release)
• Samples — runnable event sourcing and CQRS samples for the whole stack

Everything Cratis publishes today is MIT licensed and free to use.