---
title: Event sourcing in Elixir
description: "Do event sourcing in Elixir with Cratis Chronicle: an open-source (MIT) event-sourcing database with an idiomatic Elixir client on Hex — OTP-native event types, read models, transactions, optimistic concurrency, and resilient reconnection."
---

import { CardGrid, LinkCard } from '@astrojs/starlight/components';

Chronicle is an open-source (MIT) event-sourcing database and processing runtime, and `cratis_chronicle` is its idiomatic Elixir client. It exposes OTP-native constructs — `use Chronicle.EventType`, `use Chronicle.ReadModel`, `use Chronicle.Reactor`, `use Chronicle.Reducer`, and `use Chronicle.Seeder` — plus model-bound constraints, context-aware appends with process-scoped identity, correlation, and causation metadata, optimistic concurrency, transactions, jobs and webhooks, and automatic reconnection with exponential backoff. The client supervises a connection to the Chronicle kernel as part of your application's supervision tree.

With event sourcing, every state change is captured as an immutable event rather than an update in place. Chronicle stores those events, and read models are derived from them — declared model-bound projections execute server-side, or you can fold events into read models in your own process with a reducer.

The event store is not BEAM-only: Chronicle's kernel exposes a language-agnostic gRPC/protobuf boundary, and the same store is reachable from the [.NET, TypeScript, and Kotlin/Java clients](/chronicle-clients/) as well. Storage is pluggable — MongoDB by default, with PostgreSQL, SQL Server, SQLite, and in-memory providers implemented in the kernel.

## Install

```elixir
# mix.exs
defp deps do
  [
    {:cratis_chronicle, "~> 0.0"}
  ]
end
```

## A taste

An event type, a supervised client, and an append — from the client repository's quick example:

```elixir
defmodule MyApp.Events.AccountOpened do
  use Chronicle.EventType, id: "account-opened-v1"
  defstruct [:account_id, :owner_name, :initial_balance]
end

defmodule MyApp.Application do
  use Application

  def start(_type, _args) do
    children = [
      {Chronicle.Client,
        connection_string: "chronicle://localhost:35000",
        event_store: "my-app",
        otp_app: :my_app}
    ]

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

# Append an event
:ok = Chronicle.append("account-42", %MyApp.Events.AccountOpened{
  account_id: "account-42",
  owner_name: "Alice",
  initial_balance: 1000
})
```

For a local kernel to append to, the development Docker image is the quickest path: `docker run -p 35000:35000 cratis/chronicle:latest-development`.

## Where to go next

<CardGrid>
  <LinkCard title="Elixir client docs" description="Installation, connection setup, OTP integration, and the full Elixir API surface." href="/chronicle/clients/elixir/" />
  <LinkCard title="Get started with Chronicle" description="Run Chronicle locally and build your first event-sourced feature." href="/chronicle/get-started/" />
  <LinkCard title="Chronicle.Elixir on GitHub" description="The Elixir client repository — open source, MIT licensed." href="https://github.com/Cratis/Chronicle.Elixir" />
  <LinkCard title="cratis_chronicle on Hex" description="The Elixir client package for Chronicle." href="https://hex.pm/packages/cratis_chronicle" />
</CardGrid>

Looking for another language? See [Chronicle in your language](/chronicle-clients/) for the .NET, TypeScript, and Kotlin/Java clients.